How to align projects

In order to compare projects, they should be aligned properly.


If there are common ground control points (GCPs) present in the different projects

In this case, the projects are aligned. It is recommended to check that some common points around the model have the same computed positions in both projects. If the alignment is not correct, the projects should be aligned following the steps described below.


If there are no common ground control points GCPs present in the different projects

1. Select one project P1 to be used as the reference project. The second project P2 will be aligned to P1.
2. Add and mark manual tie points (MTPs) well distributed around the project P1 and reoptimize:

3. Change the type from manual tie point to 3D GCP: 

3.1. On the menu bar, click Project > GCP/Manual Tie Point Manager...
3.2. On the GCP/Manual Tie Point Table, select the Type cell of the created manual tie point, double-click and select 3D GCP.

4. Export GCPs: On the GCP/Manual Tie Point Table, click Export GCPs..., click Browse... to select the path and name to save the file, click Save and click OK.
5. Change the type from 3D GCP back to manual tie point.
6. Open the second project P2.
7. Import the GCPs file previouly exported into P2

7.1. On the menu bar, click Project > GCP/Manual Tie Point Manager...
7.2. On the GCP/Manual Tie Point Table, click Import GCPs..., click Browse... to select the created file, click Open and click OK.

8. Mark the 3D GCPs on project P2 and reoptimize:

 

The two projects will be perfectly aligned and ready to be compared.

 

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4 comments

  • Troy Fardell

    This is basically what I have done prior to reading this. I stole two 3d GCP from one and added them to the other project. I am 20m nth/sth out of alignment. 

    Question: Is there a minimum number of common 3d tie points required?

    Thanks

  • Blaž (Pix4D)

    @Troy, the workflow you follow is correct. 

    The minimum number of GCPs to georeference a project is 3. It will not work with 2. We recommend 5-8 to minimize the error as this is the number we have observed that gives the largest increase in accuracy while keeping the number small. 5 should already give good results.

  • Justin D

    How many points are recommended when aligning projects not using true 3D GCPS but using the second method, marking manual tie points and then exporting them into subsequent projects as 3D GCPs?  For example: If you are wanting to align multiple projects in order to compare acreage differences (material being removed/disappearing from the edges of the dataset over time. Think progression mapping) and the total project you are wanting to align across datasets (time) is approximately 300 acres, how many of these manual ties points imported into subsequent datasets as 3D GCPs would be recommended?  Essentially, we would be using permanent visual points beginning with the original dataset, however, without collecting their position with an Emlid or similar.  These points would only be used to align the projects and not to position the project on the earth accurately.  As this would/will be a 2D calculation and not a 3D, we only require that the datasets collected over time align over one another accurately.

    Edited by Justin D
  • Daniel (Pix4D)

    Hello Justin,

    The more points you use the better, I would say that 5-7 points properly distributed should work. 
    I do not get why you do not want to align the projects in height too. You can mark those permanent visual points including the Z coordinate too and the projects will be aligned in Z too.
    The fact that these manual points do not have precise absolute coordinates does not mean that the projects do not have a good relative accuracy.
    I mean that the project can be aligned very accurately although the absolute position is wrong.
    This article can help to understand what I mean: What is accuracy in aerial mapping

    Regards.

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