Another Leica Instrument oddity 1

[alibye]

A colleague of mine has been setting out using a Leica TC407. As an experiment they set out a point using the usual Leica 'setting out' program in the instrument using D-Hz and D-dist. Following this they scrolled down the instrument screen to the D-Length D-offset screen and put two points in 1.2m either side of the target point.

When they pulled a string line between the two offset points the target point was exactly half way along the line; but the target point was around 300mm CLOSER to the instrument than the string line. (exactly the opposite to what i would have expected if this was an error due to working with arcs).

They tried again with a second point and had the same result, both points were around 30m from the instrument. Does anyone have any experience of this, as i'm baffled

 

[msucog]

is it offsetting tangent to the arc from the points 1.5m either side of the main point (instead of offsetting tangent to the arc at the main point with a 1.5m offset to each side)? i can't think of any other reason...hmmmm.

 

[fldengnr]

Did you check to see that the prism constant in the instrument is correct for the type of prism being used. Leica uses 3 or 4 types of prisms each with a diffrent setting or constant.Also if you regularly use a third party data recorder and a non leica prism then the settings are diffrent and tricky to calculate.

 

[msucog]

wouldn't the prism constant throw all the points out (assumning all the points set at the same time)?

 

[fldengnr]

It should be constant no matter what the prism is set at,but all of your measurements will also be off . I use a TCR803 but have never used anything that was onboard. I have seen Leica instruments do crazy stuff were you staking points on a tangent line or attempting to stake a curve. I will check this out with my instrument tomorrow. I am pretty sure it is a setting somewhere

 

[alibye]

Sorry it's taken so long to pick this up again. Both myself and my colleague have been using 407's for years and are familiar with the problems of prism constants.
The 'target' pint was set out pretty traditionally, turning to the correct bearing 0deg in 'setting out' mode') and shooting the correct distance. Hence if it was a prism error, being constant throughout, the three points should have tied in. It's just an oddity. If no-one can figure it out, it's not critical, just be warned that it can crop up.

Cheers for the help

Al