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Author Topic: GCP questions:  (Read 3024 times)

andrekje

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GCP questions:
« on: January 19, 2017, 05:17:57 PM »
1:  the checkbox left  to a marker (to left for "long,lat,alt,accuracy,error"...)    - does it enable/disable a given GCP for processing ?

2: if there is a marker, (GCP) , but it is not placed in any photo ( "it's icon is not green, but grey")  does it have any influence on the processing ?

3: is it possible to have a marker only be used for altitude only ? (aka - the position is +/- 10meters, but we know that flat area had a given altitude of xx.xxx meters)

4: imagine all photos for a map contain ellipsoid (GPS) altitude, then we add some GCP's with local geoid corrected altitude, - is the result expected to be "good" ?  - or does all altitudes, (in exif too)  - need to be geoide corrected ?

Alexey Pasumansky

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Re: GCP questions:
« Reply #1 on: January 19, 2017, 05:44:53 PM »
Hello andrekje,

1. The coordinates of the elements unchecked in the Reference pane wouldn't be used for georeferencing and optimization procedures.

2. "Grey flag" indicates the possible position of the marker on the current photo where it doesn't have any manually placed projection. 3D position of the marker is projected on the photo according to their relative positions and such projection is marked with the grey flag. It was made only for the user convenience, so answering your question - grey flags are ignored (like if there's no flag at all).

3. You can use custom accuracy for the marker coordinates, like 100000/10 meaning that position of marker in XY plane is measured with 100 km accuracy and along Z with 10 m accuracy. But I still suggest to input some values close to the area of interest and do not just use zeros for X and Y. You can actually customize the accuracy for all three coordinates using two slashes: X/Y/Z.

4. PhotoScan assumes that all the coordinates in the Reference pane for the checked on elements is the coordinate system defined in the Reference pane settings dialog for the current chunk, so if the coordinate system for cameras and markers are different you need either to uncheck all cameras before optimizing the alignment or to convert camera coordinates to the coordinate system used for markers.
Best regards,
Alexey Pasumansky,
Agisoft LLC

andrekje

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Re: GCP questions:
« Reply #2 on: January 19, 2017, 08:32:11 PM »
Thank you, very informative.
May I suggest for the cjheckbox column to be named "enable" or have a tooltip telling that it's an enable/disable (as some may think it's a selection/group action feature)
Likewise, please add a tooltip to "accuracy" column, mentioning the X/Y/Z alternative

andrekje

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Re: GCP questions:
« Reply #3 on: January 19, 2017, 08:57:25 PM »
One more question about #4
you wrote "...the coordinate system for cameras and markers are different.."

My question is not about referance system, both camera and markers are in same ESPG:....   - but the altitude used in markers , is "NN1958"  - an Norwegian elevation model, made to give an proper AMSL value, compansating for geoide and gravitys effect on water.

does your answer apply to that scenario ?

Alexey Pasumansky

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Re: GCP questions:
« Reply #4 on: January 19, 2017, 09:20:28 PM »
Hello andrekje,

By default PhotoScan assumes ellipsoidal altitude, unless compound coordinate system with the different datum is used.

Coordinate system definitions with and without the geoid are different, so I meant also this case.
Best regards,
Alexey Pasumansky,
Agisoft LLC