NGVD 29 vs. NAVD 88: comparing elevations in site research

Before subtracting a flood elevation from a site elevation, check that both use the same vertical datum and units. NGVD 29 and NAVD 88 are different height reference systems. A number from one cannot be compared directly with a number from the other.

For an acquisitions team, the first job is to identify the mismatch and hand over the source documents. A spreadsheet conversion alone does not establish the property's flood compliance or replace a survey.

What the datum label tells you

An elevation needs a reference. NGVD 29 and NAVD 88 provide different vertical references, so the same physical point can have different numbers under each system. NOAA's National Geodetic Survey explains their different origins in its datum FAQ.

Keep the full label beside the number. “42.5 feet” leaves out information needed for a comparison. “42.5 feet, NGVD 29” gives the next reviewer something they can check.

Also distinguish the point being measured. A ground elevation, a finished-floor elevation, and a pipe invert describe different things. Matching their datums does not make those measurements interchangeable.

Check the documents before converting anything

Start with the survey's notes and benchmark information, the relevant flood map and Flood Insurance Study, and any civil drawings used in the comparison. Record the date and revision of each document.

FEMA's Flood Insurance Study training explains that elevation comparisons must use the map's datum. Older and newer flood products can reference different systems. Read the actual product's label rather than inferring it from its age.

Use these questions during the review:

• What physical point does each elevation represent?
• Which vertical datum and units does each source state?
• Does the document identify a benchmark or explain a local assumed datum?
• Is this the effective flood product, a preliminary product, or an older study?
• Who will confirm the reference and any conversion used in the project's design?

If a source only says “mean sea level,” or leaves the datum blank, mark the reference unresolved. Do not fill the field from a nearby property's survey.

Why there is no universal correction

NOAA's conversion method uses the site's geographic position. The correction between NGVD 29 and NAVD 88 can have either sign; treating it as a fixed subtraction is a mistake. Its sign-convention examples explain the direction:

When the supplied shift is NAVD 88 minus NGVD 29:

NAVD 88 height = NGVD 29 height + signed shift
NGVD 29 height = NAVD 88 height - signed shift

The numbers must use compatible units. Preserve the sign and conversion direction in your notes so another person can reproduce the arithmetic.

NOAA's VERTCON 3.0 documentation directs users to the NGS Coordinate Conversion and Transformation Tool, NCAT. VERTCON 3.0 is implemented within NCAT rather than offered as a separate conversion tool. Save the input location, selected references, output and tool version with any screening calculation. Ask the surveyor or civil engineer to confirm that the method is suitable for the source and intended use.

Worked example: an apparent 0.50-foot difference

This example is invented to show the arithmetic. The correction is hypothetical, not a value for any real property.

Assume the documents show:

• A site point at 42.50 feet NGVD 29.
• A comparison elevation of 42.00 feet NAVD 88.
• A verified-for-this-example shift of minus 0.85 feet, defined as NAVD 88 minus NGVD 29.

Subtracting the original numbers gives an apparent positive difference of 0.50 feet. After converting the site point:

42.50 + (-0.85) = 41.65 feet NAVD 88
41.65 - 42.00 = -0.35 feet

The sign of the comparison changed. Nothing on the property moved; the calculation now uses one reference system.

This does not determine a required floor elevation, drainage design, insurance result or whether the site can be developed. Those decisions need the appropriate measurements, applicable requirements and professional review. The example shows why a datum check belongs before the feasibility conclusion.

Copyable elevation review sheet

Use one record for each source elevation. Keep the original number even after adding a converted value.

Property / project:
Review date:
Question this comparison is intended to answer:

SOURCE ELEVATION
Document title / URL / file:
Sheet or page:
Document date and revision:
Measured point or surface:
Original elevation and units:
Stated vertical datum:
Benchmark / local reference notes:

COMPARISON
Other source and measured point:
Other elevation, units and datum:
Are the points appropriate to compare? Who confirmed?
Reference or unit mismatch:

IF CONVERSION IS NEEDED
Reviewer responsible:
Input location and coordinate reference:
Conversion tool / version / method:
Conversion direction:
Signed shift and units:
Converted elevation and datum:
Saved output location:
Uncertainty or limitations noted by reviewer:

FOLLOW-UP
Missing document or unresolved reference:
Person responsible:
Date needed:
Review outcome and supporting document:

What to hand to the project team

Send the source pages alongside the worksheet. Highlight the two elevations, their datum labels, and the decision they are being used to support. Ask for a documented common reference before carrying the result into a site plan or acquisition memo.

Keep this check with the rest of the desktop site-screening record. If the disagreement concerns area instead of height, the deeded-versus-GIS acreage guide covers a separate reconciliation process.