According to the National Oceanic and Atmospheric Administration, "a geodetic datum is an abstract coordinate system with a reference surface (such as sea level) that serves to provide known locations to begin surveys and create maps.

"Datum" is often used interchangeably with "geographic coordinate system." A datum is actually only a component of a geographic coordinate system. However, unless you are a geodetic scientist or a geographer, you can think of them as basically the same.

There are geocentric datums (such as the WGS 1984) and place-specific local datums (such as the NAD 1983, ETRS89, and the GDA2020).

**Horizontal datums**

Horizontal datums measure positions (latitude and longitude) on the surface of the Earth. Surveyors originally created these using "monuments" or "survey benchmarks." The locations of these monuments were then connected through mathematical triangulation to create horizontal datums.

## Vertical datums

Vertical datums measure land elevations and water depths. Orthometric height (or elevation) is tied to a vertical datum.

Perhaps the most important part of a vertical coordinate system is its unit of measure. The unit of measure is always linear (for example, international feet or meters). Another important part is whether the z-values represent heights (elevations). Without a corresponding geographic or projected coordinate system, you cannot define a vertical coordinate system on a dataset.

Examples of vertical datums include the NAVD 1988, EVRS, and the AHD.

**How are datums used in Propeller?**

Every coordinate reference system is based on a datum. Propeller needs to know what coordinate reference system (CRS) you are using to make your survey usable and readable.

If your surveys or designs are processed in different coordinate systems, you cannot compare them. There will be shifts and discrepancies if the CRS doesn't match.

When setting up a site, you must pick the horizontal and vertical datums appropriate for your worksite (in step 3).

Ask your surveyor what the correct datum and CRS are for your site. If you do not have a surveyor, or it does not matter, you can work with your customer success engineer to determine the best ones.

**I still can't do it!**

We wrote these articles to equip you with everything you need to get the job done on your own, but we understand that sometimes this isn't sufficient.

If you're stuck, the Propeller data success team may be able to help. You can contact our support team by clicking the support tab on the top-right pane of your user portal.