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Accuracy of GCPs

Hello, I am using a laser scan to grab GCPs for Pix4D. I use an arbitrary coordinate system in Pix4D. My GCP is in a non-geographic coordinate system that I chose in a CAD software. When I input the GCP in the GCP/MTP Manager, there is an option for Horizontal and Vertical Accuracy. I set these to 0.02 ft, the default value, because I am extremely confident in my GCP values and where I mark them in the images.

Yet, when I Rematch and Reoptimize, I find that a few of my GCPs have a very large offset from their “initial” value. I don’t want the software to change the GCP locations. I want the software to constrain its calculations based on these inputs. I have high confidence in them and I thought that’s what I was expressing with the Horizontal and Vertical Accuracy. Instead, I get multiple blocks with large displacements of the GCPs.

Why is the software overriding my accuracy inputs?

Hi @EvanH ,

When using coordinate systems in imperial units, the GCP accuracy should be changed to 0.066 ft, which is the equivalent of the default value in meters (0.02 m).

Could you post here the PDF quality report of your project?


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Hi Alice,

Thank you. I have attached the Quality Report. I should probably give you more details now that I have your attention.

This flight was done in a mountainous, forested area. The trees surrounding the roadway were thick with a mostly impenetrable canopy above the roadway. I had no choice but to fly low to the roadway, maybe 20 feet above at most, which produced a few shots that were more blurry than others. I did four runs up and down the roadway: a set of shots straight down, oblique looking downhill, oblique looking up hill, and oblique looking to the side of the roadway.

I know that usually the advice here is to re-fly with a better flight plan and more overlap, but there would be little I can do to improve this flight, other than flying extremely slowly and gathering more pictures. Although I have found that more pictures is sometimes detrimental to a project.

I have already tried four separate projects for the four separate styles of flight in an attempt to then Merge Projects. However, when I process the straight-down flight, the altitude of the roadway is clearly incorrect. It should be continuously sloping up hill. It curves upwards towards the ends. I tried fixing this by setting the horizontal and vertical accuracy to 0.1 m, as recommended in a previous post, but this only makes the data worse, as a kink then appears in the roadway.

One thing I am confused about is the arbitrary coordinate system. When I set it to arbitrary, it seems that none of the EXIF GPS data from the camera is used. I do not care where the site is geographically. I have my own coordinate system with an origin on the roadway that I work with. However, it would be nice to get the GPS data back in there to at least give a better initial guess as to the camera’s initial positions relative to one another.

Regardless, I thought that the GCPs would fix everything. I now have 6 GCPs in the project that I get from my CAD software, Rhino, where my laser scans are. Note in the quality report that apparently only two GCPs are used:

Yet I have 6 GCPs:

Note again in image properties I lost my GPS data when I set the coordinate system to arbitrary in order to accommodate the GCPs:

The reason that some images were disabled was because they didn’t record GPS data. Therefore, the images where at 0 latitude and longitude. So I disabled them.

In the end, I want the software to pin the GCPs down and constrain them, because they are 100% accurate, and then work outwards from there. Then I hope to finally sew together these multiple blocks:

Thank you.

Quality_Report.pdf (404.3 KB)

As I read more, I want to add that I am not at all interested in absolute accuracy. My application is not to survey and determine GPS points. I am only interested in relative accuracy. So whatever we can do to sacrifice absolute accuracy for relative accuracy is good with me.



Can you please try to split the project into 2 subprojects (yellow images one project, green images another project) and process again Step 1 for each of them and check the results? For processing step 1 please use the 3D Models template.

If results after Step 1 are good then you can merge the two subprojects by following instructions from this article:


Hi Nikoleta,

I have already tried this method:

There are significant issues with the altitude when I use this method. Here is an example of the straight-down flight:

I believe that I have also tried using common MTPs between projects when Merging Projects and that didn’t work either.

I’ve been trying to get this project to work for months now.

Hi Evan,

Can you share with me the quality reports of the subprojects you created?


Hi, attached is the quality report for the straight-down flight.
Straight_Down_report.pdf (183.2 KB)

Can you please

  • Set the Horizontal accuracy of the images to 2ft and Vertical accuracy to 5ft

  • Set the Internal Parameters Optimization to All Prior

and process Step 1 again?

Afterwards send us an updated quality report.