estimating gap errors

[WarrenZ]

I have two points in a survey and I want to know the gap between them 
*including an estimate of the error*.  Obviously the gap is no problem 
with aven but has anyone had a look at estimating the error?  I've 
managed to get something to work by adding in a stretchy leg of length 
zero, re-solving the network, and measuring how the gap changes, but it 
is a bit crude (see below).

Patrick

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Error estimation algorithm using survex

1. Measure H_old and V_old between the survey stations of interest (say, 
station_1 and station_2), using aven.

2. Add the following to the survex file
*begin
*sd dx dy <insert length> metres ; the length here is deltaH, chosen to 
match an estimate of what the error is likely to be
*sd dz <insert length> metres ; the length here is deltaV, similarly 
chosen to match what the error is likely to be
*data cartesian from to northing easting altitude
station_1 station_2 0 0 0
*end

3. Re-solve the survey and measure H_new and V_new.

4. Then sdH = deltaH*sqrt(H_old/H_new-1) is (an estimate of) the 
standard deviation for the horizontal gap, and sdV = 
deltaV*sqrt(V_old/V_new-1) is likewise for the vertical gap.  (I know 
these two numbers aren't the full story - one would in general expect a 
variance-covariance matrix - and of course it all assumes there are no 
gross errors!).
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