I've used a handheld GPS receiver for measuring distance. Hard to say how accurate it is, but with a good reception it must be within a few meters. The thing is, the device has only two digits. So it says 98 meter, or 99 meter, or 0.10 km or 0.11 km. So I have to set a mark at the 100 m range, and then measure the second step again in order to get a more accurate reading over hundred meters. It's not very accurate this way (two measurements equal two standard errors) but good enough for me. This methods works at any location, so also if there are no obvious markers in the field to use for google maps/earth.