Variation between contours (.10') and LAS file

PIX4Dmapper
1

So, we generally use the .dxf output for contours (.10") and generate surfaces for quantities. Usually, we compare those to proposed grading. Pretty straightforward. Anyway, I have field crews take shots with GPS to cross-check the accuracy of data. I generally see decent results but noticed a couple of areas where GPS shots were over 3-4’ off (vertically) to contour surface file. Also, there was an existing wall in place where the contoured surface shows a rounded off slope. It should’ve been closer to veritcal. Next, I brought in the .LAS file from both steps 2 and 3 for another comparison. To my surprise, the .LAS files were very different compared to the contours. Again, we contour these at .10’. While I expect some variance, this is extreme. Example attached.

Anyone ever compare generated .dxf files to .LAS files?

I’ll add this:
Step 1 is done to full rez. We do use GCP’s. This was a a large job around 50+ acres. Think we set 8-10
Step 2 mostly default settings with LAS file generated.
Step 3 default with dxf generated at .10’ Grid spacing changed to 50cm

Flown with P4P, GCP’s set with Topcon Hyper+ RTK

Doc3.pdf (635.4 KB)

2

I don’t see a way for Pix4DMapper to include breaklines in contour generation. Therefore it will be difficult to generate correct contour lines for vertical surfaces.

3

I’m not necessarily concerned with getting breaklines out of pix4d. I’m just noticing a large difference from the dxf contour file compared to the actual point cloud generated in both steps 2 and 3. This one project has a difference of close to 30,000cy between the .10’ contour dxf to las file. Massive issue when you consider haul off and/or import.

4

Forgot to mention: the wall has a slight slope and is not completely vertical. Also, there is enough exposed ground on both sides of the wall to generate info. Please see attached pic in the original post to get the full picture. If you have a smaller project, I’d be interested to see if you notice the same thing. Not sure where the disconnect between dxf and las is, nor what’s causing it.

5

Hi @Kingz, thank you for sharing all the information!

Keep in mind that contour lines are generated based on the DSM or DTM and not directly out of a point cloud. So you need to check how the DSM looks like in the area of comparisons.

Did you ever try using PIX4Dsurvey? It has the possibility to use breaklines as a constraint for the TIN and contour lines generation. For more information see TIN - Use terrain layers as breaklines

Best,

6

I figured it out. I had to change some of the default settings in step 3 to achieve a more accurate representation in contours. Hopefully, others are checking the difference between each of the output files. Definitely odd that there is such a large difference between 2 different files on the same project. Actually, it’s scary.