# Problem with Ground Control Points

**URL:** <https://community.pix4d.com/t/problem-with-ground-control-points/19089>\
**Category:** PIX4Dmapper\
**Created:** [July 23, 2021, 8:48am UTC](https://community.pix4d.com/t/problem-with-ground-control-points/19089 "2021-07-23T08:48:51Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![karinamar.banos](https://avatars.discourse-cdn.com/v4/letter/k/ce73a5/32.png) [@karinamar.banos](https://community.pix4d.com/u/karinamar.banos)\
**Post date:** [July 23, 2021, 8:48am UTC](https://community.pix4d.com/t/problem-with-ground-control-points/19089/1 "2021-07-23T08:48:51Z")

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Hi everyone.  
I have a project with 5 GCP, wich position is correct in axes x and y, but in z there is an offset in the option “Calculated Position” in Pix4D. What could be the problem?. Thanks a lot.

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**Author:** ![Nikoleta\_Dimopoulou](https://avatars.discourse-cdn.com/v4/letter/n/53a042/32.png) [@Nikoleta\_Dimopoulou](https://community.pix4d.com/u/Nikoleta_Dimopoulou)\
**Post date:** [July 23, 2021, 8:53am UTC](https://community.pix4d.com/t/problem-with-ground-control-points/19089/2 "2021-07-23T08:53:18Z")

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Hi,

Can you please send us the quality report in pdf format?

Best,

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**Author:** ![karinamar.banos](https://avatars.discourse-cdn.com/v4/letter/k/ce73a5/32.png) [@karinamar.banos](https://community.pix4d.com/u/karinamar.banos)\
**Post date:** [July 23, 2021, 11:27am UTC](https://community.pix4d.com/t/problem-with-ground-control-points/19089/3 "2021-07-23T11:27:04Z")

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Ok, here it is.

[Quality report.pdf](https://community.pix4d.com/uploads/short-url/gOPRv99N6uDA0vNUshYqHglQWSY.pdf) (2.1 MB)

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**Author:** ![Daniele\_Lecci](https://sea2.discourse-cdn.com/pix4d/user_avatar/community.pix4d.com/daniele_lecci/32/9493_2.png) [@Daniele\_Lecci](https://community.pix4d.com/u/Daniele_Lecci)\
**Post date:** [July 23, 2021, 1:00pm UTC](https://community.pix4d.com/t/problem-with-ground-control-points/19089/4 "2021-07-23T13:00:29Z")

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Hi,

Welcome to the Pix4D Community!

Do you know which vertical coordinate system was used to retrieve the coordinates of your checkpoints? Can you please confirm or reject that **EGM96 Geoid** was used?

What results are you getting if you set some of your tie points as 3D GCP and leave the other as checkpoints?

Cheers

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**Author:** ![karinamar.banos](https://avatars.discourse-cdn.com/v4/letter/k/ce73a5/32.png) [@karinamar.banos](https://community.pix4d.com/u/karinamar.banos)\
**Post date:** [July 23, 2021, 1:27pm UTC](https://community.pix4d.com/t/problem-with-ground-control-points/19089/5 "2021-07-23T13:27:45Z")

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I have done tests with the EGM96 and EGM2008 geoid, and I have the same problem.  
The 5 points are control points, so I am going to put some points as 3D GCP and others as checkpoints.  
Thanks.

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**Author:** ![Blaz](https://sea2.discourse-cdn.com/pix4d/user_avatar/community.pix4d.com/blaz/32/4344_2.png) [@Blaz](https://community.pix4d.com/u/Blaz)\
**Post date:** [July 26, 2021, 6:31am UTC](https://community.pix4d.com/t/problem-with-ground-control-points/19089/6 "2021-07-26T06:31:01Z")

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@karinamar.banos, since you are not using a drone with RTK/PPK geotags, it would be recommended to use some GCPs to georeference the project.

In general, we recommend 5-10 GCPs per project. So you should consider measuring additional points next time to be able to use them as checkpoints.

Best,