Thread: GPS absolute beginner

Mmm... Thought I might get started here. the thing is, I know almost nothing about GPS. What I want to have is a device which will give a co-ordinates position readout if I'm out there in the back of beyond and need help. Not interested in navigating through cities/ along highways. It strikes me that all I can do just now is phone in for help, but then in response to the question "Where are you?" I'd have to say "Don't exactly know".

But to get this co-ordinates position I suppose I need MAPS, and at this point i'm already lost. No-one here to explain it to me. do I have to feed maps into the device before I can get a readout to tell me where I am? If so, here in southern Oman, I don't think I can get maps.

Please direct me to a site or give a few words of advice to get me started. Right now I don't know what to do, but want to be prepared for when I'm stuck in the wilds.

Disregarding the street navigator types, AFAIK all other GPS receivers can display coordinates. Many have a number of display pages - so may need to change to another page.

The default configuration for coordinates is usually latitude and longitude and for datum it is WGS84.

If you are using topo maps, the map grid system is not latitude and longitude, but UTM (Universal Transverse Mercator) Eastings and Northings. For large scale maps (1:25000 to 1:100000) the grid lines are at 1000 metre spacing (not sure of the grid spacing of 1:250000 maps, but it will say on the map)

The GPS receiver will need to be configured (in the menu options) to display coordinates in UTM.

With UTM, the earths surface is divided into zones (each zone covers 6 deg of longitude). Also divided into northern and southern hemisphere, but this probably won't matter to you.

The far east coast of NSW and QLD is in zone 56, the lower south coast of NSW, from about Tathra down is in zone 55, the SA, NSW border is in Zone 54. So to specify position in UTM, it is important to include the UTM zone (the gps receiver will display the zone of your location).

Note that the GPS receiver will the display the UTM full Eastings and Northings in metres. Assuming the topo map grid is 1000 metres then ignore to rightmost 3 digits and read the 4th and 5th (from right) digits. These will correspond to the numbers on the maps grid lines.

Eastings (the vertical grid lines) are specified 1st. Northings (horiz grid lines) are specified last. Where these easting and northing grid lines intersect, is the lower left corner of the grid square that you are located in.

Once you find the square, the rightmost 3 digits tell you how far you are from the grid lines in metres.

Note that some topo maps use an older datum (the mathematical ellipsoid used to approximate the earths surface). The GPS use WGS84, Australia used to a different datum, but changed in about 1994 to be compatible with GPS.

The coordinates on topo maps based on the older Aus datum can be out by about 200 metres. Recent maps explain how to convert between the datums.

GPS receivers have options for selecting the various datums use in all parts of the world. If you want, you can select a datum to suit the maps you use.

Note if you want to inform searchers of the gps coordinates where you are located. You need to specify the datum used.