Handheld GPS Position Display Format

In the UK there are two main ways*, to have positions displayed on your GPS Receiver, either Lat/Long or Grid References, which you use is basically upto you as long as you setup your unit correctly. * Some GPSr's allow both!

Terms:

Position Format

The way the numbers are shown on screen, for GB either Lat/Long or Grid Reference

Map Datum

Defines the shape of the earth used to calculate the position, there are two relevent datums for GB, either OSGB36 or WGS84.

(Recommended for Geocaching) Position Format hdddºmm.mmm Map Datum WGS 84 WGS84 Longitude and Latitude Other names: Lat/Long, Degrees Minutes Secondes, DMS, Degrees and Minutes Pros International Format - works anywhere in the world works well with global websites such as Geocaching.com Cons Quite hard to mentally work with the numbers Not so many people in Great Britain are as familar with Lat/Long? (at a guess) There are a few difference position formats, that are quite easy to mix, eg dd.dddddd or ddºmm.mmm or even ddºmmºss.sss

Position Format British Grid Map Datum Ord Srvy GB Britain National Grid Other Names: Grid Reference, National Grid, BNG, sometimes even called Landranger Grid. Sometimes known by the queiviennt Datum Ord Srvy GB or OSGB36 (sometimes shortened to OSGB) Pros Is a Standard in Great Britain, therefore many places use it, the most obvious is referenceing a Ordnance Survay Map, like Landranger or Explorer (Outdoor Leisure) Quite simple to calculate rough distances in your head Cons Specific to the Great Britain (although Ireland have a very similar system)

Technical Details

A GPSr stores its positions in an internal format, these settings only affect how they display them to you the user. As long as you change BOTH THE POSITION FORMAT AND DATUM TOGETHER* then you should have no touble changing as the need arrises.

When transfering Waypoints from a PC via a cable then it doesn't matter what setting you have your GPSr set to, it will be stored correctly.

Similarlly if you use the Mark Waypoint to add a point the GPSr's memory then you can switch to the other display format and then work, then switch back etc without problems, again the only thing to ensure is that you change both settings together!

* Many newer GPSr's will handle the change of datum automatically if you change the position format or vise versa, but alway check that it has changed BOTH.

Getting it wrong

If you change one but not the other, then when referencing paper maps from your GPSr screen, or transfering position to/from a webpage like geocaching.com for example, then the position will easily be 70 to 200 meters out.

Simple rule of thumb: change the Position Format and Map Datum together. Either use Lat/Long (for example hdddºmm.mmm) with datum to set to WGS84 or British National Grid with OSGB36 (Ord Srvy GB)

Last Update: 01/12/05