Monitoring - Establishing Control

Posted by: @james-fleming

Great minds think alike.

Fools seldom differ

@james-fleming

Thanks for the recommendation! Looks pretty comprehensive but I'll get a copy.

@thebionicman

Thank you for your reply.

I appreciate that reporting displacement to sub-millimetre accuracy is unrealistic which is why we round to the nearest millimetre, which seems to be standard practice from what I have experienced/seen.

We operate under the precursor of our results being accurate to +/-1mm but present our results true to the observations. Eg, a point shows 1.6mm displacement in the eastings, we will show this as 2mm.

My question is, knowing we are operating with an accuracy of +/-1mm, would you ever allow for this within the reporting of the figures. E.g. if a point has moved (as mentioned above) by 1.6mm in the eastings, would you report this as 1mm to allow for the millimetre of accuracy tolerance? Or alternatively, would you simply report this in it's true numbers and show the displacement as 2mm, with the caveat stated on the document that all readings have an absolute accuracy of +/-1mm? We do the latter but I'd be interested to know what someone like yourself thinks, given your experience.

Posted by: @cmh89

Rounding introduces error. Not a very significant error, but that's what it is.

There is a small amount more information in 1.6 +/-1 than in 2 with a note. It is subtly different from 1.9+/- 1 or 2.2 +/-1. If your client can understand what that notation means, why not give them the best you know?

@cmh89

The manner of expression will be driven by the purpose of the monitoring. For simple jobs I would include a blanket statement of the standard statistical tests. Most would have a full report on the original and current network observations, with a spreadsheet summarizing each of the points being monitored. 

Video Published January 05, 2020, "CONTROL SURVEYING - MEETING POSITIONAL ACCURACY STANDARDS".

Video Description - MEETING POSITIONAL ACCURACY STANDARDS FOR CONSTRUCTION & MAPPING PROJECTS INVOLVING 3D LASER SCANNING, MOBILE LIDAR AND UAV MAPPING. COMBINING GNSS VECTORS, DIGITAL LEVELING OBSERVATIONS AND 3D TOTAL STATION MEASUREMENTS TO ESTABLISH SURVEY CONTROL POINTS & TARGETS.