gps help please

[JONNYX]

hi all

i'm getting to grips with a new gps unit. could anyone advise me as to which datum and position format i must use to match admiralty charts?

much head scratching

jonny

[Salar]

Um...what GPS do you have and is it a chart plotter? My Garmin uses the usual Lat and Long, didn't require any setting, thats the norm for all chart and GPS navigation. Datum settings in my set-up screen are more for altitude (for hiking and planes), tide and time.

[JONNYX]

hi

its a lowrance globalmap 100 handheld mapping gps. its quite dated (1999), but effective.

i know OS maps use "ordnance survey 1936 england and wales"as a datum but i suspect admiralty may use "WGS 84" or another as i am finding an error of around 300 meters in the northing when taking marks from paper charts. i fish the manacles a lot and dont want to get it wrong!

cheers

jonny

[chris parker]

Some Admiralty charts use OSGB 36, some use WGS 72 and the UKHO are presently updating to WGS 84 starting with the south coast charts.

 

Your GPS will use WGS 84 ( World Geodetic Survey 84 ) as it's default factory seting. It is possible to change this to WGS 72 or OSGB 36 in the system setup menu or you can put in an offset to the lat and long position which the system will apply to all positions automatically.

 

From the offset you are getting I am guessing you have a paper chart based on OSGB 36 and your GPS is set on WGS 84. If you look on the chart it will tell you the datum used and also how much correction to apply to satellite derived positions.

 

If you only use the one chart it is best to match the datum on the GPS to the chart datum. Your positions will then be accurate. If you use more than one chart make sure all the charts have the same datum, if they don't you have to remember to reset the datum used in the GPS to match whichever chart you are useing at the time, if you forget to do this you can double the error in your position.

 

Hope this makes ense to you

[JONNYX]

thanks very much chris, that does help a lot. cheers

jonny