Only 2D GCPs

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

I have a project without geotags and I could only find a source to extract 2D GCPs, so I do not have 3D GCPs for the project.
When I mark the 2D GCPs, the model ends up rotated and the GCPs have big error.
Do you know if I can mark only 2D GCPs or is this the cause of the problem?
Thanks.

2

Hi Luca,

How is it rotated? is it upside down?
I think regardless of only having 2D GCP, you need to define a 3D GCP (with a known Z value, maybe even X and Y.)
I am still trying to understand what a 2D GCP and CHECK POINT are really for.

I got this from the help, but I wish Pix4D would give us some real examples.

There are four possible point types:

PS: AAT = Automatic Altitude Triangulation, BBA is something similar (so many acronyms)

2D GCP: A 2D GCP is a point that has a known 2D position in the scene and is used for the project’s AAT/BBA.
3D GCP: A 3D GCP is a point that has a known 3D position in the scene and that is used for the project’s AAT/BBA.
Check Point: A check point is a point that has a known 3D position in the scene and is used for checking the accuracy of the reconstruction. It is not used for the project’s AAT/BBA.
Manual Tie Point: A manual tie point is a point that is identified by the user in several images. Its position in the scene is not known. Manual tie points are also created by drawing an object on the 3D View of the rayCloud. Each vertex of the object corresponds to one Manual tie point.

3

Dear all,

Using only 2D GCPs to geolocate the images when there is no image geolocation in the project is not enough.
In this case, the transformation between the arbitrary processing system and a known or a user defined system is not possible only with 2D GCPs.

There is a workaround in order to geolocate the model horizontally, when having only 2D GCPs but not image geolocation.
You can mark the 2D GCP as Manual Tie Points in the project and then convert them to 3D GCPs and reoptimize.
For step by step instructions, please check: https://support.pix4d.com/hc/en-us/articles/204951165-

Alternatively, you can use Web Map Services in order to extract 3D GCPs and mark them in the project.
In this case, you should adjust the accuracy of the GCPs to some meters, as Web Map Services are not accurate.
Also note that the absolute accuracy in this case will also be of some meters.
For step by step instructions: https://support.pix4d.com/hc/en-us/articles/202560149-

Also note that 2D and 3D GCPs are used for geolocating the model, i.e. putting the model to its correct place on a specific coordinate system.
Check points are not taken into account for processing, but they are useful for checking the accuracy of the model generated.
Manual Tie Points are points marked in many images that are useful for improving the calibration and reconstruction of the model.

Best regards,
Pix4D support