Counting Days on the Water: Maritime Geofencing, Substantial Presence, & State Tax Boundaries
An adviser's guide to how consumer geocoders perform on coastal and offshore fixes, what federal and state tax law actually says about territorial waters, and where day-count treatment remains genuinely unsettled.
Article Navigation & Contents
1. The Offshore Yacht Problem: Reverse-Address Services vs. Legal Boundaries
Consider a high-net-worth client spending a weekend anchored 5 nautical miles offshore on their yacht off Palm Beach, Florida, or cruising off Long Island, New York.
In practice, the adviser's central inquiry is rarely "Does this count as a Florida day?"—Florida imposes no personal income tax. Rather, the client asks: "After departing New York on a prior day, the client spends the entire following calendar day anchored 5 nautical miles off Palm Beach, Florida—does this day count against their 183 New York statutory residency days, and does it count toward Florida domicile or U.S. federal physical presence?"
From a legal tax perspective, the boundary position is precise:
- Under Federal Tax Law: Under Treas. Reg. § 301.7701(b)-1(c)(2)(ii) , federal physical presence includes U.S. territorial waters. While Treasury regulations do not state a numeric nautical mile width, federal sovereignty extends 12 nautical miles under Presidential Proclamation 5928 . Under the modern 12-nautical-mile territorial-sea interpretation discussed below, a vessel 5 nautical miles offshore is treated as within U.S. territorial waters for Substantial Presence Test (SPT) purposes—the primary statutory formula under IRC § 7701(b) used by the IRS to establish U.S. tax residency based on day counts.
- Under State Tax Law: State tax law application to day counts on water remains legally unsettled. The Submerged Lands Act of 1953 (43 U.S.C. §§ 1301–1315) and Supreme Court holdings in United States v. Florida , 363 U.S. 121, 129 (1960), and United States v. Louisiana , 363 U.S. 1, 64 (1960), govern statutory grants of submerged-land title, seabed ownership, and natural resource/mineral rights—establishing seabed limits of 3 nautical miles on Atlantic/Pacific coasts and 3 marine leagues (approximately 9 modern NM) on Florida's Gulf Coast and Texas. However, state constitutional boundaries can extend wider than SLA submerged-land title limits for general state jurisdiction. Most notably, under Fla. Const. art. II, § 1(a) and Benson v. Norwegian Cruise Line, Ltd. , Florida's Atlantic constitutional boundary extends to the edge of the Gulf Stream or 3 geographic miles, whichever is greater (reaching 14 NM offshore in Benson).
Maritime Unit Definitions & Measurement Standards
The Core Thesis: Address Services vs. Jurisdictional Boundary Engines
Mobile operating system reverse-geocoders (such as Apple's CLGeocoder / MapKit and Android's android.location.Geocoder) are reverse-address services engineered for consumer display, turn-by-turn navigation, and street postal lookup—not legal jurisdictional boundary engines.
Official Platform Warnings: Android's official documentation explicitly warns developers: "Geocoding services may provide no guarantees on availability or accuracy. Results are a best guess... Do not use this API for any safety-critical or regulatory compliance purpose." Apple similarly describes CLGeocoder as converting coordinates into user-friendly display strings (street, city, state, country) or broader regional descriptors depending on context, without representing administrativeArea as a statutory legal jurisdiction determination.
Because consumer geocoder outputs represent descriptive metadata rather than legal determinations, day-count software relying strictly on raw reverse-geocodes without dedicated maritime boundary GIS processing can produce conflicting results across devices.
2. What the Law Says — Federal
At the federal level, Treasury regulations clearly include U.S. territorial waters in the physical-presence calculation, although the regulations do not specify their numeric seaward extent. The Substantial Presence Test (SPT) serves as the primary statutory mechanism used by the IRS under Internal Revenue Code § 7701(b) to establish U.S. tax residency jurisdiction over foreign individuals based on physical presence day counts (requiring at least 31 days in the current year and 183 days calculated across a 3-year weighted formula). Under Treasury regulations, presence on U.S. territorial waters is explicitly classified as physical presence within the United States.
Federal Statutory & Regulatory Citations
- Treas. Reg. § 301.7701(b)-1(c)(2)(ii) defines the "United States" for substantial presence as including the 50 states, the District of Columbia, and "the territorial waters of the United States, and the seabed and subsoil of those submarine areas which are adjacent to the territorial waters..."
- Treas. Reg. § 301.7701(b)-1(c)(2)(i) establishes the general physical presence rule: an individual is present in the United States on any day that they are physically present in the US at any time during the day.
- The Airspace Exclusion: Notably, Treas. Reg. § 301.7701(b)-1(c)(2)(ii) expressly excludes the airspace over the United States. A flight passing through US airspace without landing does not constitute physical presence—a bright-line administrative rule promulgated under IRC § 7701(b).
Federal SPT Exceptions and Excluded Days
While physical presence in U.S. territorial waters creates a general presumption of a U.S. day count, the Internal Revenue Code provides statutory exceptions and excluded days that vessel owners and crew members frequently invoke:
- IRC § 7701(b)(7)(C) — The 24-Hour Transit Exception: An individual who is in transit between two points outside the United States and who is physically present in the U.S. for less than 24 hours is not treated as present in the United States on any day during such transit. For example, a private yacht cruising from Nassau, Bahamas, to Cozumel, Mexico, may qualify if the individual's total physical presence in the United States is less than 24 hours and the activities undertaken while present are substantially related to completing that foreign-to-foreign transit.
- IRC § 7701(b)(7)(D) — Foreign Vessel Crew Member Exception: An individual who is a regular crew member of a foreign vessel engaged in transportation between the United States and a foreign country or a possession of the United States is not treated as present in the U.S. on any day of such service, unless the individual otherwise engages in a trade or business in the United States.
- IRC § 7701(b)(3)(D)(ii) — Medical Condition Exception: Days an individual is unable to leave the U.S. because of a medical condition or medical problem that arose while present in the U.S. are excluded from substantial presence calculations.
- IRC § 7701(b)(3)(B) — Closer Connection Exception: An individual who otherwise satisfies the weighted Substantial Presence Test is not treated as a U.S. resident alien if present in the U.S. for under 183 days during the current year, maintaining a tax home in a foreign country, and establishing a closer connection to that foreign country (claimed via IRS Form 8840).
