Using a GPS


SEWBReC now has a Garmin eTrex H which it is making available for its data suppliers to use.  We can also provide a data-cable for use with GPS mapping programmes, such as Easy GPS.

GPS receivers can make defining the grid references for your records much easier, but there are a few things to remember when using them:

Modern receivers usually give 10-figure grid references, which imply your location can be pinpointed to a one metre square. However your receiver should also provide details of its positional accuracy (generally 3-5m with a WAAS / EGNOS enabled receiver). It is therefore important to reduce the resolution of the grid reference accordingly. If your unit says positional accuracy is:

  • ≤ 1m (highly unlikely until Galileo is fully operational) - then you can cite the full 10-figure grid reference;
  • 1m > 10m (the most likely scenario) - then you should cite an 8-figure grid reference;
  • 10m > 100m (possible in bad conditions) - then you should cite a 6-figure grid reference.

Remember, however, that when rounding off readings you should always round down, not up.
E.g. the ten figure grid reference SS 98765 98765 should be rounded down to SS 9876 9876; rounding up to SS 9877 9877 will put the record into the next 10m square.


Garmin eTrex H

GPS receivers can consume batteries fairly quickly, and so it may seem a good idea to switch them off between readings. However, older GPS receivers can take several minutes to re-connect with their satellites, so it may be a while before you get an accurate reading and you may find it easier just to leave the unit switched on.

Accuracy is also reduced when under cover (such as woodland) or in valleys, as the receiver cannot lock onto the full suite of satellites. This situation can be improved by keeping the unit switched on, holding it parallel to the ground (face upwards), and getting a clear view of the sky to the south.


The Ordnance Survey website offers some useful information regarding GPS.