How to... operate GPSRs

John Chapman gives some helpful hints on GPSRs.

Global Positioning System receivers (GPSRs) are used by many outdoor people for navigation. They have some advantages over map and compass as they work well in all weather conditions and they can record where you have been.

While they display a position to a metre, hand heldGPS units generally have a horizontal accuracy ranging from 10-15m.

Digital maps can be purchased and placed onto GPS receivers. While being very useful, often these maps don’t show some walking tracks and other details and there is often a need to relate GPS readings to a paper map.

The default settings for new GPS units are ‘Degrees/Minutes/Seconds’ for Position Format and ‘WGS84’ for Map Datum, these will need to be changed to match paper map settings. By aligning your GPS with maps means you now have both means of navigation working together.

Paper maps use a rectangular numeric grid as if they used degrees/minutes/seconds, the grid would be a series of curved tapering lines. Using straight grids makes it easier to interpolate positions between lines.

Each map has the same grid as its neighbour so the grids flow together. However, at some stage, the longitude grid lines start to slope noticeably and not point north.

The solution used by map makers is to stop the grid there and create a new 'zone' with a new grid and the pattern gets repeated.

All GPS units can show map grids. For a Garmin, open 'Setup' then 'Position Format'. Set ‘Map Datum’ first as most maps show this somewhere. This sets the zero point for the degree/minutes/seconds reading. The most common is WGS84 which is essentially the same as GDA94 which is used for recent maps in Australia. For older Australian maps, AGD66 applies and this results in around a 200m difference in position.

The other setting is ‘Position Format’ which is the actual grid system setting. The most common setting used is UTM (or UPS) and in Australia try that first. If you scroll through the list you will find a large number of country dependent grids. New Zealand has two grids and depending which map versions you have you might try both.

In a new area or country, I do this setup at a known location such as the end of the road, a hut or track junction. It can take a few minutes to find the correct settings but is worth doing.

The GPS grid reading will show something like 55H 9812345 6789123 which is a grid location to a 1m accuracy. To extract the conventional six figure grid reference simply drop the last 2 digits from both numbers and take the next 3 digits. This will give a grid position to within 100m which for most purposes is adequate.

The grid reference for the above GPS reading is GR 123891. Most government maps show how a grid reference is derived by including an example.