Thread: GPS co-ordinates

Originally Posted by veitnamcam

I had an issue with this just recently....wanted to go to some marks in not my boat...copied numbers out of my chart plotter into my handheld GPS....my plotter had one extra digit....resulting marks appeared to be about 80m out on land not water.
Which one should I set them both to and will this stuff up my existing marks?

One extra digit should not affect much IF the datum and position format are the same and both units are running the same version of the datum (the WGS84 issue where the datum needs to be updated every 5 years or so to make it match the physical features on land). You need to go to the settings in each unit for datum and confirm that they are set to the same and then double check the position format and make sure that the reference is the same and that you haven't swapped the east for south or something other odd. You **should** be able to alter the datum and the unit should alter your points to suit - I have done this with Garmin and Raymarine survived the process unscathed with both units but you do need to check this and your mileage might vary.

Also it's worth noting as an aside that a few units automatically use straight line point-point navigation, whereas some over longer course sets will automatically go to great circle navigation so you are following the shortest path along the line through the center of the earth not directly to the point. This can make you scratch your nut as you start thinking that the unit is sending you on a curved course off to one side from your destination and everything is just wrong! Garmin handhelds do this for one. Very handy for international course plotting when you are filling in your manually generated great circle course on the exam paper, saves about an hour of faffing about calculating manually! Not so useful when you want the straight line point-point distance and the GPS puts you crook.