Reading and plotting grid references
A grid reference indicates a location on a map in terms of numbered vertical and horizontal grid lines. Grid references are useful for sharing routes, and quickly communicating to emergency services. Bushwalkers often share grid references of campsites or camp caves, obstacles or challenges on a walk, where to get through a cliff line, where to cross a river or what route to take. Orienteering competitions also provide information on their flag locations using grid coordinates.
Reference systems Understanding reference systems used by bushwalkers
When sharing a grid reference, bushwalkers must be clear on the reference system being used, else the points will not necessarily match up. Maps that show the same terrain can be made using different datums and projections, meaning that the grids do not necessarily line up.
For example, compare the images of the Megalong Valley below, and notice that the location of all features relative to the grid lines is different. This is because the first map below uses an older datum than the one below that.
Any given reference system is based on a datum, a 3D representation of the earth, and datums differ in how they represent land and where the location of the central frame is. Two points can be out by as little as a few hundred metres, or as much as several kilometres if there is confusion over the reference system in use.
Maps usually have an information section that identifies the map datum and projection, along with the publisher and copyright information. When communicating a grid coordinate, state the coordinate system first, then the grid coordinates.
In NSW, the 1:25000 topographic maps use either the AMG or the MGA coordinate systems.
- AMG: The AMG is the Australian Map Grid 1966/1984 system and was used on the old series maps, mostly distributed circa the 1980s. The AMG reference system uses the Universal Transverse Mercator projection of the Australian Geodetic Datum 1966.
- MGA: The MGA is the Map Grid of Australia 1994, and is used in all the new series maps. As a general rule, all the new series maps have an aerial photo image on the reverse. The MGA reference system uses the Universal Transverse Mercator projection of the Geocentric Datum of Australia 1994 (GDA94).
The AMG system was used until roughly the mid-90s when it was replaced with the MGA system, which is more compatible worldwide as the GDA 94 datum is almost identical to the WGS84 datum used in GPS (Global Positioning Systems). The result of the change from the AMG system to the MGA system is a shift of approximately 200 metres in a northeasterly direction.
While being off by 200 metres doesn’t sound like much, in the bush this can be disastrous for an emergency operation, or cost a bushwalking party serious time delay. That’s why it’s important to communicate which reference system is being used and to know how to convert between them. In an emergency, it may be possible to get enough mobile phone coverage to contact the emergency services and communicate the location of rescue.
Converting between AGM and MGA
The shift between AGM and MGA is approximately 200m in a northeasterly direction. The images below give an example of the shift relative to the UTM Northing and Easting grid lines.
To convert between grid points, users add 100 m to the Eastings, and 200 m to the Northings. As a practical example, for a 6-digit grid reference, add 1 to the Eastings (the first three digits), and 2 to the Northings (the second three digits). So the junction of the two roads Hampton-AMG436645 becomes Hampton-MGA437647. The examples below have more information on reading grid references.
Since all objects on a map remain the same relative to each other between the two systems, bearings do not change.
Reading a grid reference from a map Learning how to read a grid reference from a map
Maps have numbered vertical and horizontal grid lines that enable the reader to identify and communicate a particular location on the map. The vertical lines are aligned with grid north, and the horizontal ones are exactly perpendicular to the vertical lines.
The Universal Transverse Mercator (UTM) system, which describes a point in terms of metres, is most commonly used by modern topographic maps. And this is the system described here.
In the UTM system, the vertical lines are called Eastings (i.e. they help locate how far east the point is), and the horizontal lines are called Northings (i.e. they establish how far north the point is). Most maps also include the Longitude and Latitude of the map at the corners (i.e. a reading in degrees, minutes and seconds).
A UTM grid reference tells the reader how East and how far North to go on the map. It consists of two parts – the Easting digits, and the Northing digits – and always has an even number of digits. Determine the number of Eastings and Northings digits by dividing the whole grid coordinate by 2. The more digits, the more accurate the grid reference.
