Version 1. - May 2006
Then Northing – do the same for the horizontal lines. Say the point is 7 tenths past the 22 line, which gives a northing of 227. The full six-figure grid reference is then 352 227. On a 1km grid this should be accurate to within 100 metres.
A more accurate eight figure grid reference can also be calculated, using a map aid called a graticule. This divides the grid squares into accurate tenths, within which you can estimate into quarters again (shown as 2, 5, 7 or 0). The above 6 figure GR 352 227 might then be more precisely estimated as 3522 2270.
Grid north is slightly different from true north, as it is difficult on a flat map to accurately represent the curved surface of the earth. The difference is usually disregarded as it is quite small.
Latitude and longitude
Latitude and longitude are divisions of the earth's surface, taking the full circle of the earth as 360 . Latitude lines are horizontal and parallel on the map, and latitude is measured north or south from the equator, which is taken as 0 . Longitude lines converge at the north and south poles like slices of an orange, however on most topographic maps the amount of convergence is very small. Longitude is measured moving east or west away from Greenwich (near London), which is assumed to be 0 . So the direction must always be shown with latitude (ie, N or S) and longitude (E or W). In Australia, you measure down first (away from the equator), then across from left to right (away from Greenwich). Eg, Mildura aerodrome is at Latitude S 34 14’, Longitude E 142 05’. Note this is different from grid references, which are measured across, then up.
On aviation charts, latitude and longitude lines are printed as the main positional reference. However on topographic maps the latitude and longitude are simply indicated at the corners of the map, and at intervals along the edges (and sometimes with small crosses in the body of the map). Note that they are NOT parallel with the map grid. So to estimate latitude on your flying map, you must look at both the left and right edges of the map, and allow for any slight difference between them. Then do the same for longitude using the top and bottom edges of the map. You must be able to calculate the latitude and longitude of a location on a topographic map to the nearest minute (1/60 of a degree).
Vertical distances are expressed in different terms according to the context (in practice these terms are often used rather loosely):
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ABF Pilot Training Manual Part 8