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Author Topic: GCP Help  (Read 6765 times)

KF7ELU

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GCP Help
« on: August 19, 2016, 06:12:44 AM »
Hello,
I need a little help on the correct way to enter Ground Control Points...
Here is the situation, Flying a mission area with a UAV. This is done in WGS84
Ground Control Points are measured in OSGB 1936

Every time I try and enter The marker coordinated and run an alignment and then create the dense cloud things go screwy. The model ends up inverted and sideways.

Anyone have a tutorial or can walk me through this procedure? Fairly new to Photoscan.

So to sum it up... Data is in WGS84 and need to output with GCP to OSGB 1936 British nation Grid.

Thanks!!!

Alexey Pasumansky

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Re: GCP Help
« Reply #1 on: August 19, 2016, 11:32:49 AM »
Hello KF7ELU,

You can use the following tutorial as a reference:
http://www.agisoft.com/pdf/PS_1.2%20-Tutorial%20(BL)%20-%20Orthophoto,%20DEM%20(with%20GCPs).pdf

Usually the common workflow has the following steps, according to your example:
- add photos, load camera position data, set coordinate system to WGS84,
- align photos,
- place marker projections, load coordinates in OSGB 1936, switch the coordinate system in the Reference pane settings to OSGB 1936,
- uncheck all cameras in the Reference pane,
- optimize camera alignment,
- proceed to the further steps: build dense cloud, build DEM, build orthomosaic, etc.
Best regards,
Alexey Pasumansky,
Agisoft LLC

Limpopo_River

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Re: GCP Help
« Reply #2 on: November 28, 2016, 02:41:45 AM »
Hi Alexey,

Under Optimize Camera Alignment on p. 9 of the the linked tutorial (PDF), it says "On the Reference pane uncheck all photos..."  What is the reason for this?  How does it optimize the camera alignment and the calibration if all the images are turned off?  Or does this simply tell Photoscan not to use the GIS data associated with the images while continuing to perform bundle adjustments to optimize camera alignment?

Also, is it ok or not recommended to change any settings in the Reference Settings pane as the optimization proceeds?  For example, after several iterations of Gradual Selection for Reprojection Uncertainty to remove selected points with higher errors, could one use a lower value for marker accuracy and tie point accuracy as the calibration proceeds?  Another expert who teaches Photoscan has told me that the tie point accuracy of 4 seems very high and it should be possible to reduce this value to 0.5 or less (he typically sets this value at 0.1).

Thanks

Limpopo_River

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Re: GCP Help
« Reply #3 on: November 28, 2016, 07:01:39 AM »
Hi Alexey,

I made an error about the tie point accuracy in the Reference Settings; the value I mentioned referred to setting the Projection Accuracy to 0.1 in an earlier version of the Reference Settings in Photoscan (http://www.agisoft.com/index.php?id=31), not the Tie Point Accuracy.  The question still stands about whether it's ok or not recommended to change any of the Reference Settings while performing iterations of the optimization steps.

A further question:  When do you recommend using Gradual Selection to select Projection Accuracy and Reconstruction Uncertainty?  Elsewhere, I've seen recommendations to use threshold values of 2 for Projection Accuracy and 10 for Reconstruction Uncertainty in the Gradual Selection dialogue box.

Thanks again!