My coordinates are wrong — but I have RTK Fix

All NTRIP correction services — including GEODNET-based services — deliver corrections in WGS84 or ETRS89. These are global reference frames used by satellites. Most countries work in a local national datum — RD New in the Netherlands, OSGB36 in the UK, GDA2020 in Australia — which does not perfectly align with WGS84.

The difference between WGS84 and a national datum is small but significant for precision work: typically 0.5 – 1.5 m in the Netherlands, and similar magnitudes in other countries. If your field software is not applying the correct datum transformation, every point you collect will be shifted by this amount.

• 1
  Check your job's coordinate system settings. Verify that both the horizontal datum and the transformation method are set correctly for your country.
• 2
  For the Netherlands: the job must use RD New (EPSG:28992) with the RDNAPTRANS™ 2018 transformation. This is a legally mandated transformation maintained by the Dutch Cadastre. Without it, positions will be off by roughly 0.6 – 0.9 m.
• 3
  For the UK: use OSGB36 (British National Grid, EPSG:27700) with the OSTN15 transformation and OSGM15 geoid. Without OSTN15, horizontal positions are off by up to 1.5 m and heights by several metres.
• 4
  Verify by occupying a known national control point. If the offset is consistent in direction and magnitude across all points, it is a datum transformation problem.

GNSS receivers measure height relative to the WGS84 ellipsoid — a mathematical model of Earth's shape. This ellipsoidal height is not the same as the height above sea level (orthometric height) that appears on maps and is used in engineering.

In the Netherlands, the difference between ellipsoidal height and NAP (Normaal Amsterdams Peil, the Dutch vertical datum) varies between approximately 38 m and 44 m across the country. If your receiver is outputting ellipsoidal height and your project uses NAP heights, every elevation you collect will be wrong by this amount.

The correction between ellipsoidal and orthometric height is called the geoid undulation. Applying the correct geoid model is what converts one to the other.

• 1
  Check whether your field software is applying a geoid model. In Emlid Flow, go to Job settings › Coordinate system and verify a geoid file is selected.
• 2
  For the Netherlands: use the NLGEO2018 geoid (included in RDNAPTRANS™ 2018). This converts ellipsoidal heights to NAP. Without it, heights are out by 38 – 44 m.
• 3
  For the UK: use OSGM15. For Germany: use GCG2016. For Belgium: use BG03. For France: use RAF18. See the country table below for a full reference.
• 4
  If the geoid file is not available in your software, download it from the national geodetic authority and import it into your field software's coordinate library.

If your positions are off by tens or hundreds of metres, or if the offset varies across your site rather than being a consistent shift, the most likely cause is a completely wrong coordinate system. This happens when the job is created with the default coordinate system (often WGS84 geographic or a UTM zone) instead of the project's required system.

• 1
  Check the EPSG code of your job's coordinate system and compare it with what your project requires. For the Netherlands: EPSG:28992 (RD New). For Germany: EPSG:25832 (ETRS89/UTM zone 32N) or state-specific systems. For Belgium: EPSG:31370 (Belgian Lambert 72).
• 2
  If the wrong system was used for existing collected data, re-process the raw observations in the correct coordinate system rather than trying to apply a post-collection offset — offsets from projection errors are not uniform across large areas.
• 3
  Create a template job with the correct coordinate system and transformation before fieldwork begins. Share it with the whole team so everyone uses the same settings.

The antenna height you enter in your field software is subtracted from the measured position to compute the ground point. An error here shifts every height measurement you collect by exactly the same amount. This is a systematic error — it affects every point equally, which makes it identifiable.

• 1
  Verify what the height is measured to. Different manufacturers measure to different reference points. Emlid heights are measured to the bottom of the receiver. Trimble heights are typically measured to the antenna reference point (ARP). Entering the wrong reference adds or subtracts the antenna offset on top of your field measurement.
• 2
  Check vertical offset in the antenna profile. Most field software stores a vertical offset for each receiver model (the distance from the measuring reference to the phase centre). Verify this value matches your receiver's published specifications.
• 3
  For tilted pole work: if you are using tilt compensation, ensure the pole bubble is calibrated. A 2° tilt on a 2-metre pole introduces a 7 cm horizontal error and a negligible vertical error — but a poorly calibrated tilt sensor may report wrong corrections.
• 4
  Test on a known benchmark. Set up over a point with a published height. If the height error equals your measured antenna height value exactly, you have entered the antenna height in the wrong direction (positive instead of negative or vice versa).

GNSS receivers do not measure position at the physical base of the antenna. They measure at the antenna phase centre — a point inside the antenna that varies by satellite elevation angle. Field software corrects for this using antenna calibration data (the antenna offset profile).

If your receiver model is not in the software's antenna database, or if the wrong model is selected, the phase centre correction is not applied and height measurements are off by a few centimetres. This is less common than datum and geoid errors but matters for the highest-accuracy applications.

• 1
  In your field software, check that the antenna model matches your physical receiver exactly — including variant (RS2 vs RS2+, for example). An incorrect model selection causes a wrong phase centre correction.
• 2
  For Emlid receivers used with third-party software, download the IGS antenna calibration file (ANTEX format) for your receiver and import it into the software's antenna database.
• 3
  If this offset is consistent and small (2 – 5 cm in height), it is worth investigating. If it is larger, the more likely cause is antenna height entry or geoid model — revisit those first.