In some countries Latitude (the angular distance above the equator) and the Longitude, (The Angular distance from Greenwich, London) is given. This is difficult to work with, and to calculate a distance between two locations for example.

In the UK we are fortunate to use the British National Grid. The Ordnance Survey have copyrighted a system of starting from an ?Origin? 130 km off Lands End, and locates any point as so many Kilometres EAST, and NORTH of that point. The OSGB system is popular and used by almost every outdoor activity enthusiast in the UK. All TrailWise locations are normally given in 'Landranger' style OS grid refs as follows...

Take Shakespeare?s Theatre in Stratford-on-Avon. It is 420,300m east and 254,700m north of the Map origin. As ?420,300 east, 254,700 north? is a confusing mouthful, each 100km Square is allocated two letters. See this gridsquares key, or look at the key on any OS Map for the 'Grid Square Identification'.

On the Landranger Map for that area the 100km x 100km Square, starting 400km east, and 200km north is 'SP' square. Therefore the reference is shortened to SP 203 547. (The 4xxx and the 2xxx are replaced in effect by SP.)

How do I know it is 4203 east? Looking at the Landranger map 151 in the four corners of the map the ?4? is shown in very small print, followed by the km squares. Count the squares along the bottom (or top) and then estimates the tenths of the distance to the next km line. The lines (spaced at 2 cm) are at 1 km intervals (hence a scale of 1:50,000). So each tenth is 100metres. It is 20km and 3/10ths. So it is in SP 203 as the easting, and likewise 54 and 7/10ths north.

This gives the area of 100m x 100m that the exact point of interest is in. Always round down. Always take the eastings first. If you remember to do this it is simple. (You go along the passage before going up the stairs!)

For most TrailWise purposes, 100m accuracy is generally good enough (six figures following the two letters). However this can look a bit misleading when the info is presented graphically. When possible, use 10m figures (eight figure) - but this will not always be practical or reliable to read off a Landranger map.

To give a map reference closer than 100m; in this case divide the 1km square into hundredths. Although a scaling device, such as a 'Romer' will assist to read paper maps, the printing process and the width of roads is a limiting factor here. It is better to go to the Explorer map at 1:25,000 or an even larger scale of map.

GPS, Navigation and Mapping software

Most GPS and software applications work 'natively' in lat/long coordinates, presenting apparent difficulties working with TrailWise information. Fortunately, many can be set to work using the 'OSGB' coordinate system.

Some mapping software will return very accurate looking, multi digit grid refs from a click on an electronic map. Beware that the underlying map may not be that accurate itself! For TrailWise purposes, accuracy to 10m is generally good enough, so round down e.g. to SU 1234 5678 form.

This also applies to GPS units, which rarely approach 1m accuracy. Round figures given from GPS down to 10m.

For further background on OSGB refs, and lat/long conversions, see the independent webpage http://www.bangor.ac.uk/is/iss025/osgbfaq.htm