Not everyone is competent with a map and compass, and the alternative – a GPS device – can be just as daunting. Geographer Lauren Kelley offers this dummies’ guide to GPS devices
About a year ago, my partner and I set off with a cracker weather forecast to walk a section of the Paparoa Range near Westport. Climbing to Buckland Peaks was stunning – sunny and clear as predicted. But the next morning, as we traversed the tops, we found ourselves wading through a thick fog and though we were just metres apart, we struggled to keep track of each other’s whereabouts. It made for a nerve-wracking scramble along a knife-edge ridge. As the day progressed, the weather deteriorated further and for the next two days navigation with a map and compass was all but impossible. Though I normally embrace the backcountry as a technology-free zone, I found myself wishing we had a GPS device to tell us where we were and to reassure us we were heading in the right direction. For some, a GPS is a complicated and confusing technological device. But it needn’t be. How does a GPS device work? Standard handheld GPS devices receive data from a network of more than 30 satellites orbiting the Earth some 20,000km overhead. This network includes satellites from the Global Positioning System maintained by the US Department of Defense (also known as NAVSTAR), and for some devices, Russia’s GLONASS satellites. Only a few satellites will be in range at any given time, and position accuracy can vary depending on landscape features such as narrow gorges and trees that obstruct the view to the sky, or atmospheric conditions that interfere with the satellites’ signal. A GPS device requires signals from at least three satellites to give an approximate position, but accuracy increases when it can pick up four or more. It calculates a position on the Earth’s surface by using trilateration – measuring the device’s distance from several satellites and determining where the satellites’ signals intersect at that distance. When a GPS device takes a long time to start up, it’s because it is searching for satellite signals and downloading information that allows it to make accurate calculations. GPS devices have a display that shows how many satellites are in range and estimates the current position accuracy. Note that word ‘estimate’ – your GPS position should only be used as a guide. Even modern handheld GPS readings can be several metres out in both position and altitude. In fact, because of the complicated mathematical calculations involved, a well calibrated altimeter will often give a more accurate altitude reading than the one derived from satellites. Some GPS devices come with a built-in barometric altimeter for this reason. [caption id="attachment_34559" align="aligncenter" width="1280"]


- NZTM coordinates: Written as northing and easting, Heaphy Huts is at N 5462341.943 and E 1525185.865.
- Decimal degrees: This shows the huts at Latitude -40.986669 and Longitude 172.110615.
- Degrees, minutes, seconds: With this format, it’s written as 40°59’12”S by 172°06’38”E.
- Grid reference: The NZTM Grid/Topo50 map reference is BP22 252 623.