Thursday, 24 May 2012

Surface Analysis Precision

This bugged me a while back and I meant to post about it.

I created a volume surface to show extent of cut/fill along my access road - red for cut and green for fill. I used the 2D Solid Level Banding surface style from the AutoCAD Civil 3D 2013 UKIE drawing template. I set my no. of ranges in the surface analysis to 2 which picks up the min and max levels on the surface and divides the level difference into two ranges.
All going well so far until I click ok and look at my surface... which appears to have 'holes' in it.
So why was this happening if I set it to automatically divide the surface into two ranges? The answer is in the surface style settings. The range precision is set to 1 which meant the max elevation for range 2 in my surface analysis stopped at 1m when in fact it was greater than that.
Setting the precision value to 0.001 and rerun the analysis solves the problem. Max elevation for range 2 now corrected to 1.293m.
The holes in my surface are no more...
The surface elevation analysis appears to begin at the absolute lowest elevation (ignoring precision) and work up to the max value (precision applied) - whereas I had always assumed that it found the absolute min and max levels first and then divided the difference into the specified number of ranges.

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