I often receive inquiries from customers who are alarmed that their coordinates in the survey database do not match their coordinates in their data collector, or that the survey database points do not match the drawing points. If I had to assign a frequency to these cases, I would say that nine times out of ten the issue is US Survey feet vs. International feet. This is a common mistake since the survey database in Civil 3D defaults to International feet and most users set up their coordinate systems in US Survey feet. It’s a simple fix, and users are working with matching coordinates in no time.
Last week, however was the one time in ten that this wasn’t the case. The user was comparing coordinates in her survey database to a point printout from Trimble Geomatics Office (TGO) and they just weren’t matching. After verifying the correct coordinate system in both the drawing and the survey database, the numbers were still off. An email to Trimble resulted in a very speedy response that told me not only how to fix it, but why it was happening. Follow the link to find out more…
In this particular case, the user was creating a FBK file using Trimble Link and then importing that FBK as a survey import event. The suggestion given to me by Trimble support was to enable sea-level correction in the survey database settings before import. This happens because of a difference in the way that Civil 3D and the Trimble Survey Controller software computes sea level correction. And with that, we get a nice math lesson:
In Survey Controller, the correction is computed as follows:
EarthRad / (EarthRad + ((StartElev + EndElev) / 2) * ObservationHorizDist = SeaLevelCorrectedDist
(The mean elevation of the survey line is used in the computation)
Survey Controller uses the WGS-84 semi-major axis length of 20925604.47sft as the earth radius value.
Civil 3D uses only the elevation of the start point instead of using the mean elevation of the survey line.
EarthRad / (EarthRad + StartElev) * ObservationHorizDist = SeaLevelCorrectedDist
This difference in computing the correction can result in small differences between coordinates from TGO and Civil 3D, but enabling sea level correction in the survey database helps to correct this. Note that I am talking about sea level correction in the survey database settings, NOT in the transformation settings found in drawing settings.
Author note: I would like to thank Scott Moffett, Land Survey Technical Support for Trimble Navigation, Ltd. for his assistance and information that helped with the writing of this article.