According to a photo released by Britain’s Ministry of Defense, the British Royal Navy used a Garmin eTrex Legend to confirm the location of the ship where 15 British troops were taken by Iranian forces. The above photo is taken from inside a Royal Navy chopper, hovering over the scene of the incident. The latitude and longitude coordinates can be seen on the eTrex’s display, confirming the exact location of the ship – a matter of some dispute by the Iranians.
You would think they could have used a Garmin 60CSx with the SiRF High-sensitivity GPS receiver instead of the cheaper eTrex Legend, you can tell the SNR is not very strong on the display.
Now we know why the British are in trouble.
When I complained to Garmin that their world map disk data was off by 0.5-1.0 km, they said the data was accurate and that the GPS must not have reported the position accurately. How can a GPS regularly measure position errors of 0.5 km? I have found this error to be frequently true in Asia and the South Pacific.
So if Garmin is correct, and I don't believe they are, you (or the British) could be off by up to 1 km.