GPS receivers are great for telling you where you are, especially if you have been to the spot before and are simply returning, of if you have been given the coordinates by someone else.
However, plotting a GPS position on a topographic map is not as easy at it could be, mostly because there is a tendency to use the wrong units.
GDA 94
The first and most important thing you must do with your GPS receiver is to choose the right datum point, or starting point, from which your GPS will calculate your position.
The whole business of datums in Australia, as elsewhere, is confusing, partly because of the similarity of the acronyms being used, and partly because the Australian datum has recently been updated. The new Australian datum to use is GDA 94, and all new topographic maps in Australia are using this standard. The acronym stands for Geocentric Datum of Australia. However, many older GPS receivers do not have GDA 94 included in their list of datums. Fortunately GDA 94 is virtually identical to WGS 84 (WGS stands for World Geodetic System).
Set your GPS receiver to either GDA 94 or WGS 84 and leave it there.
For some reason, most GPS receivers have the default setting for reporting the position in latitude and longitude. This being the standard ‘navigating’ format, it satisfies the majority of users that they are now truly ‘navigating’, even if they don’t fully understand the system.
Within the lat/long notation there is a further divide: while traditionally one used degrees, minutes and seconds of arc to describe a position, modern usage also allows decimal degrees, and degrees and decimal minutes :
degrees, minutes & seconds of arc S 35° 55’47.1”, E 148°28’42.1”
decimal degrees S 35.92976°, E 148.47835°
degrees & decimal minutes S 35° 55.786’, E 148° 28.701’
All of the above positions describe the same spot - Four Mile Hut, SE of Mt Selwyn in NSW.
Quite apart from the possible confusion caused by different formatting, lat/long positions are very hard to transcribe to a topographic map because the grid shown is of a totally different system, and because of the difficult mathematics involved in minutes and seconds of arc.
Universal Transverse Mercator (UTM)
The grid system shown on all Australian Topographic maps is UTM. In fact, as the name suggests, most topographic maps around the world use UTM, though the name is often different.
The same position (Four Mile Hut) described in UTM is
55h E 0633363, N 6022832, though most GPS receivers leave out the E & N.
Using UTM the world is divided in into rectangles, each with a letter and a number. The rectangles are unequal in size, due to the fact that the projection from a globe to a flat map with relative angles and distances reasonably accurate requires distortion.
North-South, the globe is divided into 23 zones, which are each allocated a letter of the alphabet, and east-west into zones of 6° of arc, each allocated a number between 1 & 60. This is far more complicated that the general map-reader needs to know, but a full account can be read at http://www.uwgb.edu/DutchS/FieldMethods/UTMSystem.htm
You’ll note that in our example, Four Mile Hut, and much of southern NSW and northern Victoria, lies in zone 55H.
The figures following the zone number describe the distance from the SW corner of that zone, the first seven figures being the distance east of that point, the second seven being the distance north of the SW corner.
Somewhere on the map you will find a set of boxes telling you all about the position of the particular map in relation to the rest of the world, but if you set your GPS units to UTM (often described as UTM/UPS) you can almost forget about that, because the reading will give you that relativity.
Let’s take the Four Mile Hut position 55h E 0633363, N 6022832.
55H describes the zone.
Now look carefully at the Eastings, the numbers along the top and bottom of the map (which are identical): on the edge of the map beneath the spot on the map where Four Mile Hut is you’ll see a very tiny 6, followed by the bigger numbers 30. These three numbers, 633, should really have another 0 before them to read 0633 (I’m not sure why this convention is followed, but if the first figure is 0, it is generally left off). That tells you that the spot we want is somewhere in the grid square 0633.
Apply the same logic to the Northings, reading the numbers running up the left and right sides of the map – Four Mile Hut lies in the 1000m square 6022.
Mentally divide the square into ten parts eastings and read the position of the hut (3), and the same with the Northings (8).
Now, if you already know about grid references, you’ll know that we normally pinpoint a spot with a six figure reference, the first three numbers being read from left to right, and the second three from bottom to top.
To cut a long story short, in practise you can look at your GPS reading, ignore the first two and the last two of each seven-figure set, which leaves you with two sets of 3-figure numbers… the ones you would normally use for a grid reference.
In the case of Four Mile hut, 55h E 0633363, N 6022832.
See what I mean?
The 55h and the first two numbers simply specify the map that should be used. The last two numbers, interestingly, refine the position way beyond the reach of a printed map, bringing the accuracy down to 1 metre, far more accurate than we could measure on a 1:25,000 map.
It’s only recently that we have begun to realise that our old view of the shape of the Earth was somewhat inaccurate. In fact, you’ll find a considerable discrepancy between positions plotted on the very latest series of Australian maps, and those more than ten years old.
You’ll find that the old maps are roughly 200 metres out. However, you should understand that they are only inaccurate from the point of view of an outside measuring device: something like a GPS.
Fortunately, the discrepancy is, in local terms, the same everywhere. If you plot a true GPS position on an old map, you’ll find that it puts, for instance, Four Mile Hut nearly 200 metres NE of the position on the map. In wooded country or at night this could be a problem, but generally 200 metres isn’t going to make a huge difference.
Conversely, if you are using a position taken from the old series maps, a GPS reading will put you two hundred meters to the SW. of the real position, so from the GPS position, go roughly 200m to the N.E. to get to the map position.
If you use your GPS receiver mostly to navigate with the aid of Australian topographic maps, set your machine datum to either GDA 94 or WGS84 and set the units to UTM, and leave it there… you’ll have no more trouble.
And finally, if you’re using an old map, your GPS will take you to a position around 200 metres NE of the spot marked on the map… it’s the map which is wrong, not the GPS.
Conversely, if you are using a position taken from the old series maps, a GPS reading will put you roughly two hundred meters to the SW of the real position, so from the GPS position, go roughly 200m to the N.E. to get to the map position.
© Michel Dignand 2007
Michel Dignand studied celestial navigation in the mid 1960s, and has used most available navigating systems in ocean sailing and other outdoor pursuits for more than 45 years.
The Australian Government publication ‘Map Reading Guide – How to Use Topographic Maps’ not only discusses this subject (though not as comprehensively as in this article) but also includes much valuable information about using topographic maps.