The OSTN02 extension for ArcPad is a FREE tool that automatically transforms GPS data and aligns it with Ordnance Survey (OS) background map. This easy-to-use extension removes the need to manually post-process GPS data by providing’ on the fly’ transformation of WGS84 latitude and longitude into OSTN02 corrected British National Grid coordinate. Data capture using GPS and ArcPad is usually performed using the WGS84 projection. This works for simple data collection, where the accuracy of the captured data is dependent on the GPS device, but problems can arise when trying to overlay the captured GPS coordinates with Ordnance Survey data. To align the WGS84 data from the GPS with the underlying OS map requires geographic transformation of the GPS data. The OSTN02 extension for ArcPad allows GPS coordinates to be received from a GPS device, and immediately transformed before being displayed or captured within ArcPad. This allows the GPS position to be immediately overlaid onto any OS base mapping, loaded into the ArcPad application. The coordinates will have taken into account the OSTN02 correction, and hence be properly positioned on the map.