Crucially, Treasury Regulation § 301.7701(b)-1(c)(2)(ii) includes "territorial waters" without specifying a numeric nautical mile width. The standard 12-nautical-mile interpretation stems from Presidential Proclamation 5928 (1988) , which extended the U.S. territorial sea from 3 to 12 nautical miles for international law and sovereignty purposes.
Regulatory Timing and Interpretive Limits
Treasury finalized § 301.7701(b)-1 in 1992 under T.D. 8411 (57 FR 15241), after Proclamation 5928 extended the U.S. territorial sea to 12 NM. However, substantially similar "territorial waters" language already appeared in Treasury's 1987 proposed regulations (52 FR 34252), before the Proclamation. Accordingly, regulatory drafting history does not itself establish whether Treasury intended a fixed 3-NM or dynamically defined territorial-water limit. Applying the modern 12-NM U.S. territorial sea to § 7701(b) therefore remains an interpretive conclusion rather than an expressly stated IRS rule.
Consequently, treating 12 nautical miles as the numeric width for federal Substantial Presence Test (SPT) day counting is a reasonable interpretive inference grounded in modern federal territorial-sea boundary principles, rather than an explicit statutory amendment to IRC § 7701(b).
3. What the Law Says — States
State authority over coastal waters is defined by state constitutional provisions and federal statutes, though its application to personal income tax requires careful legal analysis.
Florida Dep't of Revenue v. New Sea Escape Cruises, Ltd.
894 So. 2d 954 (Fla. 2005)
- Tax Statute Scope: Evaluated Florida sales/use tax mileage proration (Fla. Stat. § 212.02(8)) on commercial "cruises to nowhere" operating beyond state waters.
- 3-Mile Evidentiary Application: Applied a 3-mile boundary because neither party introduced evidence establishing the location of the Gulf Stream during the tax period, while expressly reaffirming the constitutional Gulf Stream rule and contrasting its facts with Benson.
- Disapproval of 12 NM Proclamation Inference: Expressly disapproved Dream Boat, Inc. v. Department of Revenue—rejecting the argument that because Presidential Proclamation 5928 extended the U.S. territorial sea to 12 nautical miles, state statutory tax jurisdiction automatically extends 12 miles offshore.
Benson v. Norwegian Cruise Line, Ltd.
859 So. 2d 1213 (Fla. 3d DCA 2003)
- State Constitutional Boundary Enforcement: Applied Fla. Const. art. II, § 1(a) , extending Florida's Atlantic boundary to the edge of the Gulf Stream (which was 14 nautical miles offshore on the date of the incident).
- Rejection of Submerged Lands Act Limitation: Held that neither the Submerged Lands Act nor international law prevents Florida from asserting state territorial jurisdiction over surface conduct on a vessel 11.7 nautical miles offshore within the Gulf Stream, unless overridden by controlling federal statute or treaty.
- Limit of Holding (>12 NM Unresolved): Because the vessel in Benson was 11.7 NM offshore, the court expressly declined to decide whether Florida can exercise state-law jurisdiction beyond the 12-nautical-mile international/federal sovereign boundary.
The Three Distinct Concepts of Coastal State Authority
To analyze maritime day counts accurately, tax advisers must distinguish between three separate boundary concepts that are frequently conflated:
- 1. State Submerged-Land Ownership Boundary: Federal statutory title grants under the Submerged Lands Act of 1953 (43 U.S.C. §§ 1301–1315) and Supreme Court rulings in United States v. Florida, 363 U.S. 121, 129 (1960), and United States v. Louisiana, 363 U.S. 1, 64 (1960), confirming state ownership of seabed title and natural resource/mineral rights out to 3 nautical miles on Atlantic/Pacific coasts and 3 marine leagues (approximately 9 modern NM) on Florida's Gulf Coast and Texas.
- 2. State Constitutional Boundary: The geographic boundary described by a state's constitution, subject to federal supremacy and the geographic reach of the particular state law at issue. Most notably, under Fla. Const. art. II, § 1(a) and Benson v. Norwegian Cruise Line, Ltd., Florida's Atlantic constitutional boundary extends to the edge of the Gulf Stream or 3 geographic miles, whichever is greater (reaching 14 NM offshore in Benson and landward of the fluctuating Gulf Stream edge off Daytona Beach / Ponce Inlet).
- 3. Geographic Reach of a Specific State Tax Statute: The territorial reach defined by specific legislative enactments. As Florida courts recognized in Kipp v. Amy Slate's Dive Center, Inc. , 251 So. 3d 941, 946 (Fla. 3d DCA 2018) and New Sea Escape Cruises, the legislature may choose to apply a particular statutory tax provision to less than the full extent of the state's constitutional territory.
The Limits of Judicial Authority & Statutory Context
Decisions like New Sea Escape Cruises (sales/use tax proration) and Benson (tort jurisdiction over surface conduct) establish state constitutional seaward limits under Fla. Const. art. II, § 1 . Their treatment of Florida's seaward boundaries is highly relevant to physical-presence analysis, but neither case is a direct holding on personal-income-tax residency day counts.
Submerged Lands Act vs. State Constitutional Boundaries
It is critical to distinguish between general state constitutional boundaries and federal statutory grants of submerged-land ownership under the Submerged Lands Act of 1953 (43 U.S.C. §§ 1301–1315) .
Under 43 U.S.C. § 1312, Congress confirmed state submerged-land ownership boundaries and title to natural resources out to 3 nautical miles (3 geographical miles) on the Atlantic and Pacific coasts. For the Gulf Coast, the U.S. Supreme Court confirmed in United States v. Florida , 363 U.S. 121, 129 (1960), and United States v. Louisiana , 363 U.S. 1, 64 (1960), that Florida (on its Gulf Coast) and Texas hold historical submerged-land ownership boundaries extending 3 marine leagues (approximately 9 modern NM) seaward based on historic statehood boundaries, whereas Louisiana, Mississippi, and Alabama were restricted to 3 nautical miles.
While the Submerged Lands Act governs underwater seabed title and natural resource ownership, state constitutional provisions (such as Florida's Gulf Stream boundary clause in Fla. Const. art. II, § 1(a) ) define state territorial limits for general statutory jurisdiction. As a result, an Atlantic vessel floating 11 nautical miles offshore off Miami may remain within Florida constitutional territory under Benson, while a vessel 5 nautical miles off Georgia sits in federal waters past Georgia's 3-nautical-mile statutory boundary.
