Converting British Lat / Long to UTM Using a GPS

These notes have been created using a Garmin GPS12 receiver, but most receivers should be capable of performing the steps documented here.

• Set your system to the relevant Map Datum - for British Admiralty charts, this will be the Ordnance Survey GB (1936) Datum.
• Set the system Position Format to that in which you want to enter your coordinates. For OS maps, this will be British Grid. For Admiralty Charts, you'll probably use hdddºmm.mmm'
• Enter your waypoint coordinates.
To convert to UTM...
• Set your systems Map Datum to WGS84 (World Geodetic System 1984)
• Set the systems Position Format to UTM/UPS

That's it. Your coordinates. will now be shown using the UTM system.

As an example, lets look at how we'd enter a charted wreck found off the coast of South Wales into the GPS.

The first thing we need to do is check the Chart Datum - for Ordnance Survey maps and for Admiralty Charts this will invariably be Ordnance Survey GB. This will always be displayed on the map/chart.

The charted wreck location is: 51º21.040' N 3º18.350' W.

To begin, set the GPS to use the correct datum for the map/chart (and this is where a lot of 'accuracy' problems are experienced). In this case, the GPS map datum gets set to Ordnance Survey GB. Now the charted wreck position can be entered.

We now change the map datum of the GPS to WGS84. If we were now to check the wreck location, we'd see it has 'moved' to 51º21.066'N 3º18.427'W or 150m SE of where it actually lies. For this very reason, it's vitally important to ensure that the correct datum is used when entering coordinates. The datum will always be found on the map/chart.
Finally, if we swap the Position Format on the GPS to UTM/UPS, the wreck location can be found at 30U 0478613 5688915. It is important when quoting locations in UTM format to ensure that the map datum is set to WGS84 and not Ordnance Survey GB - we're using a universal format, so use a universal datum.

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