GCP's and very high error values

[ak]

I have run a dataset referenced with 3 GCP's. I used the exif geographic information to align the photos then entered the 3 GCP's. I unchecked the photos and for the rest of the workflow the GCP's were used as the geographic reference. I am seeing very high error values attached to the images as you can see from the attached screenshot (ie. 131970+ meters).

I have compared my GCP coordinates with an earlier previously produced orthomosaic (georeferenced using only the image exif data) in GIS software. They are quite close- generally within 1m. When I compare the GCP georeferenced orthomosaic with the exif georeferenced orthomosaic in GIS they are also quite close as expected.

Since things check out for me I am confused as to where this large error value is coming from. Am I missing something critical here? Can you explain what this value is?

Thanks very much!

[SAV]

Hi ak,

Three GCPs are generally not enough to achieve good/accurate georeferencing results, especially when they are not evenly distributed over the area (as it is in your case, where 2 points are very close to each other and the 3rd one is quite far away).

Nevertheless, the large error is probably related to the coordinate reference system. Either you have chosen the wrong coordinate reference system for your GCPs or you might have confused X,Y / Easting,Northing. Or last but not least, you have simple mixed up your 3 GCPs, for example coordinates from GCP10 should be the one of GCP11.

Note: In order to get accurate results, you should use at least 7-9 GCPs, where 5-7 of them are used as control points (checked/enabled in the reference pane) and 2-4 as check points (unchecked/disabled in the reference pane).

Regards,
SAV