New York Statutory Boundaries: Offshore Waters (§ 7-a), NY–NJ Waters (§ 7), & NY–CT Long Island Sound (§ 2)
In addition to constitutional provisions and federal baseline rules, state legislative statutory enactments explicitly define offshore and interstate boundary lines. New York offers a prime example of statutory boundary mechanics across open ocean, riverine, and sound environments:
N.Y. State Law § 7-a: Offshore Waters & 3-Mile Limit
Jurisdiction and Ownership of Offshore Waters and Lands Thereunder
- 3-Mile Marginal Sea Baseline: Under N.Y. State Law § 7-a(1)(b), New York asserts jurisdiction over marginal seas out to 3 geographical miles from the coastline.
- Dynamic Language vs. Judicial Interpretation: While § 7-a(1)(b) includes language contemplating extension to "any other line farther seaward therefrom hereinafter defined or recognized by the United States" (and a pre-1988 opinion, 1984 N.Y. Op. Att'y Gen. 41, suggested tracking federal expansions), the Southern District of New York squarely rejected the 12 NM expansion argument in Murray v. AET Inc. Ltd. , 582 F. Supp. 3d 140, 144–46 (S.D.N.Y. 2022) (Reif, J., sitting by designation). The court held that Presidential Proclamation 5928 did not expand New York State's territorial waters to 12 NM, emphasizing that the 1984 AG opinion predated Proclamation 5928 and could not account for its explicit restriction against altering domestic statutory boundaries.
- Domicile365 Baseline Treatment: Accordingly, Domicile365 treats New York's Atlantic state boundary as capped at 3 NM under Murray absent contrary controlling state authority.
N.Y. State Law § 7: NY–NJ Interstate Water Boundary
New York – New Jersey Boundary Line Agreement of 1889
- Interstate Estuarine Partitioning: Codifies the statutory boundary agreement between New York and New Jersey in lands under water across Raritan Bay, Arthur Kill, Kill von Kull, New York Bay, and the Hudson River.
- Explicit Geodetic Coordinates: Establishes the exact interstate dividing line using absolute latitude and longitude monument coordinates (from Point A at Ward's Point / Perth Amboy through Point JJ at 40° 59' 49.74" N, 73° 53' 38.57" W on the Hudson River).
- Tax Jurisdiction Significance: Prevents territorial ambiguity in high-density shared waters, establishing precise geographic cutoff lines for vessel locations operating between New York and New Jersey (complementing New Jersey v. New York , 523 U.S. 767 (1998)).
N.Y. State Law § 2: NY–CT Long Island Sound Boundary
New York – Connecticut Boundary Line Agreement
- Long Island Sound Partitioning: Codifies the statutory boundary agreement dividing Long Island Sound and adjacent waters between New York and Connecticut.
- Geodetic Great Circle Arc: Sets the interstate boundary along a great circle arc (from Point 170 off Byram Point to Point 171 south of New London Lighthouse, through Fishers Island Sound to the Rhode Island border at Point 174).
- Exclave & Transit Impact: Defines exact state jurisdiction for vessels anchored in Long Island Sound or transiting between Connecticut and New York exclaves like Fishers Island.
Primary Legal Authorities & PDF Downloads
Access full-text statutory regulations, constitutional provisions, presidential proclamations, federal acts, and judicial decisions governing territorial limits:
Treas. Reg. § 301.7701(b)-1
Defines U.S. territorial waters and physical presence rules for federal Substantial Presence Test (SPT).
Pres. Proc. 5928
1988 Presidential Proclamation establishing 12-nautical-mile U.S. territorial sea sovereignty boundary.
Submerged Lands Act
43 U.S.C. § 1312 submerged-land ownership boundary (3 NM ocean / 9 NM Gulf).
Fla. Const. Art. II, § 1
Florida Constitution seaward boundary (3 marine leagues (approximately 9 modern NM) Gulf; Gulf Stream edge or 3 geographic miles Atlantic).
N.Y. State Law § 7-a
New York offshore waters statutory framework (3 NM baseline; subject to Murray v. AET Inc. 3 NM cap holding).
Murray v. AET Inc. Ltd.
582 F. Supp. 3d 140 holding Pres. Proc. 5928 did not expand NY state territorial waters to 12 NM.
N.Y. State Law § 7
Statutory 1889 agreement establishing absolute NY-NJ boundary coordinates in Raritan Bay, Arthur Kill, Kill von Kull, NY Bay, & Hudson River.
N.Y. State Law § 2
Statutory interstate agreement defining NY-CT boundary lines across Long Island Sound to Byram River and Fishers Island Sound.
New Jersey v. New York
523 U.S. 767 U.S. Supreme Court ruling on Ellis Island territorial boundary and filled land sovereignty.
FLDOR v. New Sea Escape
Florida Supreme Court decision (894 So. 2d 954) disapproving 12 NM tax proration logic.
Benson v. Norwegian Cruise Line
859 So. 2d 1213 holding state Atlantic territory extends to Gulf Stream edge (11.7 NM offshore).
U.S. v. Florida / U.S. v. Louisiana
363 U.S. 121 (Florida) / 363 U.S. 1 (Louisiana) holding 3 marine league (approximately 9 modern NM) Gulf boundary applies to FL/TX only.
4. Where It Is Genuinely Unsettled
Evaluating maritime day counts requires acknowledging where explicit statutory authority ends and administrative interpretation begins.
Key Unsettled Areas in Maritime Day Counting
- Absence of Published Revenue Rulings: As of September 2026, our research has not identified published New York Department of Taxation and Finance (NY DTF) or California Franchise Tax Board (CA FTB) guidance or administrative rulings addressing whether a day spent entirely aboard a private vessel anchored in state waters constitutes a physical presence day for personal income tax statutory residency.
- Transit Exceptions & Port Stops: Statutory transit exceptions (such as New York's 20 NYCRR § 105.20(c), which excludes presence in NY solely for passing through to another destination) turn on subjective facts that a location sensor cannot observe. A voluntary non-transit stop such as dining or shore recreation would generally be inconsistent with presence solely for transit, while emergency or unavoidable stops require facts-and-circumstances analysis.
