NSW Nordic Ski Club
GPS - A User's Guide
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A GPS (Global Positioning System) unit works by simultaneously receiving signals from a number of satellites circling the earth and calculating the position and the altitude of the unit. The notes here refer to the Garmin Etrex® model that weighs just 150G, is snow and waterproof and sells for under $400. Two AA batteries give up to 22 hours operation in a battery saver mode. The unit will work anywhere in the world and can be graduated for use with nautical or land maps and any distance scale. Other more expensive units offer other functions that are not essential. Cheaper units do not have as many useful features. More info at www.garmin.com
You must fully understand the use of a map and compass before attempting to use a GPS unit. They are very useful to give a position accurate to within 6 metres at best. It is uncanny how it will get you back to the same spot time after time and close enough to throw your hat on the spot. However it must be used correctly, it may not work at all under the cover of trees and its usefulness may be limited by battery life. Remember that batteries have a shorter life in cold temperatures. A GPS can be used in two ways. The map coordinates of a location can be programmed into the unit and it will then indicate a course to follow to get to that point. Alternatively it will record your track during a journey and at any time that track can be saved and followed back to the starting point. While it would be relatively easy to "follow the pointer" to get from Seaman's Hut to Thredbo it would take a lot more skill and experience to get from the end of Watson's Crags to Seaman's Hut. It is important to be thoroughly conversant with the operation and features of the GPS and to have done some trial skiing navigation in a safe area. It takes a couple of days practice to become familiar with the operation but once learnt it is possible to come back to it months later and use it without reference to the instructions.
When entering a grid reference into the GPS it is essential that the map datum used is the same as set in the GPS unit. Datums used for Australian maps are to be changed progressively as new editions are printed. The Etrex uses the UTM/UPS grid format and the WGS84 as the map datum and coordinates from old maps will have to be converted to this datum. If not converted the location will be from 100 to 150 metres out in both latitude and longitude. This is probably as close as you might get with a compass in a white out and it may be possible to get to the correct spot by observation but you may not find a hut in an emergency.
The best way to get an accurate conversion from the map datum is to measure and write down the coordinates of a known point on the map and then go to that spot and mark the spot in the GPS and read the coordinates. The difference in the two figures is the amount of correction that you have to apply for that map datum for any other location. Do a check on another location to confirm your first result. A particular location in Perisher Valley is shown as 55H 062800 (latitude) and UTM 5970950 (longitude). On the map this is measured as a point between grid 62 and grid 63 (62.8) and as a point between 70 and 71 (70.95).
The accuracy of your coordinates depends on the accuracy of your measurement and it is useful to have an easy to read ruler and a pencil for calculations and it would be easier to prepare coordinates at home than in the field. Entering your grid reference would be useful on a trip from Kiandra to Perisher or to find a location like Tin Hut in a whiteout. Double check the accuracy of the coordinates that you enter into the GPS - if you enter the incorrect information you will travel to the wrong place. If travelling in a whiteout you must be aware of the nature of the terrain as there is little margin for error if hazards are present.
The backtrack function is useful when doing a day trip from a base and the weather closes in. Such an event can occur on a trip from Charlotte Pass to Watsons Crags. It is possible to navigate back within a few metres of the outbound track; just take care not to ski too close to cornices on the way out or you may fall over them on the way back. It is a good idea to mark significant turning points on the way out, eg saddles, river junctions, peaks etc including safe turning points near a hazard.
When used to navigate to a saved or inserted waypoint the GPS will indicate the track as a straight line but there could be obstacles such as trees, cliffs, creeks etc in the way. In practice you will have to go off track but the unit will show you the way to get back on track. If you are travelling from west to east in a whiteout remember that the prevailing winds during snowfalls form lines of cornices and steep drops on ridges (eg the Kosciuszko ridge), steep drops in creek beds in a wind shadow (eg Soil Con Creek), one metre drops in flat areas (eg near Illawong lodge) and melt holes and open creeks in bog areas (eg Johnnies Plain).
Athletic types and competitive skiers will find a GPS useful for recording total training times and distances and differences in altitude. The unit can be hung around the neck, put in a shirt pocket or in a carrying pouch attached to the arm or to a pack. It will work under a layer of clothing.
In unfamiliar areas the map must be referred to frequently to confirm progress. A compass is not essential in practice but must be carried in case the unit fails or for plotting a course on the map. The pointer page on the GPS unit tells you the bearing and distance to your selected waypoint, your heading, speed, maximum speed, time to the point at your current speed, altitude, total time spent travelling, date and local time and time of sunset and sunrise as well as your current location. All this information is updated continually. The map page shows you the direct track to a selected waypoint and different distance scales allow you to see other waypoints in the area. It also shows your location in relation to the direct track and the waypoint. As the unit will tell you your distance and bearing from a known point so it is a simple matter to plot that position on the map. In some situations it may be advisable to divert from a known track and position to intercept a nearby road or pole-line that provides an easier or safer route.
When doing a trip from a hut, tent, or snow cave mark the location in the GPS to be sure of finding it on the way back. If someone is injured and a rescue party has to leave them to get assistance, mark their location in the GPS before leaving and you can be sure of finding them quickly even if returning after dark.
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