GCP are super high and turns raycloud upside sown

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

Hi there,

Iam using my Phantom 3 to take fotos 60 meters above ground level. I took 64 photos of a 1 hectare area. Overlap and keypoints are all good according to the quality report.

I have 3 Ground Control Points, plotted by a RTK Trimble GPS.
As soon as i insert the points they are displayed very high above the raycloud. On the MapView i can see that they are in the correct areas (within 5m)

I use the basic editor to link ±10 images to each of the 3 GCP`s. After reoptimizing the raycloud is upsidedown.

I then deleted all of the image geotags that the phantom3 gave it, and reoptimized again.

It helped a bit. but the image is still turend 90 degrees in raycloud view and also the cameras are now placed in almost a half moon type of effect. Also the quality report now states “157.25% relative difference between initial and final focal length” and “3 GCPs (3 3D), mean error = 4.553 m”

What am i doing wrong? The images were taken and then the memory card placed in my computer.

I notice that when uploading the images it tells me the altitude is 60m. Could it be that i need to add 1500m to each photo as i am at 1500m and so is the GCP.

2

It sounds like your image’s geotags are measured as meters above ground level and your GCPs are measured in meters above sea level. I’m not familiar with the phantom but with the UAV I use I was able to extract altitude in meters above sea level from the metadata.

3

Hi Deneys,

Duncan’s suggestion is probably right.

The reason you have a shift between your images and GCPs is because the camera location was measured above ground level (AGL). Meaning that the elevation is for instance 25 meters above the ground instead of 225 meters above the reference Earth’s surface.
The GCPs are correct.

Here is the procedure we suggest:

  1. In the GCP/Manual Tie Point Manager, compute the mean altitude of the GCPs based on the vertical coordinate. It does not have to be accurate. You could roughly estimate the mean elevation based on 3 GCPs evenly distributed.
  2. Export the image geolocation, in the Menu bar click Project > Image Properties Editor… and in Images Geolocation section click To File…
  3. Open the exported geotaggs into a spreadsheet and add the mean altitude of the GCPs to the vertical coordinate of each image that was measured above ground level. Save the modifications.
  4. Still in the Image Properties Editor, import the modified file containing the new geotaggs using the From File… option.
  5. Reoptimize the model from the Menu bar Process > Reoptimize.

Best regards,