- International Variations: Tax and maritime jurisdiction rules outside the US vary widely across sovereign nations and international conventions. Because tax treatments for foreign territorial seas and passage rights have not been comprehensively surveyed, international vessel movements require separate local legal analysis.
The Core Question Requiring Professional Guidance: An empirical location log establishes precisely where a device was physically situated relative to statutory boundaries. How that location record translates into statutory residency day counts—such as whether a day spent anchored 1 nautical mile offshore constitutes physical presence under state tax law—is a legal question that requires professional tax guidance.
5. Empirical Evaluation: iOS (CLGeocoder) vs. Android (Geocoder) in Maritime Waters
Mobile operating systems use built-in reverse-geocoding frameworks to convert GPS latitude and longitude into human-readable locations. Standard frameworks—such as Apple's CLGeocoder (CLPlacemark) and Android's Geocoder (Address)—were designed primarily for street-level postal address lookup rather than offshore maritime boundary resolution.
Platform API Evolution & Apple Deprecation Notice
At WWDC 2025, Apple formally marked CLGeocoder in Core Location as deprecated in favor of MapKit's modern MKReverseGeocodingRequest . In software engineering, deprecation serves as an official warning that an API is legacy and earmarked for eventual retirement; while CLGeocoder continues to function today, Apple may remove the feature completely in a future iOS release. Developers building residency tracking systems must account for this deprecation and transition to MapKit's MKReverseGeocodingRequest or server-side GIS resolution for long-term stability.
The Operational Reality: Typically Very Accurate, With Rare Offshore Divergence
In everyday consumer use, land navigation, and nearshore coastal settings, OS reverse geocoding is typically very accurate. Empirical testing demonstrates that both CLGeocoder and Android Geocoder perform exceptionally well across standard geographies—accurately resolving administrative state, county, and country placemarks close to shore (e.g., Volusia County 1.9 NM off Daytona) and cleanly recognizing sovereign island bubbles (e.g., Cayos Arcas off Mexico). On rare occasions, however, when coordinates cross specific offshore statutory boundaries (such as the 9 NM Gulf rule or federal territorial sea cut-offs), consumer APIs can produce non-statutory metadata or exhibit platform-specific boundary variations.
Empirical Test Methodology & Runtime Environment
Empirical evaluations were conducted using automated coordinate transect scripts executed within standard platform developer environments under US locale settings:
- iOS Simulator Environment: Executed in the Xcode iOS Simulator running iOS 26.0 using the latest Core Location
CLGeocoderand MapKitMKReverseGeocodingRequestAPIs. - Android Emulator Environment: Executed in the Android Studio Emulator (Android API level 36.1 with Google Play Services enabled) using the latest
android.location.GeocoderAPIs. - Sampling Method: Coordinates were queried programmatically at 0.5 to 2.0 nautical mile intervals across established coastal and offshore transects to observe boundary step transitions.
5.1 Domestic U.S. Maritime Transects & OS Geocoder Performance
Consumer reverse-geocoding engines are engineered for human-scale address discovery, turn-by-turn navigation, and regional metadata tagging, where they excel. Developers often wonder whether consumer APIs adhere to statutory maritime limits or simply terminate their administrative boundary polygons at the high-water baseline. Through automated transect testing across Atlantic, Gulf of Mexico, and Pacific baselines, we empirically evaluated OS geocoder behavior against three statutory maritime benchmarks:
- 1. The 3 Nautical Mile (NM) State Boundary: Submerged Lands Act of 1953 (Atlantic & Pacific States).
- 2. The 9 Nautical Mile (NM) State Boundary: Historical Gulf Coast submerged-land ownership boundary recognized exclusively for Florida and Texas under the Submerged Lands Act and United States v. Florida, 363 U.S. 121 (1960) / United States v. Louisiana, 363 U.S. 1 (1960).
- 3. The 12 Nautical Mile (NM) Sovereign Boundary: 12 NM Territorial Sea (asserted under Presidential Proclamation 5928 consistent with customary international law as reflected in UNCLOS).
Dataset 1: Key Domestic Transect Comparative Results
| Test Location | Legal & Statutory Status | iOS Output (CLGeocoder) | Android Output (Geocoder) | Observed Result & Context |
|---|---|---|---|---|
|
Cayos Arcas (Gulf of Mexico) 20.3775, -92.0550 |
< 12 NM from Mexican cay (Cayo del Oeste). Sovereign Mexican Territorial Sea. |
Country: Mexico (MX) State: nil Water: Bay of Campeche |
Country: Mexico (MX) State: null County: null Feature: Mexico |
Matches Statutory Boundary Recognized remote sovereign island's 12 NM territorial sea envelope on both platforms. |
|
Daytona Beach (Atlantic) 29.2108, -80.9700 (~1.9 NM out) |
< 3 NM from baseline. Within Florida State Submerged Lands & US Sovereign Waters. |
Country: United States (US) State: FL County: Volusia County |
Country: United States (US) State: Florida County: Volusia |
Matches Statutory Boundary Returned both state and county administrative boundaries within nearshore coastal waters on both platforms. |
|
Daytona Beach (Atlantic) 29.2108, -80.8900 (~6 NM out) |
> 3 NM Florida SLA submerged-land boundary; < 12 NM U.S. territorial sea; landward of the fluctuating Gulf Stream edge on the test date (which ranges between 35 NM and 45 NM offshore off Daytona Beach / Ponce Inlet) under Fla. Const. art. II, § 1(a). |
Country: United States (US) State: nil County: Atlantic Coastal Plain |
Country: United States (US) State: null County: null |
Matches 3-NM SLA / Diverges from FL Constitutional Boundary Both platforms drop Florida attribution beyond the SLA submerged-land line, even though the coordinate was landward of Florida's Gulf Stream constitutional boundary. Whether that constitutional location constitutes a Florida "day" under any particular state statute remains a separate legal question. |
|
Daytona Beach (Atlantic) 29.2108, -80.6500 (~18 NM out) |
> 12 NM U.S. territorial sea; > 3 NM Florida SLA submerged-land boundary. Landward of Florida's constitutional Gulf Stream boundary off Daytona Beach / Ponce Inlet. Whether Florida may exercise state jurisdiction beyond 12 NM is unresolved by Benson. |
Country: nil State: nil County: nil |
Country: United States (US) State: null County: null |
Matches Federal 12-NM / State Status Indeterminate CLGeocoder dropped US country attribution beyond 12 NM. Android maintained Country: US into the contiguous zone. |
|
Sarasota (Gulf of Mexico) 27.3364, -82.6850 (~6 NM out) |
< 9 NM. Legally inside Florida state waters under the Gulf Submerged Lands Act (U.S. v. Florida, 363 U.S. 121). |
Country: United States (US) State: nil County: Sarasota County Water: North Atlantic Ocean |
Country: United States (US) State: Florida (FL) County: Sarasota County |
iOS Diverges / Android Matches 9 NM Rule Android correctly recognized Florida's 9 NM Gulf boundary (attributing State: FL), whereas iOS applied a standard 3 NM buffer (setting State: nil).
