UNDERSTANDING TOPOGRAPHIC MAPPING
The mysteries of topographic maps explained - January 2016
If you intend to stray from the thick lines that connect major points of civilisation on road maps onto the thin or even the dotted ones, you need to understand the basics of topographic map reading – whether your ‘topos’ are on paper or on a GPS screen.
Topographic maps are produced by a number of different agencies and a complete Australia-wide topo set is produced in paper and digital format by Geoscience Australia and branded Natmap.
It’s obvious when you look at a topo map that there’s a lot more information on it than on a normal road map. Topos show roads and tracks, buildings, railways, airports and bush airstrips, administrative and state boundaries, national parks, reserves, forests, lakes, rivers and creeks.
In addition to this one-dimensional information topos have three-dimensional ‘relief’ detail. The contours shown on topo maps are useful for visualising mountains, valleys, slopes and depressions. Where the road or track you’re driving on crosses contour lines you can see if it’s about to climb or descend and also how steeply: contour lines close together show steep sections and the contour altitude numbers tell you if the track runs uphill or downhill.
The latest topo maps have ‘relief shading’ to improve the three-dimensional effect: higher ground is shaded as if by the afternoon sun.
The level of detail on topo maps is an eye-opener for people used to ordinary road maps. An example is the accuracy of sand ridge positions in the vast Australian inland: most haven’t moved substantially for thousands of years and have been faithfully reproduced on topo maps. It’s possible to plan a route through a dune field, using topo mapping to minimise the number of dune crossings, by skirting the ends of some of them.
Topographic maps serve many purposes for a wide variety of users and much of the information on them isn’t necessary for recreational bush travel. All you need most times is to be able to mark latitude and longitude from your GPS screen onto a paper topo map, or to see where the cursor indicates you are on a GPS screen topo.
An obvious difference between topo and road paper maps is an overlaid grid system, consisting of blue lines, numbers and letters. For normal map reading functions these details are unnecessary, but are vitally important if you need to relay your position to a rescue authority and you don’t have GPS coordinates.
We’ve covered the use of the topographic universal grid reference at the end of this article.
Most people have some map-reading skills, developed by looking at metropolitan street directories. Reading topo maps is no more difficult.
As with nearly all printed maps topos are oriented towards true north, so start navigating by rotating the map so that the road or track you’re driving on lines up with the direction of travel. It’s the same if you’re using GPS mapping: set the unit to show ‘track up’ rather than ‘north up’. It’s much easier to relate map details to what you’re seeing from the vehicle if the map track is aligned with the vehicle track.
Map scales vary from highly detailed 1:10,000 to less detailed 1:5,000,000, but the most popular topo maps for motoring use are 1:250,000, where 1cm represents 2.5km. It’s sometimes necessary to us a variety of map scale sizes on a trip, with less detail needed for easy sections and more detail for tricky ones.
An example is where you’re driving along a desert track, with your GPS cursor showing your vehicle on a topo map. The screen image will restrict your ‘ big picture’ view to only a few kilometres at best, so you’ll need a smaller scale paper map to give you a wider perspective of where you’re headed in respect of where you want to go.
It’s important that you pencil in your position on the larger map at intervals and particularly after intersections, so that if your GPS ‘crashes’ you know your approximate position on the map. Marking your position is simply a matter of transferring the latitude and longitude readings from GPS onto topo map, which has ‘lat’ and ‘long’ marked in black numbers and graduations along the map margins. The blue numbers and lines are not relevant for this position transfer exercise. Note that latitude increases from the north of the map to the south – we’re in the southern hemisphere, remember – but longitude increases from left to right (W to E), as you’d expect, since Greenwich in the UK is to Australia’s west.
Be aware that the GPS mapping data – normally World Geocentric System 1984 (WGS84) - may differ from the paper topo one. Typically, pre-2000 topo mapping uses Australian Geodetic Datum 1966 (AGD66) coordinates that vary by around 100 to 200 metres from more recent GDA94 figures. Recently printed topo maps have the conversion figures for lat and long adjustment to WGS figures.
If you install digital topo mapping in your GPS, make sure the mapping data match up with the GPS settings, but even then, there’s variable sensitivity from the GPS system, so don’t expect pinpoint accuracy.
If it’s important for you to measure actual distances on your topo map a map card is handy. This transparent card has measurements in kilometres marked on each edge, coloured to suit different topo map scales: blue for 1:250,000; black for 1:100,000; red for 1:50,000 and green for 1:25,000.
A more accurate way of measuring distance on a topo map is by using a digital map measuring meter. This device has a small wheel at its base and when this is run along a track line on the map, between two points, the wheel revolutions appear as a millimetre total on a small screen. An inbuilt calculator converts the millimetre figure to kilometres when multiplied by the map scale.
Be aware that topo mapping on a GPS unit is different from the mapping supplied with most GPS units. With conventional GPS mapping you get more detail as you zoom in and roads that were previously hidden emerge as the displayed area becomes smaller. What you see on the topo screen is what you get and magnifying it won’t improve detail, but just reduce the map image area. The plus for digital topo mapping is that many tracks shown don’t even appear on conventional mapping.
Using Topographic Map Grid Coordinates
When we were involved in a remote area Flying Doctor rescue a few years ago, we used a satellite phone to call Alice Springs – 500 kilometres away - and supplied our GPS coordinates. Some hours later the chopper arrived and whisked our seriously injured colleague off to hospital.
What would we have done if our GPS unit wasn’t operating and we couldn’t supply a satellite ‘fix’ to the Flying Doctor?
We’d have used the universal grid reference method of positioning that’s noted on every Natmap topo. For this task the blue letters, grid lines and margin numbers are critical. Also important is being able to pinpoint your position on a topo map and if you’ve been pencilling in your positions regularly as you travel that should be easy enough.
Step One is to write down the topo map reference number, say SF52.9 Wilson.
Step Two is to write down the two letters of the grid in which you’re positioned, say BV.
Step Three is to run your finger along the bottom margin of the map, until it lines up vertically with your actual position. Write down the blue number that’s to the left of your position and then estimate the tenths from that number to your position: if the large number is ‘7’ and you reckon about 9/10ths to your position (about 1/10th to the left of the ‘8’) then your next reference number is 9.
Step Four is to run your finger up the side margin of the map until it reaches a grid line point just below your position. Write down the blue number at that grid line, say ‘8’. Lastly, estimate the tenths from the grid line up to the position, say ‘1’.
Your position, using the grid reference system, is now SF52.9 Wilson BV7981 and rescue organisations can find you using that information.
Don’t worry that you’ll forget the steps, because there’s a sample printed on every Natmap topo to refresh your memory.
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