# Pix4d Mapper large error on Check Point

**URL:** <https://community.pix4d.com/t/pix4d-mapper-large-error-on-check-point/31021>\
**Category:** PIX4Dmapper Questions/Troubleshooting\
**Created:** [August 25, 2024, 12:07pm UTC](https://community.pix4d.com/t/pix4d-mapper-large-error-on-check-point/31021 "2024-08-25T12:07:53Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![hifaoilprojekat](https://avatars.discourse-cdn.com/v4/letter/h/f475e1/32.png) [@hifaoilprojekat](https://community.pix4d.com/u/hifaoilprojekat)\
**Post date:** [August 25, 2024, 12:07pm UTC](https://community.pix4d.com/t/pix4d-mapper-large-error-on-check-point/31021/1 "2024-08-25T12:07:53Z")

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Hello,

I’m having strange troubles processing a projects with GCPs. If I use all the known points as GCPs, i’ll usually get good accuracy on those points.

However, if I set some of those as check points, there is an error on z coordinate, around 20 cm (CP t2).

I captured this project from height of 50m and 80% overlap.

Below is geolocation lines from processing report and a link to the entire report and log file.  
[part\_report.pdf](https://community.pix4d.com/uploads/short-url/oPTCmjHNIqa7FU9Wpt9WBxKPRx2.pdf) (1.9 MB)

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**Author:** ![Kapil\_Khanal](https://sea2.discourse-cdn.com/pix4d/user_avatar/community.pix4d.com/kapil_khanal/32/304_2.png) [@Kapil\_Khanal](https://community.pix4d.com/u/Kapil_Khanal)\
**Post date:** [November 27, 2024, 9:35pm UTC](https://community.pix4d.com/t/pix4d-mapper-large-error-on-check-point/31021/2 "2024-11-27T21:35:26Z")

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Hello, I would say remark your GCPs and checkpoint and it is not recommended not to have GCPs at the edge of the area of mission where the overlap is low.