WHERE ON EARTH
WORDS BY MARK ALLEN PHOTOGRAPHY BY MARK ALLEN AND ROBB COX
Global Positioning System - GPS - enter the sci-fi-world of electronic-navigation. Don’t be scared though - it really is an easy hi-tech system to get the hang of, so long as you know the basics and are prepared to learn a few extra terms along the way.
A modern GPS receiver has more functions than most people will ever use or need. Many functions, such as hunting, fishing, high and low tides are all related to the moon phases; it’s just been the imagination of the programmers to provide more and more seemingly fancy outputs from a basic set of information that has always been available.
GPS’s can make remote travel far safer, in that you’ll be more likely to know exactly where you are – even if you do run into strife. They can provide hours of outdoor entertainment through the multitude of hide-and-seek (geocashing) type games, and they can provide a heap of useful and interesting information while travelling away from street signs. |
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HOW DOES GPS WORK
The world wide Global Positioning System is solely reliant on car-sized satellites, orbiting earth, emitting signals to our ground-based receivers – the ones we carry in our hands. These signals are at work 24/7 and battle on regardless of the weather. The part most civvies don’t realise is that there is a third party to the whole system, comprised of ground-based control-stations, ground-based antennas and a master control station, which are scattered around our planet.
Triangulation calculations are performed in the wink of an eye by our hand-held units, to determine longitude, latitude and heights (or RL’s – Reduced Levels) above (or below) sea level. This pinpoints exactly (or very close to) where you are standing on planet earth.
It’s then up to the user to convert the information and translate it to either a paper (manually) or digital (automatically) map.
The more satellites that are viewed (received electronically) by your hand-held GPS, the higher the accuracy of the output information will be. There are at least 24 satellites orbiting the earth, and most GPS’s will have a screen to tell you how many you are tracking at any one time (maximum of 12 or 14 if you have a WAAS enabled unit). Any ‘obscuring of view’ via buildings or foliage will inhibit the tracking of satellites with a corresponding lower accuracy.
Initially, America had the sky to themselves with their GPS satellite system (NAVSTAR - NAVigation Satellite Timing And Ranging) but we now have the Russians with their own system in action (GLONASS - GLObal NAvigation Satellite System).
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DATUMS, GRIDS AND COORDINATES
There are many different datums and grids in use around the world. You need to use the one relevant to your country (most likely Australia if you’re reading this) and the map you are reading from. If your datums and grids do not match, you may as well pack your GPS away – the numbers it’ll provide will be useless to you.
In Australia, most maps and surveying work were on the AGD66 (Australian Geodetic Datum 1966) and the AMG66 map grid (Australian Map Grid 1996), and their very similar counterparts AGD84 and AMG84, which were used in some parts of Australia.
Australia now uses the GDA94 datum (Geocentric Datum of Australia 1994) for latitude / longitude and the MGA94 map grid (Map Grid of Australia 1994) for UTM (Universal Transverse Mercator) coordinates. This change in map grid results in UTM coordinates moving roughly 200 metres (depending exactly where you are in Australia) to the North-East. |
GDA94 is the same(ish) as the WGS84 (World Grid System 1984) for most practical purposes. The differences are in the order of a few centimetres.
So, in Australia you need to be sure the datum / grid on your GPS receiver is set to GDA94 (or WGS84), provided you have the newest style of paper/electronic maps.
UNIVERSAL TRANSVERSE MERCATOR
Universal Transverse Mercator (UTM) is based on a military grid system to make pin-pointing an exact spot on a map quick and easy, and in Australia has 30 zones. Each zone is referred to by: a region of longitude (which is 6 degrees ‘wide’) and a letter to refer to a region of latitude (which is 8 degrees ‘high’). The East-West coordinate (Easting) is referenced to the central meridian of longitude (i.e. 3 degrees from each boundary), where it has a value of 500,000 metres. For the North-South co-ordinate (Northing) (in the Southern hemisphere) the 0 value is referenced to the latitude which is 10,000,000 metres south of the equator.
So, a typical set of full coordinates are UTM zone 55H, Datum GDA94, E0394250, N6464600. On your GPS unit, you’ll only be concerned at reading the E and N values (provided the datum and grid have been set correctly), so that should make life a little easier with less numbers to worry about.
LAT / LONG OR EAST / NORTH
The easiest way to plot a point on a map, in two dimensions, is with two lines that cross. One line that goes up and down (north / south) on the map and another that goes left to right (east / west). Where the two lines intersect, is the point of interest.
Whether you quote (or use) latitudes and longitudes, or Eastings and Northings is dependant on the map you are using. Look along the sides (and in the legend) of the paper map to check and then set your GPS to the same system.
