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. The result of the change from the AMG system to the MGA system is a shift of approximately 200 metres in a northeasterly direction.

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 MGA reference system uses the Universal Transverse Mercator projection of the Geocentric Datum of Australia 1994 (GDA94). As a general rule, all the new series maps have an aerial photo image on the reverse. MGA: The MGA is the Map Grid of Australia 1994, and is used in all the new series maps.The AMG reference system uses the Universal Transverse Mercator projection of the Australian Geodetic Datum 1966. AMG: The AMG is the Australian Map Grid 1966/1984 system and was used on the old series maps, mostly distributed circa the 1980s.In NSW, the 1:25000 topographic maps use either the AMG or the MGA coordinate systems. When communicating a grid coordinate, state the coordinate system first, then the grid coordinates. Maps usually have an information section that identifies the map datum and projection, along with the publisher and copyright information. 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. 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. The six-figure grid reference for the church is 817098. In the diagram above the four-figure grid reference for the square marked X is 8109. In this instance, the tourist information office is located at 476334. You now have a six figure grid reference.Write this number after the last two digits.

Next, estimate how many tenths up the grid square your symbol lies.Write this number after the first two digits. Estimate or measure how many tenths across the grid square your symbol lies.First, find the four-figure grid reference but leave a space after the first two digits.The distance between one grid line and the next is divided into tenths. In this case you can imagine that each grid is divided into 100 tiny squares. Sometimes it is necessary to be even more accurate.

Grid coordinates map