Using Topographic Maps II
- Published: Sunday, 28 December 2014 11:28
- Written by Lance Hartley
- 1 - 2 crew
- 1:25000 topographic map
- Bega Map
- Magnetic compass
A Quick Guide to Using UTM Coordinates
Standing at the junction of the two fire trails, marked with the star on the topographic map pictured below, a GPS unit set to display position in UTM coordinates, would report a location of:
The 55 H represents the zone you are in. The zone is necessary to make the coordinates unique over the entire globe.
The top set of numbers, 0565175, represent a measurement of East-West position, within the zone, in metres. It's called an easting.
The bottom set of numbers,5931290, represent a measurement of North-South position, within the zone, in metres. It's called a northing
The map has Universal Transverse Mercator (UTM) grid lines spaced every kilometre or 1000 metres. The vertical grid lines determine East-West position and the horizontal grid lines determine North-South position.
The UTM system can be useful on medium and large scale maps because the coordinates are metre based and a grid overlay is square and regular – commonly 1 kilometre. If a map is marked with a 1 kilometre grid it may be the practice in a radio net to give local locations as a 6 digit grid reference. The appropriate numerals are underlined in the diagram below – 565 312.
Shorthand for UTM Coordinates
Most land navigation activities focus on a very small portion of the globe at any one time. Typically the area of interest to a fire fighter for example, is less than 20 km on a side. This focus on a small area allows us to abbreviate UTM coordinates.
The zone information and the digits representing 1,000,000 m, and 100,000 m are dropped. The 1m, 10m and 100m digits are used only to the extent of accuracy desired.
A GPS unit might read
Using a notation similar to the one found on a topographic map, this would be written as: Zone 55 H 0565175 mE. 0531290 mN.
An abbreviated format for the same coordinates would look like:
Remember, the 100m abbreviated format, 651 312, and the 10m abbreviated format, 6517 3129, are the most commonly used. Notice that the easting is reported first, followed by the northing. Remember the phrase "read right up" to help you remember to read the easting from left to right, followed by the northing from the bottom up.
Also notice that when you abbreviate coordinates you should not do any rounding. 0565651 becomes 656 not 657. This ensures that your position is still within the reported square. As accuracy decreases, the square gets bigger.
Using a UTM grid overlay tool (Roma)
If you want to find your location with more precision than is available from the grid lines on the map, you will need a tool that is marked in finer divisions. One such tool is a grid overlay. The grid overlay is placed on the map with its edge aligned with the grid lines. Then the position of the mark can be determined using the tool's additional precision.
Additional precision is available either by "eyeballing" or by using a UTM Corner Ruler with finer markings. You will find one of these useful tools in your Firefighters Pocketbook.
For many land navigation situations 100m precision is quite adequate. It also fits well with the 100 m accuracy of civilian GPS units. The example shown below locates the Red Star Symbol to a precision of 100m. The 10,000m and 1,000m digits of the coordinate are taken from the map. Thus the coordinates 65 31 locate the 1,000 metre square containing the star. The grid overlay is placed over the grid and the 100 m digit is determined. Remember to read the Easting followed by the Northing.