Pix4D topology different than the previous surveys

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

I completed a project where the quality report shows that the data is within 1.5 cm of the ground control points. I exported a topology drawing to the client and they reported back to me that the topology is very different than their previous survey shows. I don’t know if there’s something wrong with my project or if maybe the contour drawings of Pix4D are not as accurate or robust as the LAS file? I need help before I lose this client.

Any help is greatly appreciated.

My current monthly license is up in 4 days and I need to move very quickly on this.

thanks

Dave King

2

Did you adjust the contour settings to be more detailed?
i.e. set ground height, elevation interval, resolution, vertices.My outputs can be almost perfectly overlaid on our surveyors reports.

3

HI Steven

Thanks for your reply. I “tried” to enter the numbers but I don’t know if they are correct.
I went into the Mosaic Editor-DSM color map-parameters to get the min and max altitude of the project
Min altitude is 1124.235
Max alt is 1206.952
But I am not sure what to set the contour base to- Should I set it to 0?
Elevation interval is 50
Resolution is 100cm
Min line size is 20

4

Mine is set to about 1/4 of the default and gives good results. I am only mapping about 100 x 200 m area. Big areas might take too long with tight settings.

5

Would contour base or any other contour setting change the accuracy of the topology?

6

My guess would be the coordinate system. Your client may be comparing with a different system from the previous survey.
I’ve never changed the Base Contour Setting, and the models I’ve created matched LiDaR contours flown from manned aircraft perfectly. My GCP’s are located with RTK GPS in State Plane Coordinates.

You can shift the Geoid Height under the Advanced Coordinate Options if you need a quick fix.
My guess is that your client is not comparing with the same Horizontal and/or Vertical Coordinate Systems you modeled with.

7

Hi David,

I would firstly check (as Ryan said) what is the coordinate system used by your clients. Do they check using the same horizontal and vertical datum?

Then, I would check the differences that they give you.
If you have a constant difference of few meters for example, then very probably, you and your clients use different vertical datum (case 1)
Give us more information about the differences

Then you can check the overall accuracy of your project by adding some mtps and check their reprojection on the images
Olivier gives some hints here:
https://www.youtube.com/watch?v=zK9zCnxy6Ok&list=PL8ZbncaV3f_bg3lShz615skNYnRG4DBEU&index=19
about how to check the accuracy of a project

what about the report? Does it indicate any error?

finally, I would check the data of my clients. How accurate is their previous survey? Do they use laser scanner data, photomaps, few surveyed points (how many?)?

Cheers
Marc