How this works

Methodology

Every figure in a property report comes from a named public source with a date attached. This page explains where the data comes from, how the risk score is calculated, how often each source changes, and, just as importantly, what this assessment cannot tell you.

Where the data comes from

We do not create flood data. We retrieve it from public authorities, normalize it, and show it with its provenance. The geographic relationship column matters: some sources describe your property, others describe your county or a nearby gauge.

Data sources, what each provides, how often it updates, and its geographic relationship to the property.
Source	What it provides	Update cadence	Relationship to your property
FEMA National Flood Hazard Layer (NFHL)	Flood zone designation, Special Flood Hazard Area status, FIRM panel identifiers, base flood elevation where published.	Updated by FEMA as map revisions are issued. We read the current published layer and show its effective date.	Queried at the property coordinates.
FEMA Disaster Declarations	Federally declared flood disasters affecting the county.	Updated as declarations are issued; we cache for 24 hours.	County level, not property level.
USGS Water Services	Real-time stream and river gauge readings from nearby monitoring stations.	Near real time, typically every 15 to 60 minutes.	Nearest gauges within a search radius. A gauge is not at your property.
USGS 3DEP Elevation	Ground elevation at the property and across a surrounding grid.	Static between national elevation dataset releases.	Point and grid samples at and around the property.
NOAA / National Weather Service	Active flood watches, warnings, and advisories.	Live. The national alert count is cached for up to two minutes.	NWS forecast zones, which are larger than a parcel.
Open-Meteo	Current conditions, precipitation forecast, and historical precipitation.	Hourly forecast data; historical series updated daily.	Interpolated to the property coordinates from a gridded model.
NLCD (MRLC)	Land cover classification used for impervious (hard) surface analysis.	Published periodically by MRLC, typically every two to three years.	Measured across a defined radius around the property, not the parcel alone.
US Census Bureau	Population, median household income, and social vulnerability context.	Annual American Community Survey releases.	Census tract level.
Esri World Imagery Wayback	Historical satellite imagery for visual comparison over time.	As Esri publishes new imagery versions.	Imagery tiles covering the property.
ArcGIS Geocoding	Address normalization and coordinates.	Continuous.	Resolves the address you typed to a canonical property identity.

How the risk score is calculated

The score is a 0–100 summary built from the FEMA flood zone, then adjusted for elevation, coastal proximity, and documented flood history. It is a convenience summary layered on top of the evidence, not a replacement for it. The flood zone and its effective date are always shown alongside.

Step 1: Base score from the mapped flood zone

Base score by FEMA flood zone category.
Category	FEMA zones	Base
High risk	A, AE, AO, VE, V	75
Moderate risk	X500, B	45
Lower risk	X, C	15

Step 2: Adjustments

Score modifiers applied after the base score.
Factor	Adjustment
Elevation relative to base flood elevation	up to ±20
Within roughly half a mile of the coast	+10
Documented flood history in the area	+15
Community participates in the NFIP	−5

The result is clamped to the 0–100 range.

When required evidence is missing, we do not guess.

If a source needed for the score is unavailable, the report says Data unavailable or Insufficient evidence rather than falling back to a low-risk value. A missing measurement is not the same as a reassuring one.

What the dates mean

These are deliberately kept separate, because collapsing them into one vague “updated” would hide real differences.

Map effective: The date the official flood map took effect. Often years old, which is normal.
Observed: When a measurement was actually taken, such as a gauge reading or rainfall total.
Updated: When the provider last refreshed the underlying dataset.
Fetched: When we last retrieved the value from the provider.
Report generated: When this particular report snapshot was assembled.

How the written summary is produced

Pro reports include a written risk narrative generated by a language model. It is given the normalized facts from your report and asked to explain them. It does not retrieve data, and it does not calculate risk.

The model receives the same structured figures shown on the page, never raw source responses.
Numeric claims are validated against those figures before display.
If validation fails, we show the structured facts and omit the narrative.
When the underlying data changes, the narrative is regenerated rather than reused.

