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We have generated a large contour file on a 14 mile long project, encompassing 1500 acres flown in several missions. We have generated one large project, then split it into 3 projects. We have about 80 GCP’s done by surveyors that include elevation.

The issue is that the large single project has contour lines that are off, some by hundreds of feet vertically, while most are spot on. However, it seems that the contours to the split projects are all fine. What could be the reason? Aren’t the contour elevations derived from the GCP elevations? Each contour  file also has the contour lines going different ways. If it is mapping the topography, shouldn’t they all lay the same way?

Hello Merlin,

In order to help you diagnose your issue, could you please state a few items of information?

1.  How many flights, how many photos, what do your quality report(s) look like - specifically RMS for GCPs? (Need to be .05-.1ft or better).  How many marks are you placing on each GCP?  How many total photos per GCP?

2.  Are you processing in a State Plane Coord Sys or in Arbitrary?

3.  Consider building a pyramid. Start at the bottom tier. Build projects with two or three flights each - verify your GCPs, and your contours are tight.  Then Merge Two Projects (4-6 flights) - verify your GCP’s and your contours are tight. And so on. Pay close attention to the horizon as you merge projects, verify that the horizons are correctly aligned - this should fix your contours.




1 )20 flights. 2894 images. Had a bad RMS I think due to one GCP being identified incorrectly in one photo  , but not near where the contour line elevation issues were.Most GCPs have 10+ photos. The least amount on a GCP is 4. What do you mean marks?

2)We are processing with a state plane output coordinate system

  1. Sean, I had started doing it that way, but was advised to do it all in one project and then split in order to save time. YOu htink that would do better then?

It does seem that the point cloud has each pixel with the correct elevation…

I have made contours out of photographs captured with a drone, but there are lots of extra lines in DEF forme. how to get rid of them?

Hello Rashin,

1.) Can you show the Ortho?

2.) It looks like you have trees in the project?

3.) It also looks like you have some ‘noise’ at the boundary of the project - I would use the point cloud editor, and delete the edge points (clean up the edges). 

4.) It looks like there may be ‘noise’ (incorrect/invalid) points ‘underneath’, and possibly ‘above’ the point cloud.

           - Did you process with Step 2 - minimum number of matches - 3 or higher? (If this is 2, it will produce noise under the point cloud).

5.) After you do that - then go to contours:

           - Resolution  - (The longer you make this the ‘smoother’ your contours will be) def=100, consider using 457.

           - Min Vertices  - (The less of these {min 4} you have, the more contours you have at the ‘top’ of an object (like a tree or a pile volume).

Basically, it looks like you have issues with your point cloud.  Fix those issues first, and then go into contours and adjust your contour parameters.

Hope all goes well.

Sean Erickson.