Time has come to assign my least accurate survey data a lower accuracy! I'm curious as to how Therion deals with the std deviations. Had a look, but can not find much more than is in the Therion book. First, what are the default values adopted by Therion? Currently I have only one survey in a large(ish) dataset that has been allocated sd's. From a casual look at the distortions it clearly works - in that a nice result is obtained with more distortion in the lower accuracy area. I guess that if the default values are zero, then all the distortion would be forced to an area given non-zero sd's; Is this how it works? In this case it would seem that if one survey parameter has a sd setting, all the surveys should all have their respective settings allocated for this parameter to more accurately model a realistic distribution of corrections. Second, has anyone made any attempt to associate sd values with recognised cave survey grades (bcra grades for example). Here is my off the cuff assessment of what in NZ some people call bcra5 but is really well short. grade NZBCRA4 # Bruces assessment of the BEST typical NZ surveying practice MIGHT achieve tape length 0.1 meter # ie -0.2m to +0.2m for measurement to chest to 0.1m compass bearing 1 deg # ie -2 deg to +2 deg for sighting to light of person on station to 1 deg gradient clino 1 deg # ie -2 deg to +2 deg for sighting to light of person on station and sometimes accounting for 'eye to light' offset to 1 deg endgrade I suspect the sd's may often be larger than this. The actual sd for polar measurements probably depends a lot on the average length of the legs I expect Thanks Bruce