Fix: FIX, but your points are in the wrong place

What you see: a solid FIX, but your points are off from a known mark, last year's survey, or the client's CAD file. Heights are the most common problem. Sideways shifts usually come from a different reference frameReference frameA precise version of a datum, defined by real stations on the ground. ITRF2020 and NAD 83(2011) are examples.Full definition.

Why it happens

• Different datum. A datum is the agreed-on starting point for coordinates. NAD 83(2011)NAD 83(2011)The official US datum for horizontal position right now. Most state networks give coordinates in it.Full definition and ITRF2020ITRFInternational Terrestrial Reference Frame. A worldwide datum. In the US it's about a meter different from NAD 83.Full definition are both used in the US and differ by about a meter or more depending on where you are (NGSNGSNational Geodetic Survey. The part of NOAA that runs the official US coordinate system, CORS and OPUS.Full definition). If your network sends one and your project uses the other, everything shifts.
• Geoid missing or applied twice. GPSGPSThe US satellite navigation system. People often use "GPS" to mean all satellite positioning.Full definition measures height above a smooth math shape (the ellipsoidEllipsoidA smooth, slightly squashed ball shape that stands in for the Earth in GPS math.Full definition). Elevations like NAVD 88NAVD 88The official US datum for elevations. Heights in it are roughly "height above sea level."Full definition need a geoid model such as GEOID18GEOID18NOAA's model that converts GPS heights to official NAVD 88 elevations in the US.Full definition to convert. If it's skipped, or both the network and your app apply it, heights will be off by many feet (datums and geoid basics).
• Wrong antenna heightAntenna heightThe straight-up distance from the ground mark to the antenna's reference point. Getting it wrong shifts every height you measure.Full definition or type. The receiver can't catch this for you.
• Your own base has a rough position. Fix it with OPUS.

Fix it

1. Ask your provider which datum and epochEpochOne moment of GPS measurement. A receiver logging at 1 Hz records one epoch every second.Full definition their correctionsCorrectionsLive info from a base station about how far off the satellite signals are, so your rover can cancel out the error.Full definition use. FixFinder lists it on the provider page where it's published.
2. Set your app or software to the same datum, and choose GEOID18 for NAVD 88 heights.
3. Check that only one step applies the geoid.
4. Shoot a known NGS mark or checkpointCheckpointA surveyed point you leave out of processing, then use to test how accurate your finished map really is.Full definition before every job.