Survey with laser instruments

[Andy Waddington]

Sometime before sending, Graham Mullan typed (and on Wednesday 2008-07-16 sent):

> I have no idea what sort of instrument you used for your surface 
> survey in Austria but I don't quite understand what you mean by the 
> "angular accuracy being very small". Can you re-phrase?

Not our surface survey, but the Austrians'. They used laser theodolites to
measure distance and relative angles to fixed points in the National mapping
system, and thus produce a series of fixed reference points in our area.
Since some of the leg lengths involved were of several kilometres, I am
assuming that the angular resolution - both precision and accuracy - is
very small, since these are supposed to be accurate fixes, and a tiny angular
error over a multi-km leg will produce a significant error in space. (0.1 deg
error over 6 km is still 10m on the ground, and I was given to understand that
the error bounds on the fixes was more in the order of centimetres, which
implies milli-degree accuracy).

Obviously, since they were using theodolites to measure relative angles to
two (or more) known points, we don't have to worry about all the usual sources
of error for magnetic surveys (solar flares and so on). The one blunder that
we spotted was something like a ten degree error in one of the measured
angles ...

Andy