# vertical co-ordinates GCP

**URL:** <https://community.pix4d.com/t/vertical-co-ordinates-gcp/6510>\
**Category:** PIX4Dmapper Questions/Troubleshooting\
**Created:** [May 11, 2018, 2:36am UTC](https://community.pix4d.com/t/vertical-co-ordinates-gcp/6510 "2018-05-11T02:36:44Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Patrick\_Malone](https://avatars.discourse-cdn.com/v4/letter/p/5e9695/32.png) [@Patrick\_Malone](https://community.pix4d.com/u/Patrick_Malone)\
**Post date:** [May 11, 2018, 2:36am UTC](https://community.pix4d.com/t/vertical-co-ordinates-gcp/6510/1 "2018-05-11T02:36:44Z")

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Hi, I was wondering if anyone knows how to transform vertical co-ordinates from GCPs measured in the AHD (Australian Height Datum) to Pix4D accepted datums e.g. the MSL EMG 84/96/08. The surveyor suggested to use the arbitrary units option which doesn’t sound correct to me?

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**Author:** ![Christina](https://sea2.discourse-cdn.com/pix4d/user_avatar/community.pix4d.com/christina/32/303_2.png) [@Christina](https://community.pix4d.com/u/Christina)\
**Post date:** [May 11, 2018, 1:54pm UTC](https://community.pix4d.com/t/vertical-co-ordinates-gcp/6510/2 "2018-05-11T13:54:57Z")

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Hi Patrick,

In this case, the best way to define your GCP’s vertical coordinate system is, indeed, to select the _Arbitrary_ option:

Pix4Dmapper will “respect” the GCPs´altitudes and give coordinates on the entire model based on these altitudes.&nbsp;

By selecting the arbitrary system, you may lose a bit of accuracy for very big projects, where the altitudes cannot be calculated by&nbsp;a simple interpolation, but the difference should be small and present only on big projects (some square&nbsp;kilometers).

As a quality check, you could use some of your GCPs, as check points&nbsp;in order to assess the accuracy of your project, by checking the altitudes that Pix4Dmapper gave to these points after processing (you should not use them as GCPs, you should use the label Check Points, see [Type](https://support.pix4d.com/hc/en-us/articles/202558179-Menu-View-rayCloud-Right-sidebar-GCPs-and-Manual-Tie-Points))

I hope it helps 🙂  
Christina

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