NAVD 88, the North American Vertical Datum of 1988, is the vertical reference system FEMA uses to set Base Flood Elevations on modern Flood Insurance Rate Maps. If you own property in or near a mapped flood zone, every elevation number that affects your insurance premium, your building permits, and your ability to challenge a flood zone designation is tied to this datum. When your survey data and your flood map use the same datum, the comparison works. When they don’t, and you fail to convert between them, the resulting flood risk determination is meaningless, and the mistake can cost you thousands of dollars a year or leave you thinking you have protection you don’t have.
What NAVD 88 Actually Is
NAVD 88 is a gravity-based vertical control datum established in 1991 through a joint adjustment of leveling observations from Canada, Mexico, and the United States. It holds fixed the height of a primary tidal benchmark at Father Point/Rimouski in Quebec, referenced to the International Great Lakes Datum of 1985 local mean sea level value.1National Geodetic Survey. North American Vertical Datum of 1988 (NAVD 88) In 1993, it was affirmed as the official vertical datum in the National Spatial Reference System for the contiguous United States and Alaska.
It replaced the National Geodetic Vertical Datum of 1929 (NGVD 29), which was built on a network of 26 tidal gauges and assumed the sea is level. Ocean surfaces actually vary with currents, wind, barometric pressure, water temperature, and seafloor topography, and those variations introduced distortions that compounded over distance. NGVD 29 also couldn’t account for ground movement from earthquakes, post-glacial rebound, and regional subsidence. NAVD 88 corrected these problems by incorporating satellite technology and gravity measurements, producing orthometric heights that reflect the actual shape of the Earth’s gravity field.
How NAVD 88 Sets Your Base Flood Elevation
FEMA uses NAVD 88 to set Base Flood Elevations (BFEs), which define the expected water level during a 1-percent-annual-chance flood event. Every Flood Insurance Study and FIRM containing hydrologic and hydraulic analysis is referenced to a specific vertical datum. Before the mid-1990s most used NGVD 29; after that point, NAVD 88 became the standard for new flood studies and map production.2Federal Emergency Management Agency. Guidance for Flood Risk Analysis and Mapping – Vertical Datum Conversion
Your property’s relationship to the BFE drives nearly everything about your flood insurance obligation and your floodplain permit compliance. If your lowest floor sits below the BFE, you’re in a high-risk zone. If it sits above the BFE, you may be able to have the property reclassified. That comparison only works if both numbers, your building’s elevation and the BFE, are measured against the same datum. A building surveyed in NGVD 29 and compared to a BFE published in NAVD 88 produces a meaningless result.
When Your FIRM Still Uses NGVD 29
Plenty of older FIRMs in communities that haven’t been restudied still reference NGVD 29. When a property owner in one of those areas submits elevation data for a map change or insurance application, the survey data and the map’s BFE need to be in the same datum. If they aren’t, the applicant is responsible for providing the conversion.3FEMA (Federal Emergency Management Agency). MT-1 Application Forms and Instructions
The offset between NGVD 29 and NAVD 88 varies by location. There is no single, universal number you can apply. In some coastal areas the shift is less than half a foot; in other regions it exceeds three feet. The direction changes too: in some places, NAVD 88 elevations are higher than NGVD 29; in others, they’re lower. A foot or two of error is enough to push a property from above the BFE to below it, flipping the insurance and compliance picture entirely.
The National Geodetic Survey publishes two tools for datum conversions:
- VERTCON computes the modeled difference in orthometric height between NAVD 88 and NGVD 29 for a given latitude and longitude. Enter your coordinates and it returns the offset to apply. This is what most property owners and surveyors need when converting older flood map data or survey records.4National Geodetic Survey. VERTCON
- VDatum is a broader transformation tool that converts among tidal datums, orthometric datums (including NAVD 88 and NGVD 29), and ellipsoidal datums. It’s designed for geospatial professionals merging datasets from different reference systems, such as land survey data with coastal tide gauge measurements. More capable, more complex.5NOAA. VDatum 4.8 – Vertical Datums Transformation
For a straightforward NGVD 29 to NAVD 88 conversion on a flood map application, VERTCON is the right choice. You’ll need the latitude and longitude of the site and the existing elevation value from the older survey or map.
What This Means for Your Flood Insurance
Under FEMA’s Risk Rating 2.0 methodology, which took full effect in April 2023, the National Flood Insurance Program uses its own tools and data sources to determine a building’s elevation, and an Elevation Certificate is no longer required to purchase coverage.6Agents FloodSmart. Frequently Asked Questions – Risk Rating 2.0 FEMA’s automated elevation data isn’t always right, though, and property owners can still obtain an Elevation Certificate and submit it to the insurer to see whether it produces a lower rate.