A coordinate of say, E0394250, N6464600 in UTM zone 55H will have a Latitude and Longitude expressed as -31º 56’ 58.69890” Lat 145º 52’ 51.83780” Long. Eastings and Northings tend to be easier for most people to understand as they are based on the deci-system (ones, tens, hundreds, thousands, etc) as compared to the Lat and Long is based on Degrees Minutes and Seconds, of which is made up of 60 units and multiples thereof. This is derived from 1 degree of longitude, at the equator, being equal to 69.1722 miles (111.2981 km), one minute is 1.1528 miles (1.8549 km) and one second is 101.45 feet (30.9m). |
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DISTORTED VIEWS / INACCURACIES
As the earth is not perfectly round and even (or flat) all datums and grids are based on a ‘best fit’ scenario, which does provide small errors in some places – more so the heights than the coordinates. For our 4WDing and camping uses, these errors can be disregarded as they are insignificant. Picture that old adage of trying to lay an orange peel onto a flat surface – there will be a few wrinkles in some places.
This is why we need to use a datum and grid that is best suited to us – in this case GDA94 datum and MGA94 map grid for Australia.
PAPER MAP AND COMPASS – DO YOU STILL NEED THEM?
Yes. Nothing beats laying a map out on the bonnet, while the whole family looks on, to see where they’ll be at days end and for viewing larger portions of land.
Batteries and electronics fail, and the whole GPS system could fail for some unknown reason! While the latter would be highly unlikely, the point is that a backup system should be kept handy – paper, compass and the ability to use them, is the best backup available.
WHICH GPS FOR YOU
There really is no one single answer as to which GPS is best on the market. It’s like being asked which 4WD is the best. It all depends on your needs, wants and uses. There are many facets of a GPS that should be considered before handing over your plastic.
Here are just a few points to consider: Battery life and 12V adaptor availability, size and weight, antenna type, mapping compatibility, software compatibility, vehicle mount availability, electronic compass, way points, routes and track logs, accuracy, coordinate system choices, water proofing, 12-channel or WAAS compatibility and price.
Go forth, map and conquer the earth with your new found toy. Don’t be concerned about getting geographically challenged; you will always know where you are lost with a GPS. Just be sure you have the means to get ‘un-lost’ and can contact the outside world at all times.
The sky is not the limit!
WHAT’S IT ALL MEAN?
WAYPOINT: a GPS memory stored ‘position of interest’
GOTO: A command to calculate the direction and distance to a point of interest (such as a waypoint). It’s generally given as a ‘straight line calculation’, so you need to take into account twists and turns en route.
ROUTE: A series of Waypoints to take you from one geographic place to another, such as the process needed to take you from Birdsville to Innamincka, via the French Line.
TRACK LOG: The track (or route) that has been recorded on a map in the GPS memory, by your GPS while in motion.
BREADCRUMB TRAIL: Similar to a track log, but utilised to re-trace a route. Extremely helpful in finding your way back to camp or out of a maze of tracks, exactly the reverse direction to what you original took
COORDINATES: Usually referred to in Eastings and Northings (how far north/south and east/west a point of interest is)
LONGITUDE AND LATITUDE: Similar to a coordinate, although quoted in degrees, minutes and seconds, or degrees, minutes and decimals of that minute.
MAP DATUM: A reference plane (or line), or orientation or start line, from which all other readings are taken from. The datum of your GPS must match the datum of the map you are referring to.
ELECTRONIC COMPASS: The ability for the compass to work while stationary. Standard compasses on a GPS require constant movement to enable a reading to be recorded and displayed
SELECTIVE AVAILABILITY: Denial of full accuracy from the satellites to earth’s GPS units.
INITIALISATION: When you first turn a GPS on, it goes through an initialisation process to determine where in the world you are. The initialisation time depends on the time and distance from your last initialisation.
COLD START: Turning on a GPS after a longer period of inactivity will require a longer time to track satellites and determine your position
WARM START: Are much quicker at finding your position, as they already know ‘roughly’ where in the world you are from a recent use.
12 CHANNEL RECEIVER: This is the standard technology now inbuilt into a GPS, which receives and processes information from up to twelve satellites simultaneously. Even though some GPS’s have an extra two channels (WAAS enabled) you do not gain any accuracy – unless you use it in USA, Europe or Japan.
WAAS: Wide Area Augmentation System is for improved GPS readout accuracy. Not currently operational in Australia. When enabled, WAAS is claimed to provide better than three metre accuracy, 95 percent of the time.
EXTERNAL ANTENNAE: is attached to a vehicle or building to provide superior reception.
DGPS: Differential GPS involves a remote (to your position), known point to be recording (at exactly the same time as you). Both yours and the second GPS readings are used to calculate higher accuracies.
GEOCASHING: Playing hide and seek with your GPS.
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