When reading a grid reference, Eastings always come first, followed by the Northings. A simple way to remember this is the saying ‘cross the creek before going up the tree’, that is, go horizontal first, then vertical.
Example 1: Describing the location of the hill labelled 619. The 619 refers to the height of the hill in metres. Often, bushwalkers describe a hill by its height, unless another name is written on the map (typical for popular and distinctive hills). In this example, since the hill has no name it can be called Hill 619.
On the 1:25,000 map above, each square is 1km wide and high. The Northing and Easting lines are all numbered. Ignoring the small numbers either side of the black numbers (explained later), notice that the lines increase by one unit along the bottom (68, 69, 70, 71…) and up the left-hand side of the map (05, 06, 07, 08…).
A four digit grid reference (2 digits for the Easting, and 2 for the Northing) is a crude measure of the location with an accuracy of one square kilometre. It describes the intersection of two lines. The closest intersection to Hill 619 is the intersections of the Easting line 69 with the Northing line 06. Hence Hill 619 has a four digit coordinate of 6906 plotted as a white circle below.
However, since the margin of error in a four digit this grid coordinate is 1km square (outlined in orange), this reference could describe any of the features from the hill in question, to a gully or a creek junction. To more accurately describe Hill 619, it’s best to use a six-digit grid reference.
Draw perpendicular lines from Hill 619 to intersect with the Easting and Northing lines. Then measure how far along the grid the lines are. For the Easting, the line crosses 6/10th of the way along the grid. Hence the Easting coordinate is 686. For the Northing, the line intersects 1/10th of the way up. Hence the Northing coordinate is 061. The Hill has a six-digit grid reference of 686061. Since the map is an old map (‘1:25,000 Colo Heights’) and uses the AMG reference system, the six-digit grid reference is ‘Colo Heights AMG-686061’.
A six figure coordinate has six figure grid reference an accuracy of 100 m2, indicated in the orange square box. This area of error is much smaller than the search area provided by a four-digit grid reference.
Example 2: Bonnum Pic.
Care must be taken to quote all the zeros in the right place. In this example, the Northings have a zero at the front. The 6 figure grid reference is therefore 477060. Since the map is a new edition map and uses the MGA reference system, the six-digit grid reference is ‘Hilltop MGA-477060’.
Example 3: Hill 2122. The hill lies on a grid line meaning that for a six-digit grid reference, the Easting has to include a zero at the end. Hence the six-digit grid reference of Hill 2122 is 010977. Since the map is an old map (‘1:31680 Caoura’) and uses the AMG reference system, the six-digit grid reference is ‘Caoura AMG-010977’.
Example 4: Hill 731.
An 8-digit grid reference is even more accurate than a 6-digit one and has an accuracy of 10 m2. Supplying accuracy to the nearest 10 metres involves splitting up the grids into 100-tick increments.
Hill 731 has an eight-digit grid reference of 49029349. Since the map uses the MGA reference system, the eight-digit grid reference is ‘Hilltop MGA-49029349’.
UTM zones: Identifying the region
UTM divides the earth into 60 zones each with 6 degrees of longitude, and 20 designators each with 8 degrees longitude.
Australia falls between zones 50-56, and Sydney is in 56H.
A 6-digit grid coordinate system works well among bushwalkers to communicate points of interest and routes, and Bushwalkers use a shortcut to identify the region in question. They refer to the name of the map in question, a practical and quick solution for sharing information.
However, on a global scale, there would be many identically-numbered grid locations unless the specific UTM zone is also reported, along with a context for the Easting and Northing lines.
In the corner of all maps, the Eastings and Northings are given additional small numbers before the main ones that identify the lines.
