Modern Mapping Tech, Vertical Datums & Map Interpretation

Modern Mapping Tech, Vertical Datums & Map Interpretation

Vertical Datums: The Foundation of Elevation

Floodplain management is fundamentally about comparing two elevations: the elevation of the water (the Base Flood Elevation) and the elevation of the ground or a building. To make accurate comparisons, both elevations must be measured from the exact same starting point, or "zero."

A Vertical Datum is a base measurement point (or set of points) from which all elevations are determined. It represents a hypothetical surface of zero elevation. In the history of the NFIP, two primary vertical datums have been used:

1. National Geodetic Vertical Datum of 1929 (NGVD 29)

Created in 1929, this datum was based on the mean sea level measured at 26 tide gauges in the US and Canada. For decades, it was the standard for all topographical maps and older FEMA FIRMs.

2. North American Vertical Datum of 1988 (NAVD 88)

As surveying technology improved and the continent's crust naturally shifted over decades, surveyors realized NGVD 29 was no longer accurate. In 1988, a new, highly accurate datum was established based on a single origin point (Father Point in Quebec, Canada). Today, NAVD 88 is the official vertical datum of the United States, and all modern FEMA maps are produced using it.

The Danger of Datum Mixing

A Base Flood Elevation of "100.0 feet" in NGVD 29 is not the same physical height as "100.0 feet" in NAVD 88. Depending on where you are in the country, the difference between the two datums can be a few inches or several feet.

A critical mistake made by inexperienced practitioners is mixing datums. For example, if a FIRM specifies a BFE of 105.0 ft (NAVD 88), and a surveyor measures a house's first floor at 106.0 ft using an old benchmark based on NGVD 29, you cannot directly compare them. The surveyor must convert their field data to NAVD 88 before confirming the house is compliant.

FEMA provides conversion factors for every county to mathematically translate elevations from NGVD 29 to NAVD 88.

Modern Topographic Mapping: LiDAR

Historically, the topographic data used for FIRMs was gathered via aerial photogrammetry (measuring contours from stereo photographs). This method struggled in heavily forested areas and generally only provided 2-foot or 4-foot contour intervals. This lack of precision often led to mapping errors and the need for LOMAs.

Today, the industry standard is LiDAR (Light Detection and Ranging). LiDAR data is collected by aircraft firing millions of laser pulses at the ground and measuring the time it takes for the light to bounce back. LiDAR can penetrate forest canopies to strike the bare earth, generating incredibly dense and highly accurate digital elevation models (DEMs).

Modern FIS hydraulic models (using HEC-RAS) run on LiDAR data, resulting in vastly more accurate Base Flood Elevations and precisely drawn SFHA boundaries.

The Digital Transition: DFIRMs and the NFHL

In the early 2000s, FEMA launched Map Modernization, an effort to convert thousands of paper FIRMs into a digital format.

DFIRMs

A Digital Flood Insurance Rate Map (DFIRM) is a FIRM that has been created in a Geographic Information System (GIS) environment. DFIRMs overlay the flood hazard data onto aerial photography and modern road networks. This makes determining whether a specific house is in the floodplain exponentially easier than the old paper maps, where users had to guess based on poorly drawn street lines.

The National Flood Hazard Layer (NFHL)

The culmination of map modernization is the National Flood Hazard Layer (NFHL). The NFHL is a seamless, national database of all digital flood hazard data (DFIRMs) for the United States.

The NFHL is a living database. When a LOMR or LOMA is approved, the changes are digitally incorporated into the NFHL. Users can access the NFHL online via FEMA's Map Service Center, print out a Firmette (a customized, full-scale portion of the map), and instantly see the most up-to-date flood risks, complete with all active LOMCs.

Step-by-Step Map Interpretation

When a property owner asks, "Am I in the floodplain?", a CFM should follow a structured interpretation process:

1. Locate the Property: Use the NFHL or local GIS to pinpoint the exact location of the structure.
2. Identify the Effective Map: Ensure you are looking at the current, effective panel, noting the panel number and effective date.
3. Determine the Zone: Identify if the structure touches the SFHA (Zone A, AE, V, VE) or is in an X Zone.
4. Find the BFE: If in an AE or VE zone, identify the regulatory Base Flood Elevation. If on a river, refer to the FIS Flood Profiles for the exact interpolated BFE.
5. Check the Datum: Note the vertical datum of the map (usually NAVD 88) to ensure any future surveying work uses the correct reference.
6. Check for LOMCs: Search the FEMA database to see if a LOMA or LOMR has already removed the structure from the SFHA.