A DatumThe starting point and ruler that coordinates are measured from. The same spot has different numbers in different datums.Full definition is the starting point and ruler that every coordinate is measured from. The same spot on the ground has different numbers in different datums. It's like giving your address using two different street maps. If you mix datums, your map lands in the wrong place.
In the US, these are the ones you'll run into:
- NAD 83(2011)
- The official US datum for horizontal position (and 3-D position) right now. It belongs to the NSRSNational Spatial Reference System. The official US system of coordinates, heights and survey marks.Full definition (NSRS), the country's official coordinate system. Most state networks use it, but ask yours to be sure.
- NAVD 88
- The official datum for elevations (heights above "sea level") in the lower 48 states and Alaska. These are called Orthometric heightHeight above the geoid, which is what most people call elevation or height above sea level.Full definition.
- GEOID18
- A model from NGSNational Geodetic Survey. The part of NOAA that runs the official US coordinate system, CORS and OPUS.Full definition that turns GPSThe US satellite navigation system. People often use "GPS" to mean all satellite positioning.Full definition heights into NAVD 88 elevations. GPS measures height above a smooth math shape called the EllipsoidA smooth, slightly squashed ball shape that stands in for the Earth in GPS math.Full definition. The GeoidA lumpy, imaginary "sea level" surface around the whole Earth. Elevations are measured from it.Full definition is a lumpy "sea level" surface. GEOID18 gives the gap between the two at every spot.
- ITRF / WGS 84The worldwide datum GPS uses by default. It's very close to ITRF.Full definition
- Worldwide frames. Many global services, and OPUSOnline Positioning User Service. A free NOAA website: upload a GPS file and it emails back the precise position of that spot.Full definition alongside NAD 83, give positions in ITRF2020. In the US these differ from NAD 83 by about a meter or more. The gap slowly grows because the continents move.
Simple rules
- Know which datum your CorrectionsLive info from a base station about how far off the satellite signals are, so your rover can cancel out the error.Full definition use. Your RoverThe receiver that moves around the job, like a survey pole or a drone. It uses the base's corrections to find its exact spot.Full definition takes on whatever datum the base uses.
- Drone GPS normally gives Ellipsoid heightHeight above the smooth math shape (the ellipsoid). It's what GPS measures, and it can be very different from elevation.Full definition. Apply GEOID18 to get NAVD 88 elevations. Only apply it once.
- Set your base, GCPGround control point. A target on the ground with a surveyed position, used to line up a drone map with the real world.Full definition (GCPs), CheckpointA surveyed point you leave out of processing, then use to test how accurate your finished map really is.Full definition and software to the same horizontal datum, vertical datum, geoid and units. Watch out: the Survey footAn old US foot that is slightly longer than the international foot (2 parts per million). Mixing them up causes errors over big distances.Full definition and the international foot are slightly different.
- Change is coming. NGS plans to replace NAD 83 with four new Reference frameA precise version of a datum, defined by real stations on the ground. ITRF2020 and NAD 83(2011) are examples.Full definition (such as NATRF2022One of the new US reference frames NOAA plans to use in place of NAD 83.Full definition) tied to ITRF2020, and to replace NAVD 88 with a new height datum. Check NGS for the timeline.
Mixed-up datums are a common reason a drone map ends up off by a meter sideways, or by tens of meters in height. See Troubleshooting.
Sources (checked Oct 16, 2026):NOAA NGS: DatumsNOAA NGS: NAD 83NOAA NGS: NAVD 88NOAA NGS: GEOID18NOAA NGS: New Datums (NSRS modernization)NOAA NGS: About OPUSDiagrams are FixFinder illustrations. Always follow your equipment's current manual.