|
|
Sarasota (Gulf of Mexico) 27.3364, -82.7700 (~10.5 NM out) |
> 9 NM, < 12 NM. Outside FL waters; inside US sovereign territorial sea. |
Country: United States (US) State: nil County: Sarasota County Water: North Atlantic Ocean |
Country: United States (US) State: null County: null |
Matches Statutory Boundary Set State to nil beyond state waters, while maintaining county attribution on iOS into federal waters. |
|
New York Bight (Atlantic) 40.4700, -73.6000 (~7 NM out) |
> 3 NM, < 12 NM. Outside NY state waters; inside US territorial sea. |
Country: United States (US) State: Mid-Atlantic States County: NY Metropolitan Area Water: North Atlantic Ocean |
Country: United States (US) State: null County: null |
Returns Regional Descriptor iOS populated regional and metropolitan statistical area descriptors (Mid-Atlantic States, NY Metropolitan Area) in administrative fields beyond 3 NM state boundary. |
|
South of Long Island (Atlantic) 40.5300, -73.0000 (~6 NM out) |
> 3 NM, < 12 NM. Outside NY state waters; inside US territorial sea. |
Country: United States (US) State: Mid-Atlantic States County: The East Coast Water: North Atlantic Ocean |
Country: United States (US) State: null County: null |
Returns Regional Descriptor iOS populated informal geographic descriptors (Mid-Atlantic States, The East Coast) in administrative fields. |
Dataset 2: Georgia Offshore Stepped Transect (Latitude 31.9900° N)
| Offshore Distance | Coordinate | Statutory Status | iOS Output (CLGeocoder) | Android Output (Geocoder) |
|---|---|---|---|---|
| ~1.5 NM | 31.99, -80.815 | Inside GA State (< 3 NM) | Country: US State: GA County: Chatham County Water: North Atlantic Ocean |
Country: US State: GA County: Chatham |
| ~2.8 NM | 31.99, -80.790 | Edge of GA State (< 3 NM) | Country: US State: GA County: Chatham County Water: North Atlantic Ocean |
Country: US State: GA County: Chatham |
| ~6.0 NM | 31.99, -80.727 | Federal Waters (> 3 NM GA, < 12 NM US) | Country: US State: nil County: Atlantic Coastal Plain Water: North Atlantic Ocean |
Country: US State: null County: null |
| ~10.5 NM | 31.99, -80.639 | Outer Territorial Sea (< 12 NM US) | Country: US State: nil County: Atlantic Coastal Plain Water: North Atlantic Ocean |
Country: US State: null County: null |
| ~16.0 NM | 31.99, -80.531 | Outside US Sovereignty (> 12 NM); Contiguous Zone | Country: nil State: nil County: nil Water: North Atlantic Ocean |
Country: US State: null County: null |
| ~35.0 NM | 31.99, -80.157 | Deep EEZ (> 24 NM, < 200 NM) | Country: nil State: nil County: nil Water: North Atlantic Ocean |
Empty Result (Null List) |
Dataset 3: New York / New Jersey Regional Coastal & Boundary Transect
To evaluate platform performance across complex estuarine, island, and multi-state boundary geographies, we executed a 6-point empirical evaluation across New York and New Jersey coastal waters:
| Test Location & Coordinates | Legal & Statutory Status | iOS Output (CLGeocoder) | Android Output (Geocoder) |
|---|---|---|---|
|
[1] Ellis Island (South Landfill) 40.6975, -74.0415 |
State of New Jersey (Hudson County — NJ v. NY , 523 U.S. 767 (1998); N.Y. State Law § 7 1889 interstate line). | Country: US State: NJ County: Hudson County Locality: Jersey City |
Country: US State: New Jersey County: Hudson County Locality: Jersey City Feature: New Hospital Extension |
|
[2] Fishers Island (Land Exclave) 41.2580, -72.0150 |
State of New York (Suffolk County / Town of Southold), adjacent to CT coast (N.Y. State Law § 2 NY-CT line). | Country: US State: NY County: Suffolk County Locality: Fishers Island |
Country: US State: New York County: Suffolk County Locality: Fishers Island Feature: 288 |
|
[3] Long Island Sound (Water) 41.0300, -73.0700 |
New York Internal Waters (Suffolk County / Town of Brookhaven; N.Y. State Law § 7-a(1)(a) ; N.Y. State Law § 2 ). | Country: US State: NY County: Suffolk County Locality: Port Jefferson Water: Long Island Sound |
Country: US State: New York County: Suffolk County Locality: Brookhaven Feature: Brookhaven |
|
[4] Fire Island ~1.5 NM Offshore 40.6150, -73.1600 |
< 3 NM from baseline. Inside NY State Submerged Lands & US Waters (N.Y. State Law § 7-a(1)(b) ). | Country: US State: NY County: Suffolk County Locality: Islip Water: North Atlantic Ocean |
Country: US State: New York County: Suffolk County Locality: Islip Feature: Islip |
|
[5] Fire Island ~5.5 NM Offshore 40.5500, -73.1600 |
> 3 NM New York state boundary; < 12 NM U.S. territorial sea. Outside NY state territorial waters under the reasoning of Murray v. AET Inc. Ltd. ; inside U.S. territorial sea. | Country: US State: Mid-Atlantic States County: New York Metropolitan Area Locality: nil Water: North Atlantic Ocean |
Country: US State: null County: null Locality: null Feature: United States |
|
[6] Montauk Point ~16.0 NM SE 40.9200, -71.5800 |
> 12 NM. Outside US Sovereign Waters; inside US EEZ / Contiguous Zone. | Country: nil State: nil County: nil Locality: nil Water: North Atlantic Ocean |
Country: US State: null County: null Locality: null Feature: United States |
Key Technical Takeaways for Compliance Engineers
1. Sovereign 12 NM Limit vs. Offshore Provider Fallbacks
In testing off the Georgia coast (~16 NM offshore), iOS transitioned country attribution to nil past the 12 NM territorial sea boundary. In contrast, Android maintained Country: US past 16 NM until dropping to an empty result between 16 NM and 35 NM offshore (encompassing the 24 NM contiguous zone). Both reflect platform backend design choices, though tax compliance applications require strict adherence to 12 NM sovereign territorial sea limits.
