I set up two tripods, with a tribrach on one and the Sokkia PL1 level on the other and measured the relative height difference between the Sokkia AP01 prism holders. The prism holder connect two ways, one via a V groove and shoulder into a tribrach carrier, the other via a 5/8-11" UNC thread.
So it appears I can significantly improve the vertical centring error by selecting the most accurate holders for the traverse kits, and shimming or stoning the remaining for pole mounting.
So it appears I can reduce my prism vertical centring error within 0.1mm or +-0.05mm, instead of +-0.5mm by calibrating my prism holders.
Photographs of level calibration of prism holder height. Note I've used one tribrach, one Topcon carrier (with thread attachment) and Sokkia AP41 carrier with v-grove and shoulder attachment.
Factors affecting instrument height v's levels; tape measures in windy conditions, prism pole points pentrating marks, worn points, different tools used to measure instrument height.
Level differences are included in least squares, instrument heights errors aren't.
Maybe a rod tip design that consists of a flat, used as a height reference datum, with a spring loaded tip that drops / retracts to align to the centre of the mark, the weight of the prism pole suffienient to cause the tip to retract. It would likely fill with compacted dirt and become stuck.
If I can use fixed height rods over control points, it minimises transcription of height errors, then level resected instrument positions, it saves the hassle of measuring instrument height and marking positions, it's much faster levelling from a tape (or small wood measuring staff) attached to a prism target board.
Cheers,
Peter.
@ Norman_Oklahoma
For the record, I’ve long felt that the common box tape is way too floppy to accurately measure the instrument height. Especially in any kind of breeze. For that reason I’ve long used a folding wooden ruler, which is much stiffer.
That's funny because I think it's just the opposite. I routinely subtract 0.01' from all height measurements.
What first order level are you using to measures reliably down to 0.02mm?
I too will cut a hundreth.
@mulambda382 "What first order level are you using to measures reliably down to 0.02mm?"
Sokkia PL1
A quick update, I managed to organise the prism holders and carriers, so I had a 0.16mm range in height for the traverse kits. For the prism poles I filed and stoned the high prism holders (two were +0.45mm) so they were all within a range of 0.2mm. The remaining three prism holders were 0.2 to 0.25mm too short, these I placed on our magnet mounts that we attach to structures.
I measured the SX12 instrument height using the level it is 196mm off the tribrach, I don't know where I got it into my head that it was 195.5mm. The Sokkia APS12 traverse kits are adjustable and set to 195.5mm off the tribrach.
I also checked prism offsets, I had a total variance of 0.7mm for the holders I'd selected for the traverse kits and 1.13mm for the holders on prism poles, I might look into improving variance by shimming the holders so they have identical offsets. I have been using prism centring accuracy of 0.5mm in my LSA, but it appears these kits are slightly worse than that.
I checked the pole tip for penetration into a Myzox No.4 Survey Nail, it was 2.8mm.
I also cut slots in some of the prism target boards from the traverse kits and filed them until they were the same height as the prism centre (as viewed from the level). So now I can hang tapes from the prism boards and measure differential levels from resected positions by forced centring. Maybe we'll get second or third order level accuracy, it will tighten up my verticals, while eliminating the need to mark my scan positions on the ground, measure instrument height and perform a level traverse after. This actually speeds up my scanning workflow significantly and I can measure levels while the GLS is scanning.
For control I'll use prism poles with the prism placed close to the ground, check tip penetration and subtract it from rod height. Maybe an easy way to mark the rod tip penetration is to sit it in position and give it a light spray with marker pain.
We had a job last year where the engineers wanted +-2mm accuracy (std dev), it wasn't achievable, so we quoted as accurate as practically achievable. We did get parts of our control network down as low as 3mm @ 95% confidence in 3D, but we also had some higher than that as well, our biggest errors were all vertical, which I put down to small differences in occupation height measurements.
We use the GLS to check the accuracy of point clouds of other scanners used or as a scaffold for registering to.
It's difficult to estimate point cloud accuracy, but if we have prisms attached to the structures we're scanning and we include the scanner's observations in LSA, it goes a long way to providing a good final estimate.
