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Understanding absolute accuracy vs check point accuracy

 I’m trying to understand my absolute accuracy error after using arbitrary coordinate system and loading the proper parameters in advanced coordinate system settings. They reduce my absolute error but still leave x=1.19% y=1.37% z=1.84%. My GCP and check point errors are within acceptable limits. Usually within’ 50mm. Would my absolute accuracy be considered an “accurate survey”? We’re a construction company that will have our data scrutinized by other engineering companies validating our quantity. Looking to understand how to explain these errors as a percentage if requested to do so. Thanks. 

Hello John,

I am not sure to which absolute accuracy you refer. Could you please specify?

In general, the absolute accuracy of a project depends on many parameters and it is not global for all the points. For more information about the accuracy of the Pix4D outputs, you can take a look here.
The check point errors indicate the accuracy of the model in the area of the check point. Thus, to better evaluate the accuracy of the model you should use check points in several locations throughout the project.

To better comment on your results, could you please send me the quality report of the project? 

Regards,

What format is best to attach the report?

Hi John,

Please attach the PDF quality report that you can find in the results folder under: …\project_name\1_initial\report\project_name_report.pdf

Regards,

Antigoni

Is there another way to attach .PDF? I am getting an error…

Hello,

You can upload it to Dropbox or any other data sharing platform and post the link here.

Alternatively, you could submit a support request sharing the pdf quality report. If you do this, please refer to this Community post.

Regards,

https://1drv.ms/b/s!AsvXZJE-mU5ngbZt3tjA0NHvfZOqig

 

Not the same project I spoke of but similar. my checkpoints are looking good but absolute z error is 4%. What kind of Absolute error can I expect in this situation?

Hello John,

The Absolute Geolocation Variance table displays the percentage of geolocated and calibrated images with a geolocation error in X, Y, Z within a predefined error interval. When there is a high percentage of images with high error, this could be an indication of problems in the GPS device or errors in the geotagging process.

In your case, since you are using a DJI drone for capturing the images the large errors are due to the limited accuracy of the GPS receiver that is mounted on the drone and is used for image geolocation information.
The images that are saved on the drone’s SD card are geotagged by the manufacturer. Lat./Long. coordinates are reliable in the image EXIF, however, there might be some inaccuracies for the altitude depending on the location where you are mapping. We made testing in our office here and we found that the vertical coordinate is off by several meters that can reach an error of 100 meters. Note that this is just an offset meaning that the within the model, the accuracy is not affected, only the absolute location is. 

Using GCPs fixes the issues of the absolute accuracy of the project. The values that you should check in such cases are the GCPs errors, both the individuals GCPs and the mean and RMS errors, and the check points errors. These values look good in your Quality Report.

Please note that for sub-centimeter GSD projects, it is harder to get the ideal conditions. Problems such as the perspective, vibrations of the camera, blur effect, depth of field, etc. are magnified. It is recommended to do very careful testing and planning for such projects, in order to get the expected accuracy.
Additionally, the accuracy of the GCPs is crucial for the absolute accuracy of the final output and it should be better than the expected final accuracy.

I hope this helps you. Please let me know if something is not clear.

Regards,