2. Regional Descriptors Beyond 3 NM State Limits
Past the 3 NM state boundary, iOS does not simply set State: nil everywhere. In regional coastal waters like the NY Bight (~7 NM) and Fire Island (~5.5 NM), iOS returns broad regional descriptors like State: Mid-Atlantic States or County: New York Metropolitan Area. This presents a worse failure mode than nil for naive parsers, which may write `"Mid-Atlantic States"` into a state database field or misattribute state residency.
3. The 9 NM Gulf Coast Boundary Rule
Off Sarasota (~6 NM in the Gulf), Android correctly recognized Florida's 9 NM Gulf boundary (attributing State: FL), whereas iOS applied a standard 3 NM buffer (setting State: nil). Under Supreme Court holdings in United States v. Florida, 363 U.S. 121 (1960) and United States v. Louisiana, 363 U.S. 1 (1960), Florida's Gulf coast and Texas hold statutory 9 NM submerged land limits, whereas Atlantic states default to 3 NM.
4. Florida Atlantic Coastal Progression
Our Daytona transect, together with Benson, illustrates a 3-step legal progression: at 6 NM, consumer geocoders drop state attribution to nil matching the 3-NM SLA line (though landward of the fluctuating Gulf Stream edge); at 11.7 NM, Benson confirms state territory within the 12-NM limit; at 18 NM (past the 12-NM federal limit), federal sovereignty drops to Country: nil, while Benson explicitly leaves state jurisdiction past 12 NM unresolved.
5.2 Global Non-Standard Territorial Sea Limits & International Anomalies
While Article 3 of the United Nations Convention on the Law of the Sea (UNCLOS) sets 12 nautical miles as the maximum standard territorial sea limit, global tax compliance and maritime resolution engines must handle significant international statutory anomalies where nation-states assert narrower, wider, or treaty-based sovereign water limits:
| Region / Jurisdiction | Asserted Maritime Limit | Legal Authority, Treaty Framework & Compliance Context |
|---|---|---|
| Greece & Turkey (Aegean Sea) |
6 Nautical Miles | In the island-dense Aegean Sea, both Greece (Greek Law 230/1936) and Turkey (Law No. 476 of 1964 and Law No. 2674 of 1982) assert a 6-nautical-mile territorial sea. Although Greece (an UNCLOS signatory) claims the legal right to extend to 12 NM, Turkey maintains a formal 1995 casus belli declaration against unilateral extension. Note: Greece asserts standard 12 NM in the Ionian Sea (since 2021), and Turkey asserts 12 NM in the Black Sea and Mediterranean. |
| Caspian Sea States (Azerbaijan, Iran, Kazakhstan, Russia, Turkmenistan) |
15 Nautical Miles | As an inland, landlocked body of water, the Caspian Sea operates outside standard UNCLOS regimes. Under Article 7 of the 2018 Aktau Convention on the Legal Status of the Caspian Sea, the five littoral states established a 15-nautical-mile territorial waters zone plus an adjacent 10-nautical-mile exclusive fishery zone (totaling 25 NM of national maritime jurisdiction). |
| El Salvador & Peru (Constitutional Maritime Dominions) |
200 NM Dominion | Non-UNCLOS states assert constitutional sovereignty over extended maritime zones. El Salvador claims a 200 NM territorial sea (Salvadoran Const. art. 84), while Peru constitutional law defines a 200-nautical-mile maritime dominion (Mar de Grau; Peruvian Const. art. 54; 1947 Decree No. 781) in which Peru exercises sovereignty and jurisdiction while preserving international navigation freedoms (recognized in the 2014 ICJ Peru v. Chile judgment). |
| UNCLOS Modern Realignments (Somalia, Benin, Togo) |
12 Nautical Miles (Aligned from 200 NM / 30 NM) |
Nations that historically claimed extended territorial seas—such as Somalia (formerly 200 NM under 1972 law), Benin (historically listed by DOALOS at 200 NM under 1976 Decree No. 76-92 before enacting its 2011 Maritime Code), and Togo (formerly 30 NM under 1977 ordinance)—have modernized domestic maritime laws to align with UNCLOS Article 3 (adopting a standard 12 NM territorial sea and a 200 NM EEZ, as confirmed in Somalia's 2014 Maritime Law and ICJ jurisprudence). |
Empirical Case Study: Peru 200-NM Mar de Grau Transect & Cross-Platform Evaluation
To evaluate how consumer reverse-geocoders handle extended constitutional territorial claims in practice, we executed an 8-point empirical transect extending from ~2.5 nautical miles to 350 nautical miles off the coast of Callao/Lima, Peru (-12.06° S baseline). The results reveal a profound cross-platform divergence between iOS and Android:
| Test Location & Distance | Legal Status & Maritime Framework | iOS Output (CLGeocoder) | Android Output (Geocoder) |
|---|---|---|---|
|
[1] Callao Nearshore -12.060, -77.202 (~2.5 NM) |
Inside UNCLOS 12 NM Territorial Sea & Peru Mar de Grau. | Country: Peru (PE) State: Callao Water: Pacific Ocean |
Country: Peru (PE) State: null County: null Feature: Peru |
|
[2] Lima Offshore -12.060, -77.310 (~8.8 NM) |
Inside UNCLOS 12 NM Territorial Sea & Peru Mar de Grau. | Country: Peru (PE) State: Callao Water: Pacific Ocean |
Country: Peru (PE) State: null County: null Feature: Peru |
|
[3] Beyond 12 NM Limit -12.060, -77.467 (~18.0 NM) |
Beyond conventional 12-NM territorial-sea limit; within Peru's 200-NM constitutional maritime dominion (Mar de Grau). | Country: Peru (PE) State: Callao Water: Pacific Ocean |
Empty result (Null address list) |
|
[4] Mid-Continental Shelf -12.060, -78.012 (~50.0 NM) |
Beyond conventional 12-NM territorial-sea limit; within Peru's 200-NM constitutional maritime dominion (Mar de Grau). | Country: Peru (PE) State: Callao Water: Pacific Ocean |
Empty result (Null address list) |
|
[5] Deep Pacific Ocean -12.060, -79.716 (~150.0 NM) |
Beyond conventional 12-NM territorial-sea limit; within Peru's 200-NM constitutional maritime dominion (Mar de Grau). | Country: Peru (PE) State: Lima Water: Pacific Ocean |
Empty result (Null address list) |
|
[6] Outer EEZ Arc -12.060, -80.739 (~210.0 NM) |
