Data sources and known limits — Suffolk County, NY

Sources used for Suffolk County, NY

• NLCD 2021 — the developed-land mask that every areal figure is measured over
• NLCD Tree Canopy Cover 2025 — share of each area’s land under tree canopy, measured over the same boundary as every other areal figure with open water excluded. Used only where it bears on storm power outages; see below.
• NLCD Fractional Impervious Surface 2025 — share of each area’s land under hard surface, same boundary and same open-water exclusion. EPA finds surface runoff roughly doubles once impervious cover passes 10–20% of a watershed, which is why this is measured over total land rather than over built-up land alone. Both NLCD annual layers are modeled products derived from the Landsat record, not fresh yearly surveys.
• USGS 3DEP — lidar elevation
• USGS SIM 3326 — the 2013 Long Island depth-to-water raster. Regional: this survey covers Long Island and nothing else. Another region needs its own groundwater source.
• FEMA NFHL — National Flood Hazard Layer
• OpenFEMA FimaNfipClaims v2 — snapshot frozen 2026-06-01, filtered to county code 36103, aggregated by reported ZIP
• USDA SSURGO — soil survey
• US Census ACS 2019–2023 5-year estimates, and the 2020 Decennial Census for population
• Suffolk County DPW — sewer district boundaries, filtered to lifecycle status Active
• Suffolk County OEM — hurricane evacuation zones and emergency shelters
• EPA NPDES register — permitted wastewater treatment works, used to confirm unsewered findings
• USGS GNIS — named hydrographic features

How streets are counted

Street lists come from Suffolk County's published address points, one point per street address, placed by the county, usually on the building. They are addresses, not homes: a shop or an office counts the same as a house, and a building with many apartments counts once, because units inside a building are not counted separately.

• Flood zone: an address is counted when its point falls inside a FEMA special flood hazard area on the current flood map. That means the address is mapped in the zone. It does not mean the building has flooded, and an address outside the zone is not guaranteed dry.
• Sewer main: an address is counted when an active sewer main in the county's records runs within 45 meters of its point. That means a main passes the property, not that the property is connected. A street is listed when at least four in five of its addresses meet that test.
• Groundwater depth: the USGS depth-to-water map for Long Island (2013), read at each address point. It was drawn from 335 monitoring wells measured in one spring, so it is a map of that season, not a live reading, and groundwater rises and falls through the year. A street is listed by the middle value under its addresses.
• Whether groundwater can reach a basement: a basement floor is about 9 ft down, and USGS well records for the county show the water table running up to about 5 ft above its spring 2013 level in wet periods (192 wells, the 90th percentile for ground of this depth). So an address counts as within reach of groundwater when it is mapped 14 ft down or less. A sump pit is cut about 2 ft below the floor, so for sump pumps the figure is 16 ft. This does not cover water held above clay layers on the north shore, which those wells do not measure.
• Proposed sewers: mains the county's records mark as proposed. That is a plan on file. It carries no construction date and no commitment that the main will be built.
• Flood zone letters: AE and A are the 1%-annual-chance floodplain, VE is the coastal zone with wave action, AO is shallow flooding. All four are special flood hazard areas.
• The street lookup: the same counts, for any street with three or more addresses. It runs in your browser against a list of that area's streets; the street you type is not sent to us or to anyone else.
• What is left out: the county's sewer records do not include every town- and village-run system, so a street missing from a list may still have a sewer. Streets with fewer than five addresses are not listed.

What Suffolk County, NY measures

Across the 155 areas covered:

• Water table: median 31 ft below the surface, ranging 4–153 ft.
• Wastewater: 109 unsewered (cesspool/septic), 30 partially sewered, 16 sewered.
• Heating fuel and building decade: Census ACS 5-year estimates for each Census place. They come from a sample, so each carries a margin of error; a page shows the figure only where that margin is narrow enough to support it, and prints the margin beside it.
• Hurricane Sandy high-water marks: 69 areas carry a mark USGS surveyed after the 2012 storm, in feet above NAVD88. Each is a reading at one point. An area without one was not surveyed; that does not mean it stayed dry.
• FEMA zone over developed land: 97 at AE (100-yr floodplain), 35 at X (minimal), 13 at VE (coastal high-velocity), 9 at A (approximate 100-yr floodplain), 1 at X-shaded (500-yr).
• Soil drainage: 109 well drained, 40 excessively drained, 6 moderately well drained.
• Hurricane evacuation zones: 117 not mapped, 19 in Zone A, 10 in Zone B, 6 in Zone C, 3 in Zone D.
• Shelters: 27 distinct shelters serve these areas, so neighboring ones frequently share a shelter.
• ZIP codes: 104 in total, and 81 areas share a ZIP with at least one neighbor — which is why NFIP figures are attributed to the ZIP and never to a single area.

What we do not know about Suffolk County, NY

Published because a dataset with no known limits has usually not been checked.

• Three village sewer systems are unresolved. Greenport, Ocean Beach and Sag Harbor operate village-owned systems that do not appear in the county district layer, so we have no measured coverage figure for them, only the recorded status of sewered. We have not found a source for how much of each village the system serves. Do not rely on our sewer figure for those areas.
• Three areas have no distinct boundary of their own. Lloyd Neck, San Remo and South Jamesport are not separately delineated in either the Census place layer or the county hamlet layer, so their figures are inherited from the polygon that contains them. That is permanent rather than a backlog item, and none of them carries a neighbor comparison — comparing an inherited number against the neighbor it was inherited from would compare a number with itself.
• 21 areas have no named water feature. GNIS lists nothing hydrographic inside those boundaries. That is a sourced finding, not a gap, and those pages do not mention a water body.
• No neighbor comparison for 3: Lloyd Neck, San Remo and South Jamesport — either too few nearby areas in our data to average, or an inherited boundary.
• Everything here is a snapshot. FEMA remaps, sewer districts expand, the ACS re-estimates annually. The verification date above is when the set was last checked; it is not a claim of continuous monitoring.
• No figure here is a substitute for an inspection. Every number is an average or a classification across an area. Your property may differ from all of them.

Corrections

If a figure is wrong about your area we want to know, particularly if you have a source. Call (631) 440-4144. Corrections are made against the source data and republished, not patched on a single page.