GPS Accuracy, Datums and Why Your Grid May Be a Few Metres Off

TL;DR

Your grid is exactly as good as the iPhone's GPS fix. The Horizontal Accuracy tile shows the fix's uncertainty in metres. MGRS and UTM are computed on WGS84. Use fewer figures when accuracy is poor, and set a datum only for latitude/longitude from older maps.

The problem

You mark a point and the colleague who goes there ends up 30 m away. The maths was right. Either the GPS fix was weak, or the two of you were using different datums or different precision.

Read the accuracy tile

On Location, add the Horizontal Accuracy tile. It shows the phone’s estimate in metres with a coloured dot, plus a fix badge near the top of the readout. A smaller number is better. If it says ±25 m, a 10-figure (1 m) grid is false precision.

Rule of thumb: report no more figures than the accuracy supports. ±30 m → 6 figures; ±8 m → 8 figures.

Improve a fix

  • Move to open sky, away from walls and dense canopy.
  • Stand still for 20–30 seconds.
  • Remove thick or metal cases if the signal is poor.
  • Wait for the badge to settle before marking.
  • Take two readings a minute apart and compare.

Datums in this app

  • MGRS, USNG, UTM/UPS: computed on WGS84. This matches GPS and modern maps.
  • OSGB: computed on OSGB36.
  • Latitude/longitude: you choose WGS84, NAD27, OSGB36, ED50 or AGD66 and the app converts, for both showing and typing.

Older paper maps may be in NAD27 or ED50. If you read lat/long off one, set that datum for the lat/long slot to compare like with like. If you need MGRS in a different datum, this app does not provide it.

Common grid mistakes

  • Reading from the wrong corner of a square: MGRS references the south-west corner.
  • Swapping easting and northing (easting first, always).
  • Mixing zones near a zone boundary.
  • Using 4 figures in a briefing when 8 were needed.

Limits

Not for surveying, aviation, marine or live-fire use. The app says so on first launch and in the store listing.

FAQ

What accuracy can I expect? Open sky: typically a few metres. Under trees, in valleys or among buildings: much worse. The Horizontal Accuracy tile shows what the phone itself believes.

Why does my grid disagree with a paper map? Usually a datum mismatch, a different grid zone, or reading the map to the wrong corner of the square. Check the map’s datum note.

Does the app apply NAD27 to MGRS? No. MGRS and UTM are always on WGS84. The datum setting applies to latitude/longitude.

How do I get a better fix? Get a clear sky view, hold still for a short time, and let the fix badge settle before marking an important point.

The app MGRS GPS →