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Site last updated by RMH
14 Aug 2015, 13:50

Maps and measuring distances
A succesful record claim is far more likely if you have a suitable flight recorder on board your microlight or paramotor during a record attempt. None the less, it is still possible to make a succesful claim using the 'traditional' methods of barograph, turnpoint photos and map but there are several things to watch for when you are using a map to measure distances.

For most microlight or paramotor records the 1/4 million aviation map is simply not detailed enough to substantiate a claim so you will have to use the 1:50,000 ordnance survey map. Annotated copies of these maps should form the core evidence of your claim.

Any distances which constitute part of a record claim cannot be simply measured off a map, (or read off a GPS), they must be calculated in a specific way. To do this you will need to find the exact latitude and longitude of every significant point on your route (start, turnpoints, finish) and enter them in the distance calculator to find the total distance.

It is not so easy as it might at first seem to extract accurate WGS84 latitudes and longitudes from OS maps.

The UK National Grid
The Ordnance Survey of Great Britain is a Transverse Mercator projection. This is a particularly ideal choice for a tall thin area like Britain in that distances, areas and angles are well mapped with minimal error.

The OS projection is based on a true origin at 49 degrees North, 2 degrees West which is near the Channel Islands.

From this origin, a square grid is imposed on top of the ‘converging’ lines of longitude and the ‘curved’ lines of latitude. Points are located by measuring distances in metres east and north from this true origin. Since this process will introduce distortion as you move away from the central longitude of 2 degrees west, a scale factor of 0.9996012717 is introduced which reduces this distortion - two lines of longitude with zero distortion are thus created. Furthermore, to eliminate the inconvenience of negative values of easting, the concept of a false origin is introduced. A value of 400000 (400 Km) is added to eastings and 100000 (100 Km) subtracted from northings so placing this false origin near the Isles of Scilly. Irish OS maps (which includes Northern Ireland) are based upon a grid with a different false origin off Co. Kerry.

For more info, see Phil Brady's OSGB FAQ or visit the Ordnance Survey.

What does this mean?
OS maps all have a latitude and longitude key in the border, but the lines of latitude and longitude across the map are NOT straight and do NOT follow the grid lines in either axis and what's more, even if you look very carefully at your 1:50,000 OS map and find the small blue + at every 5 minute intersection of lines of latitude and longitude to get a reasonably accurate position, the one you find is not a correct one relative to the WGS84 datum anyway (it is to the OSGB36 datum which is slightly different).

The only way to get WGS84 latitudes and longitudes is to extract OS grid positions from the map (an explanation of how to do this is on every OS map) and mathematically convert them. There are various utilities out there which will do this, for example Phil Brady's Excel spreadsheet does the trick nicely for British (not Irish) grid references.

When you have got your WGS84 positions, simply enter them into the distance calculator to find the "correct" distances between your turnpoints.

By rmh updated 3 Jan 2003
OS information courtesy of Phil Brady and the excellent gps.gov.uk