Support Website Contact Support Blog

Need examples of using different point types please (maybe a webinar?)

Hi guys, I understand the concepts of the 4 types of points that we can have in pix4d but I am having a hard time finding a real life scenario in which some of those types are applicable.

Specifically the 2D and Check Points.
Thanks.

There are four possible point types:

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.

Hello DDS,

To explain in more detail the 4 types:

  • 2D GCP: It is a Ground Control Point measured in the filed, with good accuracy but which does not have the altitude coordinate, or a fake GCP obtained via web services, as well, without altitude value. Most users directly use 3D GCPs, but we allow the use of 2D GCPs if needed. They help in the reconstruction of the model and positioning the model in the right position in the earth.
  • 3D GCP: It is a Ground Control Point measured in the filed, with good accuracy with X, Z, Y coordinates, or a fake GCP obtained via web services, as well, with X, Z, Y coordinates. Most frequently used and it is the suggested type. They help in the reconstruction of the model and positioning the model in the right position in the earth.
  • Check Point: It is a 2D or 3D GCP, but when marking it as control point, it will not be used for the reconstruction, it will be used to check the accuracy of the model regarding the real words, so the coordinates that the user input for the point are compared with the coordinates of the 3D point obtained by marking the check point in several images.
  • Manual Tie point: They help in the reconstruction of the model, but not in the positioning the model in the right position in the earth. By marking a point in several images, we indicate that certain point should be reprojected as closer as possible in the 3D model, considering all the images reprojections. They are very useful when several blocks are created in the reconstruction. For more information: https://support.pix4d.com/hc/en-us/articles/202559369-Reprojection-Error

Please note that: Ground Control Points (GCPs) are points from the area of interest with known coordinates. Their coordinates have been measured with traditional surveying methods or have been obtained by other sources (LiDAR, older maps of the area, Web Map Service).

Regards,

Thank you guys for taking the time to answer this post.
It appears to me that the 2D GCP is same as using a 3D GCP (fake) with its elevation set to Zero ?

I can see using a 2D GCP to define the scale of the model in X,Y based on a know pattern of the floor or object like a parking space. ( 9’x19’) correct?

Hello DSS,

The elevation for 2D GCPS is not set to zero, it directly doe snot use elevation, set up elevation to zero would be consider an elevation for the point.

Using 2D GCPs helps to set up the scale X,Y, the results is neraly the same than the X,Y using 3D GCPs.

What is important is to check the accuracy of the GCPs measurements and place that accuracy when adding the GCPs.

The following links as well have useful information:

Regards,

I completely understand now. Since they have no elevation they can be used “in plan” to reference any object regardless of elevation. It seems like a pretty useful tool.