Within NW Peruvian Headland 200-NM Arc; High Seas relative to local baseline. | Country: Peru (PE) State: Lima Water: Pacific Ocean |
Empty result (Null address list) |
|
[7] True High Seas -12.060, -81.850 (~275.0 NM) |
Unambiguous International High Seas (> 200 NM from all Peruvian land). | Country: nil (nil) State: nil Water: South Pacific Ocean |
Empty result (Null address list) |
|
[8] Deep Basin High Seas -12.060, -83.100 (~350.0 NM) |
Undisputed Open Ocean High Seas. | Country: nil (nil) State: nil Water: South Pacific Ocean |
Empty result (Null address list) |
Empirical Provider Divergence Across Extended Maritime Dominions
In our Peru transect, iOS continued returning Country: Peru and Peruvian administrative areas (State: Callao / State: Lima) well beyond 12 NM out to ~210 NM offshore, while the tested Android Geocoder returned no address beyond approximately 12 NM. These results demonstrate materially different provider-side geographic attribution—not that either platform is making a legal determination about Peru's maritime claims. For tax compliance applications, relying on unparsed consumer geocoder data creates severe cross-platform audit inconsistencies where identical offshore positions are recorded as national territory on iOS and open sea on Android.
Architectural Recommendation for Advisers & Developers
Consumer reverse-geocoders (Apple’s CLGeocoder and Android’s Geocoder) are typically very accurate for everyday location tracking, onshore addresses, and nearshore placemarks. However, because tax compliance platforms, digital logbooks, and statutory residency trackers require strict legal boundary enforcement, developers building maritime apps should supplement consumer geocodes with dedicated maritime GIS layer resolution to handle specialized offshore boundaries seamlessly.
6. What Domicile365 Now Does
To resolve these coastal ambiguities, Domicile365 built a specialized server-side PostGIS maritime geocoding engine that evaluates raw GPS coordinates against a multi-zone GIS boundary cascade—incorporating U.S. Census Bureau TIGER/Line 2025 state shapefiles (where our inspection of TIGER/Line 2025 state geometries shows internal waters, 3 NM Atlantic/Pacific state limits, and Florida's 9 NM Gulf boundary), Natural Earth 10m global land and sub-national administrative layers, and Marine Regions v4 maritime polygons (defining internal waters, archipelagic waters, and 12-nautical-mile territorial seas worldwide)—rather than relying on standard consumer reverse-geocoder address tables.
In standard operation, the Domicile365 mobile app utilizes native iOS and Android reverse-geocoding platforms and supplements those results with data from our engine whenever native iOS or Android platforms return empty or incomplete results. Domicile365 subscribers can request to have their location records run directly through our specialized server-side maritime geocoding engine at any time.
Multi-Zone Boundary Cascade
Coordinates are resolved against a 4-zone GIS cascade combining U.S. Census Bureau TIGER/Line 2025 (Zone 0 for US states), Natural Earth 10m (Zone 0 for 239 global countries), and Marine Regions v4 maritime polygons (Zone 1 internal waters, Zone 2 archipelagic waters, and Zone 3 12 NM territorial seas), supported by Natural Earth Admin-1 for foreign subdivisions.
State vs. Federal Partitioning & Differing Boundaries
Positions are evaluated against state boundary layers. For most Atlantic/Pacific SLA boundaries this is 3 NM, while special rules—including Florida's 9 NM Gulf boundary and Atlantic Gulf Stream clause—are explicitly accounted for. Where underlying GIS source shapefiles contain omissions (such as TIGER 2025 omitting Texas's 9 NM Gulf SLA line) or dynamic boundaries cannot be captured by static polygons, the engine flags coordinates for manual review rather than asserting false certainty.
Boundary Proximity & Audit Trail
Every determination logs how the match was made and the distance (in meters) to the nearest jurisdictional boundary (res_edge_m) and confidence bands (res_conf), making borderline locations transparent rather than asserting certainty.
Versioned Maritime Datasets
Boundary datasets are version-controlled so an audit report generated today remains reproducible and defensible years later if underlying GIS layers are updated.
The Adviser's Decisive Result: Partitioning Federal Waters
For tax advisers, this partitioning produces the exact structural clarity needed in high-stakes residency audits. When a taxpayer anchors a vessel 10.5 nautical miles west of Sarasota, Florida, Domicile365's engine resolves the coordinate as U.S. Federal Territorial Waters (State: None / Unassigned). This fix establishes that:
1. Outside Florida Territorial Boundary: The coordinate is beyond Florida's 9-NM Gulf constitutional/SLA boundary and therefore is not attributed to Florida by the location engine.
2. Within U.S. Territorial Sea: The coordinate lies inside the 12-NM federal territorial sea and is treated by Domicile365 as U.S. federal physical presence for SPT purposes under Treas. Reg. § 301.7701(b)-1(c)(2)(ii), subject to applicable statutory exclusions under IRC § 7701(b)(7).
Core Thesis Insight: Consumer Geocoders vs. Specialized Legal Boundary Resolution
Raw GPS coordinates are independent of reverse-geocoder metadata and can provide meter-level accuracy under favorable conditions, subject to the device's reported location accuracy. Consumer reverse-geocoders are not faulty; they answer a consumer location-description question using provider-specific geographic and administrative datasets. Our testing shows that those underlying datasets can differ by platform (such as Android recognizing Florida's 9-NM Gulf boundary while iOS applies a standard 3-NM buffer) and need not correspond to the legal maritime boundary relevant for tax compliance. Dedicated maritime GIS processing resolves this gap by evaluating coordinates against multi-layered legal boundary datasets directly.