While the small numbers aren’t usually quoted by bushwalkers for six- or eight-digit grid references, they will appear on a GPS reading. GPS Eastings and Northings always include an accuracy down to the nearest metre. Hence for every square kilometre, the accuracy has to be to 3 digits for Eastings and Northings, something that is hard to do when reading off a map, but if given these grid coordinates, it’s possible to plot accurately.
Hence, the full coordinates of Hill 731 are Zone 56H, 0269210mE, 6316890mN. This is the same eight-digit grid calculated in example 4 above in orange.
Plotting a grid reference on a map Learning how to plot a grid reference on a map
Plotting a grid reference on a map is the reverse process of reading it on the map.
Example 1: Plotting the grid reference ‘Caoura AMG-041948’, the junction of a creek with a major river.
The reference 041948 is a six-digit grid reference with a 041 Easting and 948 Northing. Find the 04 Easting line and follow it 1/10th of the way further east. Then find the 94 Northing line, and follow it 8/10th of the way further north. The grid point is the junction of Paradise Creek with the ShoalHaven.
Example 2: Plotting the grid reference for Mt Wangandarry ‘Hilltop MGA-48259765’, a trig point.
The reference 48259765 is an eight-digit grid reference with a 4825 Easting and 9765 Northing. Find the 48 Easting line and follow it 25/100th of the way further east. Then find the 97 Northing line, and follow it 65/100th of the way further north. The grid point is the top of a knoll on a large flat hill top.
Coordinate plotting tools can aid plotting grid points on maps. Grid tools enable the user to keep an exact right-angle position as they find the coordinates they’re looking for, and the user does not need to draw lines on the maps.
A UTM plotting grid is one option. Alternatively, a corner style tool or simple ruler. Ensure that any map plotting tool has the right scale (note that American sellers stock 1:24,000 tools, not to be confused with the more common 1:25,000 series used in Australia).
Using a GPS to identify and plot locations How to use a GPS to identify and plot your location on a map
GPS units are an excellent tool for identifying and plotting locations along a bushwalk. GPS units can also be loaded with a basic topographic map and route plan at home, and users can follow the route in the field. Alternatively, users can identify specific locations along the walk and translate them back onto a topographic map to double check their location and accuracy of navigation.
GPS units act as an additional navigation tool to traditional map and compass. They typically give accuracy up to 20 m but rely on good satellite coverage. GPS units fail in regions where the sky is partially or wholly obstructed (caves, cliff lines, canyons).
Before use set the GPS unit to the correct Geographic Coordinate System, based on the type of maps being used. The most common system that bushwalkers use is the UTM/UPS system (not latitude/longitude). Then select the appropriate datum: for new series maps use WGS 84 (or GDA 94 as they are essentially the same); for old series maps use AGD66. If unsure, read the fine print in the publication details of the map. All datum and reference modifications on a GPS can be made in the ‘settings’ section.
Reading a grid reference on a GPS
A GPS grid reference looks something like this:
It includes information on the Geographic Coordinate System used (UTM), the location zone (56H), the Eastings (0706832) and the Northings (4344683). Eastings are always given first.
The ‘706’ part of the Easting information refers to a major Easting line, and the ‘832’ gives the location down to the nearest metre. The ‘4344’ part of the Northing information refers to a major Northing line, and the 683 gives the location down to the nearest metre in the grid.
Translating a GPS reading onto a map
Example: Finding the GPS coordinate Zone 56H, 0304920mE, 0695440mN.
The map below is in Zone 56H. Major Easting lines are 04, 05, 06, etc. and all have a ‘3’ at the start. Major Northing lines have 95, 96, 97, etc. Eastings have a ‘3’ prefix shown on the map, which is ‘03’ in the GPS coordinates. Similarly, the Northing lines have a ‘6’ prefix shown on the map, which is ‘06’ in GPS coordinates.
Hence, the 8-digit grid reference is 04929544. Follow the Eastings along to the 04 line, and then 92/100th further east. Follow the Northings to the 95 line and 44/100th further north.
The location is Mount Woolnough.