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Generating accurate Contours

Hello,

I have been using the Pix4D mapper software and wanted to see if I could get some help with the contour lines part of the process. I have been using ground control points, but when the contours are generated, it is not assigning elevations in my project area. I have attached a screen shot of how I have been running my uploads. I am wandering if by starting at a base contour of 0 it is calling the lowest point that value and assigning values in contours from the lowest known point. I should be getting elevations around 2,000 feet in my project and I am getting values in the 0-200 foot range.

My question is since we are using GCP’s is there a away to produce contours with their real assigned elevation and not an auto generated value. I want the finished product to generate 2 foot contours, which I have assigned in the processing options. I am also curious what the resolution and minimum line size are and if I should be changing something in those values as well.

Hi David,
Could you please send us :
▸ The quality report (.pdf format): …\project_name\1_initial\report\project_name_report.pdf.
▸ The 3D Coordinates 1-2 of your highest and lowest points in the Raycloud.

Note that the Contour Base is used as a starting point for contour lines generation. Contour lines are generated, based on the selected Elevation Interval, upwards and downwards from the Contour Base.

For example, a project with an elevation between 315 and 350 meters with:

  • Contour Base = 0 and Elevation Interval = 1 the first contour line will be at 315 meters, the second contour line at 316 meters and the last one at 350 meters.
  • Contour Base = 0.5 and Elevation Interval = 1 the first contour line will be at 315.5 meters, the second contour line at 316.5 meters and the last one at 349.5 meters.
  • Contour Base = 2 and Elevation Interval = 10 the first contour line will be at 317 meters, the second contour line at 327 meters and the last one at 347 meters.

You can take a look here for more information.

Best,

I am using GCP’s with known elevations, however when flying over the entirety of the job, I can’t determine the lowest spot on the flight in order to enter that on the starting base elevation. I can estimate what that lowest elevation of the flight area, but if a GCP is not actually placed on the lowest location of the map area, I am just guessing at what that elevation is. I guess that is why I put 0 as the starting base elevation with my upload.

Mark Energy 03-29-21 Flight 2_report.pdf (1.6 MB)

Hi David,

Note that you do not have to enter the lowest point of your project in order to be used as the starting base elevation.
Your GCPs are used in order to georeference your project, not to determine the lowest 3D point of the mapping area.

The Contour Base is used as a starting point for the contour lines generation.

For example, if your project has an elevation between 1000 and 2000 meters:

  • When you set Contour Base = 0 and Elevation Interval = 2 the first contour line will be at 1000 meters, the second contour line at 1002 meters and the last one at 2000 meters.

I checked your quality report and everything seems good. Did you experience any problems?

The created file contains the contour lines along with the associated elevation values.

When I generated contours with my past uploads I had started by base elevation as 0 even though I did provide GCP’s. My output contours started at 0 when I did this and did not match up with the GCP elevations marked in the field. When I upload these projects I never know the lowest elevation on the flight. I do know the GCP elevations however. I’m wandering if I should just use the GCP elevations and just subtract a few hundred feet off and use that as my base elevation?

Hi David,
Your GCPs are used only for georeferencing your project during step 1. and not to determine the lowest elevation point of the project. When you set Contour Base = 0 have in mind that this is a relative value and it doesn’t mean that your project starts from 0m height. It means that the software will start the reconstruction from the lowest elevation point of your project. Also, you said that your output contours started at 0, did you import in another software the contour lines and noticed a zero (0) value?
Could you please send us :
▸ The quality report (.pdf format): …\project_name\1_initial\report\project_name_report.pdf.

Best,