The savings can be substantial. In one documented case, a New Jersey policyholder who had never provided an Elevation Certificate to their carrier was paying $13,144 per year. After submitting the certificate, the carrier discovered the building had been misrated, reduced the annual premium to $4,680, and issued five years of refunds totaling $23,366.7Federal Emergency Management Agency. Periodic Report: January to March 2022 That result is not typical, but it shows what accurate elevation data can be worth.
Elevation Certificates also remain necessary for floodplain management building requirements. Communities in the NFIP must keep official records showing that new construction and substantial improvements in Special Flood Hazard Areas are properly elevated.8Federal Emergency Management Agency. Elevation Certificate Expect to encounter one during new construction, major renovations, or a map challenge.
Getting a Valid Elevation Certificate
An Elevation Certificate must be completed and certified by a licensed land surveyor, registered professional engineer, or certified architect authorized by law to certify elevation information. The professional conducts a field survey to measure the building’s lowest floor elevation, the lowest adjacent grade, and other features relative to the datum shown on the applicable FIRM.3FEMA (Federal Emergency Management Agency). MT-1 Application Forms and Instructions Costs typically run between $300 and $800, though prices vary with location, property complexity, and site conditions like dense vegetation or difficult terrain.
The certificate must specify which vertical datum the elevation measurements reference. If that datum differs from the one on the current FIRM, the certificate needs to include or be accompanied by the datum conversion. Submitting a certificate in NGVD 29 when the FIRM uses NAVD 88, without converting, produces an incorrect flood risk comparison. Before hiring a surveyor, confirm they’re licensed in your state and familiar with the current FEMA form.
Using NAVD 88 Data to Get Out of a Flood Zone
If your property’s actual elevation is at or above the BFE, you can request a Letter of Map Amendment (LOMA) from FEMA to officially remove the property from the Special Flood Hazard Area. FEMA does not charge a fee to review a LOMA request.9Federal Emergency Management Agency. How to Request a Letter of Map Amendment (LOMA)
The key measurement is the Lowest Adjacent Grade (LAG), the lowest ground elevation touching the structure. For FEMA to remove a structure from the flood zone through the LOMA process, the LAG must be at or above the BFE. For undeveloped lots, the lowest point on each lot must meet the same threshold. If the property is clearly outside the mapped flood boundary based on the map alone, FEMA calls it “out as shown” and no elevation data is needed.
When elevation data is required, you submit it using FEMA’s MT-EZ form (for simpler cases) or the MT-1 form (for more complex requests). You can also submit an Elevation Certificate that includes the LAG data. The elevation information must be certified by a licensed land surveyor or registered professional engineer, and you must identify the vertical datum used for the survey and provide a conversion if the survey datum differs from the datum on the current Flood Insurance Study.3FEMA (Federal Emergency Management Agency). MT-1 Application Forms and Instructions
Watch for Subsidence in Older Benchmarks
Where the ground is sinking, published benchmark elevations become unreliable over time. Subsidence is non-linear and spatially variable, so two benchmarks a few miles apart can sink at different rates. The published NAVD 88 elevation on a benchmark reflects the height at the time it was last surveyed, not necessarily the height today.10U.S. Army Corps of Engineers. Standards and Procedures for Referencing Project Elevation Grades to Nationwide Vertical Datums (EM 1110-2-6056) A property that sat above the BFE when it was built may sit below it now if the ground around it has dropped.
In high-subsidence regions such as parts of Louisiana, Texas, and California, primary control benchmarks are date-stamped to indicate when they were last surveyed. For LOMA applications in these areas, the most recently adjusted Elevation Reference Mark must be used, and benchmark elevations should share the same re-leveling date as the benchmarks used to develop the BFEs on the FIRM. If you’re working with elevation data in a subsidence-prone area, check the survey date on any benchmark you’re referencing. A benchmark last surveyed in 2005 may no longer reflect current ground elevations, and using stale data can make the property look higher than it really is.
The Coming Switch to NAPGD2022
NAVD 88 has known shortcomings. The zero-elevation surface it implies disagrees with the latest international geoid estimates by roughly half a meter to two meters.11National Geodetic Survey. New Datums The National Geodetic Survey plans to replace it with a new geopotential datum called the North American-Pacific Geopotential Datum of 2022 (NAPGD2022).
The rollout of the modernized NSRS is scheduled for 2025 or 2026, with the Federal Geodetic Control Subcommittee expected to vote on approval in 2026. If approved, NGS will publish a Federal Register Notice and begin transitioning modernized components to the official website over several months. During the rollout and testing phase, NAVD 88 remains the official datum of the United States.12Federal Register. Updated Implementation Timeline for the Modernized National Spatial Reference System (NSRS)
The transition will change existing orthometric heights by as much as four meters in some places. Every flood map, engineering design, and elevation record tied to NAVD 88 will eventually need to be referenced to the new datum. For property owners today there’s nothing to do differently, but if you’re commissioning a survey or filing a LOMA in the next couple of years, keep clear records of the datum used. When NAPGD2022 takes effect, every BFE and Elevation Certificate in the country will need to be understood in the context of the new reference surface.