Release (Z) from (XY) GCPs coordinates

[Yoann Courtois]

Hi !

I would like to know if it could be possible to add this feature at some point.

Indeed, sometimes GCPs have Z-coordinate inaccurate or even missing.
Then, a point could be use as (X,Y) GCP only.

As it's not an daily usage, it could be hide somewhere.
For example after a right clic on a couple of points, we could set them as "none 3D point" and then check or uncheck each coordinate.

Regards

[Alexey Pasumansky]

Hello Yoann,

You can use very low accuracy for Z coordinate, it will almost neglect the impact of the altitude value of the corresponding point or points.

[Yoann Courtois]

I remember I saw this somewhere but...
Where can we exactly do it ? Is it also possible to set particular Z-accuracy to a particular point like we can do it for 3D accuracy ?

Regards

[Alexey Pasumansky]

Hello Yoann,

The general accuracy values can be set in the Reference pane settings dialog. You can override the general setting for each marker or group of markers by selecting them in the Reference pane and using Set Accuracy option from the Context menu. The following options are supported:
single value for all coordinates: XYZ
individual accuracy for each coordinate: X/Y/Z
individual planar and vertical accuracy: XY/Z
For the latter two options use "slash" symbol as the value separator.

[Yoann Courtois]

Oh nice ! Didn't know that point !
Thanks one more time for those quick and precise help !

Regards

[n@sk]

Hello Yoann,

You can use very low accuracy for Z coordinate, it will almost neglect the impact of the altitude value of the corresponding point or points.

"almost" may be good enough for an over-constrained bundle adjustment but, correct me if I am wrong, it will not work for a 7-parametric similarity transformation using 2 3D points and one more coordinate.

[Yoann Courtois]

Hi n@sk,

As you said you will be able to apply a 7-parametric similarity transformation using 7 known-coordinates, which could be two 3D-points and one more coordinate.
Using three 3D points in PS will give you 9 coordinates. But if you define two points with a millimeter accuracy and the last one with a kilometer plannimetric accuracy and a millimeter altimetric accuracy (1000 / 0.001 in accuracy column), you will almost* get only 7 coordinates-based transformation.

Just remember that it's the theory, but in practice the most known coordinates the better 7 parameters determination.

Regards

*(meaning at about a millionth of millimeter in the final adjusted coordinates)

[n@sk]

Hi Yoann,

In theory yes,
but last time I tried (before the transform object menu in v1.4) using 7 coords and guessing the other two
(or even worse choosing an arbitrary origin/horizontal orientation and specifying a vertical difference)
PhotoScan did not  solve a minimally constrained problem but initialised a very bad solution, in which case the small weights would/could not make it right.