What this cannot tell you

This is not an official FEMA flood determination. Lenders and insurers require a determination from an authorized provider. Use the FEMA Map Service Center for regulatory decisions.
Being outside a mapped high-risk zone does not mean water cannot reach the property. Intense rainfall, blocked or undersized drainage, upstream development, and dam or levee failure all cause flooding outside mapped zones.
Maps lag reality. A flood map can predate nearby construction, channel changes, or a recent flood event.
Some evidence is areal, not parcel-specific. Gauges, census figures, and disaster declarations describe an area around the property.
We do not model your specific structure. Finished floor height, foundation type, grading, and drainage on the lot materially affect outcomes and are not in any of these datasets.
Community reports are unverified unless explicitly marked otherwise, and are never merged into the official risk classification.

Corrections

If a figure looks wrong for a property you know well, tell us. Include the address and what you expected to see.

Report a data problem

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Where the data comes from

Data sources, what each provides, how often it updates, and its geographic relationship to the property.
Source	What it provides	Update cadence	Relationship to your property
FEMA National Flood Hazard Layer (NFHL)	Flood zone designation, Special Flood Hazard Area status, FIRM panel identifiers, base flood elevation where published.	Updated by FEMA as map revisions are issued. We read the current published layer and show its effective date.	Queried at the property coordinates.
FEMA Disaster Declarations	Federally declared flood disasters affecting the county.	Updated as declarations are issued; we cache for 24 hours.	County level, not property level.
USGS Water Services	Real-time stream and river gauge readings from nearby monitoring stations.	Near real time, typically every 15 to 60 minutes.	Nearest gauges within a search radius. A gauge is not at your property.
USGS 3DEP Elevation	Ground elevation at the property and across a surrounding grid.	Static between national elevation dataset releases.	Point and grid samples at and around the property.
NOAA / National Weather Service	Active flood watches, warnings, and advisories.	Live. The national alert count is cached for up to two minutes.	NWS forecast zones, which are larger than a parcel.
Open-Meteo	Current conditions, precipitation forecast, and historical precipitation.	Hourly forecast data; historical series updated daily.	Interpolated to the property coordinates from a gridded model.
NLCD (MRLC)	Land cover classification used for impervious (hard) surface analysis.	Published periodically by MRLC, typically every two to three years.	Measured across a defined radius around the property, not the parcel alone.
US Census Bureau	Population, median household income, and social vulnerability context.	Annual American Community Survey releases.	Census tract level.
Esri World Imagery Wayback	Historical satellite imagery for visual comparison over time.	As Esri publishes new imagery versions.	Imagery tiles covering the property.
ArcGIS Geocoding	Address normalization and coordinates.	Continuous.	Resolves the address you typed to a canonical property identity.

How the risk score is calculated

Step 1: Base score from the mapped flood zone

Base score by FEMA flood zone category.
Category	FEMA zones	Base
High risk	A, AE, AO, VE, V	75
Moderate risk	X500, B	45
Lower risk	X, C	15

Step 2: Adjustments

Score modifiers applied after the base score.
Factor	Adjustment
Elevation relative to base flood elevation	up to ±20
Within roughly half a mile of the coast	+10
Documented flood history in the area	+15
Community participates in the NFIP	−5

The result is clamped to the 0–100 range.

When required evidence is missing, we do not guess.

What the dates mean

These are deliberately kept separate, because collapsing them into one vague “updated” would hide real differences.

Map effective

The date the official flood map took effect. Often years old, which is normal.

Observed

When a measurement was actually taken, such as a gauge reading or rainfall total.

Updated

When the provider last refreshed the underlying dataset.

Fetched

When we last retrieved the value from the provider.

Report generated

When this particular report snapshot was assembled.

How the written summary is produced

The model receives the same structured figures shown on the page, never raw source responses.

Numeric claims are validated against those figures before display.

If validation fails, we show the structured facts and omit the narrative.

When the underlying data changes, the narrative is regenerated rather than reused.

What this cannot tell you

This is not an official FEMA flood determination. Lenders and insurers require a determination from an authorized provider. Use the FEMA Map Service Center for regulatory decisions.
Being outside a mapped high-risk zone does not mean water cannot reach the property. Intense rainfall, blocked or undersized drainage, upstream development, and dam or levee failure all cause flooding outside mapped zones.
Maps lag reality. A flood map can predate nearby construction, channel changes, or a recent flood event.
Some evidence is areal, not parcel-specific. Gauges, census figures, and disaster declarations describe an area around the property.
We do not model your specific structure. Finished floor height, foundation type, grading, and drainage on the lot materially affect outcomes and are not in any of these datasets.
Community reports are unverified unless explicitly marked otherwise, and are never merged into the official risk classification.