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S8 BS/FS Distance differences of up to 5mm @100m when measuring to reflective tape or surface but not prism?

Question asked by Angus Keane on Oct 22, 2020

As the title says im getting significant differences at times with my BS/FS when measuring to an effectively perpendicular surface, and i cant figure out the cause. The key points here:


  • done a comp calib, hz, vert, and trunnion adjustments with very good residuals
  • when measuring to a perpendicular point even in survey basic, there is a slope distance difference between f1 and f2 of 3-7mm
  • these are good surfaces to measure off of  (reflective survey targets or at times a clean shaded slab)
  • have used dr and +0prism modes on reflective tape (no differences between these 2)
  • the edm constant is set to 0 (but shouldn't affect it?) 
  • when measuring to a prism (autolock or otherwise) there is negligable difference (0-1mm) 
  • resection has a sub 3" residuals on 3 control points. Resection error has been known to affect f1/f2 MTA calcs, but this shouldnt affect the SD on survey basic


I feel like ive tried everything to narrow down the cause and at this point it may be something simple, or very technical with the instrument. Im hoping to hear either!