Hi folks, I guess this could just as easily be a Survex question as a Therion question. I have GPS readings taken on a pole, with the position being given for the top of the pole. Subsequent Disto readings are taken from the bottom of the pole. The pole is brought to vertical using a spirit level. I have checked the accuracy of the spirit as being 0.125°. I have the (measured) standard deviations of the GPS positions, so I can specify the standard deviations for the location fix. What I need to work out is how to specify the standard deviation for the pole. It's not a plumb measurement, because it is not purely gravitational. So although I could use "DOWN" that is not actually accurate, since it is really a nearly-vertical measurement depending on the accuracy of the spirit level. It's really a length and clinometer reading. #length of the pole accuracy measurements sd length 0.001 metres #accuracy of the spirit level sd clino 0.125 degrees But when working with a supposedly vertical position, even the slightest error in the clinometer angle of the pole could cause the compass bearing to flip to any angle at all. But this seems extreme: sd compass 360 degrees So what would be the right way to specify it? Cheers, Tarquin