Land records · Parcel maps · October 7, 2026
What Is a Geodetic Datum? Mapping Reference Basics
What is a geodetic datum? The reference model that ties coordinates to the earth. Horizontal vs vertical datums and why they matter for plats and GIS.
What is a geodetic datum? A geodetic datum is a reference framework that defines the size and shape of the earth and the origin and orientation of a coordinate system, so that positions measured on the ground can be expressed as coordinates. Horizontal datums, such as NAD27, NAD83, and WGS84, are used for latitude and longitude. Vertical datums, such as NGVD29 and NAVD88, are used for elevations. Because different datums produce different coordinates for the same point, knowing the datum is essential when comparing plats, surveys, flood maps, and GIS data. This page is general land-records background, not legal advice.
Horizontal vs vertical datums
| Type | Examples | Used for |
|---|---|---|
| Horizontal | NAD27, NAD83, WGS84 | Latitude, longitude, grid coordinates |
| Vertical | NGVD29, NAVD88 | Elevations and flood heights |
See what NAD27 and NAD83 mean on a plat and what is NAVD88.
Why datums change
Older datums were based on limited measurements and a reference point on the ground. Modern datums use satellite data and a model centered on the earth. As measurements improve and continents move, agencies release updated datums.
Effects on coordinates
The same point can shift by tens of meters between NAD27 and NAD83 in some areas, and by a meter or two between NAD83 and WGS84. Elevations can differ between NGVD29 and NAVD88 by a foot or more.
Datums on plats and surveys
Modern surveys state the basis of bearings and coordinate system, such as “NAD83 State Plane, Zone X.” See what is a state plane coordinate system. Flood elevations on certificates must use the same vertical datum as the flood map.
Datums in GIS
GIS software stores the datum with each dataset. Overlaying data with mismatched datums can shift layers. Proper transformations align them.
Upcoming changes
The U.S. National Geodetic Survey plans new reference frames to replace NAD83 and NAVD88, which will change coordinates and elevations. Surveyors and GIS users will need to convert data.
Example
An engineer compares a 1970s plat that lists coordinates in NAD27 with a modern GIS layer in NAD83. Without converting, the plat lines appear shifted dozens of meters. After applying a datum transformation, the lines align closely.
Datums and flood elevations
A house’s elevation certificate must state the vertical datum. If a flood map uses NAVD88 and an older survey used NGVD29, a conversion is needed to compare them.
Practical tips
Note the datum on every coordinate or elevation you record. Ask surveyors which datum they used.
Datum vs projection
A datum and a projection are different things. The datum ties coordinates to the earth. A projection, such as state plane or UTM, flattens the curved surface onto a grid. A complete coordinate description names both, for example “NAD83, State Plane Zone X, US survey feet.”
Bottom line
A geodetic datum is the reference model that ties coordinates and elevations to the earth. Horizontal and vertical datums differ, and mixing them causes shifts. Always check the datum on plats, surveys, flood maps, and GIS data. Explore parcel maps via the Platbookmapper map.
What is a geodetic datum FAQ
What is a geodetic datum?
A reference model for coordinates and elevations.
What is the difference between NAD27 and NAD83?
They are different horizontal datums with shifted coordinates.
What is a vertical datum?
A reference for elevations, such as NAVD88.
Why do datums matter?
Mixing them causes position and elevation errors.
Are datums changing?
Yes, new U.S. reference frames are planned.