Author Topic: Question about Orthophotos and Ground Control Settings/measurement accuracy  (Read 5575 times)


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I am generating an orthophoto and DSM from imagery collected with no camera GPS location and very few ground controls. X,Y Ground control is derived from 0.1 m orthoimagery from a few months ago, and Z ground control is from 2-year-old 1 meter Lidar DEM.

In Ground Control Settings:

/Camera accuracy - is this for camera position? Does this setting matter if I haven't specified GPS locations for camera? I have left at default 10 meter for now. When/why would I change this?

/Marker accuracy - I estimated 0.25 meter since my imagery and the orthoimagery have similar (~0.1 m) resolution. Does this value represent my certainty that I picked the right pixel for my markers?

/Projection accuracy - this is at 0.1 pixels. What does it mean? when would I change this?

Finally, if I change these values (and if not), do I want to optimize photo alignment after I add GCPs?

If I understand the manual and the process in PhotoScan Pro correctly, I think that I want to accurately constrain my error, optimize photo alignment, then produce the orthophoto and DSM. I want to see if I can use the imagery to measure volumes of eroded sediment (from a dam removal project), so I am very concerned about accuracy and will be comparing to Lidar that will be flown in a couple weeks. I think that some explanation and clarification could save me hours of trying different settings.

The imagery is of a river, so the three GCPs that I have defined are oriented mostly linearly (see screenshot). I have considered adding a fourth, but I have poorer quality coverage (fewer photos, more noisy surface) further from the river.

I am using ground features like road centerlines and centers of manhole covers and boulders as control, and I am trying to understand the proper settings for measurement accuracy in the ground control settings. Also trying to determine whether I want/need to recalibrate camera positions.

Alexey Pasumansky

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Hello andyroo,

Camera and Marker accuracy parameters are not used if no cameras or markers (correspondingly) are checked in the Ground Control pane.
Projection accuracy is only used if markers are included into optimization process.

For camera/marker accuracy parameter values it is recommended to use higher accurate values. For example, if GCPs are measured precisely with 1-2 cm accuracy we recommend to set Marker accuracy parameter to zero.
But in the described case I think optimization is required if the reconstructed model has visible "bowl effect", but using zero value for Marker accuracy is not recommended due to low number of GCPs used. I assume that values 5-10 cm could be used.
Best regards,
Alexey Pasumansky,
Agisoft LLC