This capability provides an empirical location record of where the device moved on the water; it does not dictate how state tax authorities will treat that day legally.
7. Technical Limits & The Baseline Measurement Problem
Here is where maritime geofencing reaches its natural technical boundaries:
| Scenario / Geography | Technical Limit & Audit Treatment |
|---|---|
| Dynamic Oceanographic Boundaries (Florida Atlantic Gulf Stream) | Florida's constitutional Atlantic boundary (the edge of the Gulf Stream under Fla. Const. art. II, § 1(a)) is inherently time-dependent and fluctuates continuously (e.g. National Weather Service forecasts placing the west wall of the Gulf Stream off Ponce Inlet between 37 NM and 44 NM offshore over a recent two-week period we reviewed). |
| Inconsistent GIS Source Data & Omitted State Boundaries | Published GIS boundary shapefiles often contain internal omissions regarding differing state boundaries. For example, U.S. Census Bureau TIGER/Line 2025 shapefiles encode Florida's 9-nautical-mile Gulf submerged-land boundary, but omit Texas's 9-nautical-mile Gulf boundary—despite both deriving from the same 1960 Supreme Court ruling (United States v. Louisiana ). |
| The Statutory Baseline Problem | Maritime boundaries (3 NM, 9 NM, 12 NM) are measured seaward from official statutory baselines—which under official NOAA Office of Coast Survey standards consist of the normal low-water line marked on large-scale nautical charts, juridical bay and river closing lines, and coastal accretion/erosion dynamics. Measuring from the physical low-water shoreline vs. straight bay/river closing baselines (e.g. Tybee Island / Savannah River entrance in Georgia, or Florida Bay / Ten Thousand Islands in Florida) is a central technical challenge in maritime boundary work, as natural shoreline movement continuously alters baseline geometry. |
| Aegean Sea (Greece & Turkey) | Both Greece and Turkey assert a 6-nautical-mile territorial sea in the Aegean Sea under regional decrees, whereas standard published datasets model 12 nautical miles. Fixes in this 6-to-12 mile band are flagged for manual review rather than definitively attributed. |
| Uncharted Archipelagos | Certain heavily indented coastlines lack high-resolution charted baseline detail. Coordinates in these zones are marked with lower spatial confidence indicators. |
| Intent & Transit Reasons | The Domicile365 software platform records device speed, position, altitude, and accuracy; it does not infer why the vessel was moving or whether a stop constituted a tax-defeating non-transit event. |
| Disabled Location Services | If location services are turned off while afloat, no position data exists. A day on the water with disabled location services cannot be attributed at all. |
8. Conclusion & Adviser Summary Loop
A day spent on coastal waters is still a day spent somewhere.
Resolving the Opening Adviser Scenario
Returning to the high-net-worth client scenario introduced in Section 1—who, after departing New York on a prior day, spends the entire following calendar day anchored 5 nautical miles offshore off Palm Beach, Florida—our legal and empirical analysis delivers three practical conclusions for tax advisers:
- New York Statutory Residency — Not a New York Day: Assuming the taxpayer spends no part of that calendar day physically within New York State, the day does not count toward New York's 183-day statutory-residency threshold. The separate Murray analysis becomes relevant when determining whether a vessel physically located offshore from New York remains within New York territorial waters.
- Florida Domicile / State Income Tax: Florida imposes no personal income tax, and domicile itself is not established through a simple day-count formula. The offshore location may form part of the taxpayer's overall factual domicile record, but a day spent five nautical miles off Florida does not by itself establish Florida domicile.
- U.S. Federal Presence — Included Under the 12-NM Interpretation: Under the 12-nautical-mile territorial-sea interpretation discussed in Section 2, a position five nautical miles offshore falls within U.S. territorial waters for SPT purposes and is treated as a U.S. physical-presence day, subject to applicable statutory exclusions such as the less-than-24-hour transit rule.
By resolving raw coordinates against versioned maritime boundary layers, tax advisers gain a clear, defensible record that separates federal territorial presence from state waters. Existing Domicile365 subscribers with recorded water fixes can request their location history to be re-derived against our maritime resolution engine at any time.
Legal & Tax Audit Disclaimer
This article provides general legal, statutory, and GIS technical analysis for informational purposes only and does not constitute formal legal, accounting, or tax advice. Personal tax residency determinations depend on individual facts and circumstances and applicable federal and state statutes. Taxpayers should consult a qualified tax attorney or CPA regarding their specific location records and audit compliance strategy.
Defend Your Residency Status
Don't rely on inadequate records or unverified geocoder logs. Use the Domicile365 App to create a defensible, GPS-based log of your location.
Sign up for a free 60-day trial and start tracking today.
Quick Reference
Federal Limits & Exclusions
US Territorial Waters = 12 Nautical Miles (Pres. Proc. 5928 ). Treas. Reg. § 301.7701(b)-1(c)(2)(ii) includes territorial waters, excludes airspace. IRC § 7701(b)(7)(C) excludes < 24-hr transit.
State Limits & Statutes (Submerged Lands Act )
Atlantic & Pacific States: 3 Nautical Miles (or edge of Gulf Stream for FL Atlantic under Art. II § 1(a) ; NY offshore under N.Y. State Law § 7-a ; NY-NJ boundary under N.Y. State Law § 7 ).
FL Gulf Coast & Texas: 3 Marine Leagues (approximately 9 modern NM).
Consumer Geocoder Limits
Consumer reverse-geocoders excel on land and near shore, but rare offshore statutory boundaries (like the 9 NM Gulf rule or federal territorial limits) require specialized maritime GIS layer processing.
Key Case Law
Murray v. AET Inc. Ltd. , 582 F. Supp. 3d 140 (S.D.N.Y. 2022); New Jersey v. New York , 523 U.S. 767 (1998); Fla. Dep't of Revenue v. New Sea Escape Cruises, Ltd. , 894 So. 2d 954 (Fla. 2005); Benson v. Norwegian Cruise Line, Ltd. , 859 So. 2d 1213 (Fla. 3d DCA 2003); Kipp v. Amy Slate's Dive Center, Inc. , 251 So. 3d 941 (Fla. 3d DCA 2018); United States v. Florida , 363 U.S. 121 (1960); United States v. Louisiana , 363 U.S. 1 (1960).