Hi WIlliam, I have a small hand held device called an Alba Windwatch made by Silva for this job and calculating hydro heads. The specs claim an accuracy to +- 1.0m. It uses barometric pressure and so far I have found it pretty good. Only downside is it is not waterproof so falling in streams can be a problem!
I always check it out before using it by putting it on the ground, letting it stabilise and then making sure it registers a change as I raise it above waist height. One thing to be careful of is the pressure sensor is mounted behind the battery and the compartment is vented to allow exposure to the atmosphere so heat from the hand can upset the pressure reading. It needs to be held by the edges with the fingertips to be accurate. Bruce Geddes PowerOn A small place called New Zealand Accurate enough to size water pumping systems. I imagine 3 meters total, or +/- 1.5 meters. I don't need absolute measurements, just relative between two points. William
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