GPS Coordinate Systems
Edit on GitHubDecimal degrees, degrees and decimal minutes, UTM: the coordinate formats the beacon can use and how to convert between them.
GPS Coordinate Systems
A position can be written several ways. The beacon transmits in decimal degrees, but maps, radios and rescue teams use other formats. You should be fluent in all three.
Decimal degrees
45.8325 N, 6.8650 E
One number per axis, signed or with a hemisphere letter. This is what the firmware transmits. At the precision used by the payload (5 decimal places), resolution is about 1.1 m at the equator.
Degrees and decimal minutes (DDM)
45 49.950 N, 6 51.900 E
The GPS module’s native format and the format used on most topo maps. To convert DDM to decimal degrees: divide the minutes by 60 and add to the degrees.
45 + (49.950 / 60) = 45.8325UTM
32T 323450 E 5077600 N
A metric grid projected onto the earth. UTM is what rescuers often use because distances are in meters and easting/northing pairs are simple to plot. The zone (32T here) matters: the same pair of numbers in another zone is a different place.
Which one to use
| Situation | Format |
|---|---|
| Beacon payload | Decimal degrees (firmware default) |
| Topo map plotting | DDM or UTM |
| Reporting to a rescue center | Whatever they ask for; read back their format |
| This wiki’s map tools | Decimal degrees |
Conversion tools
The firmware’s CONFIG mode includes a coordinate preview that converts the live fix between formats on the OLED. Online converters exist, but do not rely on a network in the field; practice the DDM to decimal conversion until it is automatic (see Coordinate Conversion).
Related pages
- Coordinate Conversion for worked examples
- GPS UTM vs Decimal for the trade-offs
- Coordinate Plotting and Maps for plotting practice