When Survey Elevations Disagree, Check the Datum Before Changing Grades

When a field elevation disagrees with a grading plan, resist the urge to make the numbers match by adding an offset. First, ask whether both elevations use the same reference system and compatible benchmark information. A vertical datum defines the reference for heights; without that information, the disagreement is not yet a reason to change the design.

The practical first step is to gather the reference notes and ask the project surveyor to reconcile them before anyone applies a correction.

 

Start with the vertical reference—not just the elevation

A vertical datum is the reference system to which elevations relate. The North American Vertical Datum of 1988, or NAVD 88, and the National Geodetic Vertical Datum of 1929, or NGVD 29, are different systems. NOAA advises identifying both the datum of the existing data and the datum needed for the intended use.

Compare the survey notes, grading-plan notes, and accompanying data documentation side by side. Look for an explicit vertical datum name. A note saying NAD 83—the North American Datum of 1983—does not, by itself, establish the vertical reference. NOAA specifically cautions against inferring the height reference from the horizontal datum.

Check what kind of height is reported

An orthometric height is measured relative to the geoid, a physically based reference surface associated with Earth’s gravity. An ellipsoid height uses a mathematical model of Earth’s shape instead. They are not interchangeable.

The National Geodetic Survey (NGS) expresses their relationship as H = h − N: orthometric height equals ellipsoid height minus geoid height. If one record reports ellipsoid heights and another reports orthometric heights, ask the surveyor to identify the height type and any geoid model used before comparing the values.

Match the benchmark description to the actual mark

A benchmark is a physical reference mark with an assigned elevation. Compare more than its abbreviated label: request its identifier, description, recorded elevation, and supporting control documentation. Vertical control is the set of reference points used as the elevation basis for a project.

The California Department of Transportation’s (Caltrans) control-survey guidance illustrates why those details matter. Its project control documentation includes vertical monument designations, elevations, descriptions, and the accuracy classification of the control network. For applicable Caltrans projects, subsequent surveys requiring elevations are based on the project’s vertical control.

Also ask whether the mark was positively identified and what condition it is in. NGS warns that ground survey marks can deteriorate, be destroyed, or go unmaintained, leaving associated height and position information outdated or inaccurate. Do not accept a benchmark identification solely because an object looks familiar.

This is general troubleshooting guidance, not a nationwide surveying standard. The Caltrans provisions are agency-specific. Requirements vary by state and jurisdiction; confirm those applicable to your project with your state licensing board, county, or a licensed surveyor.

Compare units and reference notes explicitly

For each conflicting value, ask:

 

• • What units are stated in the original record?

• • What units were selected when the data were imported or exported?

• • Does the documentation identify a named datum or a local reference?

• • Was any conversion or adjustment already applied, and where is it documented?

Do not fill gaps by guessing. If the comparison includes a USGS digital elevation model—a digital dataset representing elevations—read its documentation rather than assuming its reference. USGS says these products typically use NAVD 88, but identifies other vertical references in some areas outside the conterminous United States.

A consistent-looking difference is a clue, not permission

NOAA explains that the differences between NAVD 88 and NGVD 29 form a complex surface, rather than a simple offset and tilt. There is no single correction to assume for every location.

That does not mean a local offset is always inappropriate. NGS has reported that a single local bias can be estimated in many areas for most mapping applications. But that finding is not blanket approval to shift construction elevations or revise a grading plan.

Ask the surveyor to evaluate whether a correction is appropriate for this project and its intended use. If additional common points are available, send those comparisons too; do not select a correction solely because it makes one point agree.

Send a reconciliation packet, not just a screenshot

Before requesting a correction, assemble:

 

• • The conflicting records: full survey and plan sheets, their dates or revision identifiers, and the relevant digital files.

• • The reference information: datum notes, stated units, height type, and any documented geoid model or conversion.

• • The benchmark evidence: identifiers, descriptions, elevations, control diagrams, and available photographs.

• • The comparisons: point locations, both reported elevations, and the source of each value.

• • The intended action: whether the question affects layout, a data import, or a proposed grading revision.

Request a documented explanation of the mismatch and the elevation basis to use going forward. Keep any proposed offset separate from the working grading data until the surveyor has evaluated it and the responsible engineer has addressed any design implications.

Featured image: Image by PublicDomainPictures from Pixabay (Pixabay Content License)

Footnotes & Sources

 

1. 1. Datums, Heights and Geodesy, NOAA National Geodetic Survey. — Vertical datum and height definitions, including the relationship between orthometric, ellipsoid, and geoid heights.

1. 1. NOAA/NOS’s VDatum: A tutorial on datums, National Oceanic and Atmospheric Administration. — Datum identification, distinction between horizontal and vertical references, and the spatially complex relationship between NAVD 88 and NGVD 29.

1. 1. 9 – Control Surveys – Caltrans, dot.ca.gov. — Caltrans vertical project control documentation and the role of vertical control in subsequent project surveys.

1. 1. New Datums | National Geodetic Survey, NOAA National Geodetic Survey. — Potential deterioration, destruction, and lack of maintenance of ground survey marks.

1. 1. What is the projection, horizontal datum, vertical datum, and resolution for a USGS digital elevation model (DEM)?, U.S. Geological Survey. — Typical and regional variations in USGS digital elevation model vertical references.

1. 1. Results of the General Adjustment of the North American Vertical Datum of 1988, NOAA National Geodetic Survey. — Qualified use of estimated local datum bias factors for most mapping applications in many areas.