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Author Topic: v1.6.4 and previous - ground altitude vs capture distance (Reference Settings)  (Read 1918 times)

andyroo

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I noticed that the manual (Page 25, Alignment Parameters/Reference Preselection) says,

"For oblique imagery it is necessary to set Ground altitude value (average
ground height in the same coordinate system which is set for camera coordinates data)
in the Settings
dialog of the Reference pane to make the preselection procedure work efficiently."

But the dialog box in Reference/Settings calls the value I think it is referring to "Capture distance"

Which value is the dialog box expecting? For example if I am flying at an altitude of 1000 m above sea level and a 45° oblique angle, the capture distance is 1414m, but if I am imaging land at sea level, the ground altitude value is 0

andyroo

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Anybody have any idea on this? Am I missing something obvious? wouldn't surprise me given the way the year's been going...

Arie

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Hi andyroo,
just as a quick thought: Entering the altitude above sea level would not be useful for the actual purpose this value serves. Quote from above "... to make the preselection procedure work efficiently."
For estimating which overlapping images could possibly match, it would make a lot more sense to have the actual distance from image to surface (as well focal length etc.).
Does that make sense? It definitely has been a wild year...
Cheers!

andyroo

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Hi Arie!

That's exactly why I was confused by the dialog. The manual and the software dialog say different things.

The dialog says "capture distance" which is something I would have to calculate for oblique photos, but I know the average ground altitude (relative to the ellipsoid) roughly over the area where we're flying, and the plane altitude is in the GPS data, so I could easily put in a value for ground altitude (would be even cooler if I could put in a rough range, since we fly cliffs sometimes)

Arie

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Ah, it took a while but I finally understand your predicament. Excuse my naive input!
Since the value is used to preselect images, have you tested the ones stated in your example above (or similar ones with a large variability) and checked the matching results? The amount of matches should be a good indication for the type of input expected, or?

Alexey Pasumansky

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Hello andyroo,

The idea of "capture distance" is quite simple. If it is not set, the Reference preselection consider the source XYZ coordinates from the Reference pane when checking for the camera neighbors.

When the Capture Distance parameter is used and camera orientation angles are loaded to the Source tab of the Reference pane the preseelction is based on the new 3D points calculated as original 3D point + vector in the direction of the camera view with the length equal to the input Capture Distance value.

So the use of the Capture Distance parameter may be important for aerial surveys captured with highly oblique angle, when the Reference preselection should be used (thousands of images).

P.S. the information in the Manual seems outdated and describes the meaning of the Ground Altitude parameter that is no longer available. We'll try to fix the description soon.
Best regards,
Alexey Pasumansky,
Agisoft LLC

andyroo

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Thanks Alexey. I thought that was probably the case, but was confused by the manual.

mcstieg

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Hello andyroo,

The idea of "capture distance" is quite simple. If it is not set, the Reference preselection consider the source XYZ coordinates from the Reference pane when checking for the camera neighbors.

When the Capture Distance parameter is used and camera orientation angles are loaded to the Source tab of the Reference pane the preseelction is based on the new 3D points calculated as original 3D point + vector in the direction of the camera view with the length equal to the input Capture Distance value.

So the use of the Capture Distance parameter may be important for aerial surveys captured with highly oblique angle, when the Reference preselection should be used (thousands of images).

P.S. the information in the Manual seems outdated and describes the meaning of the Ground Altitude parameter that is no longer available. We'll try to fix the description soon.

A question about cameras that don't write orientation angles:

Does Metashape benefit from an expected pitch and roll angle?
In my data sets all the cameras have nearly the same pitch and roll angle. only yaw changes.
So if I would manually write expected pitch & roll angles to the set - Does that speed up the process?

Thank you very much!  ;D