GCPs from Google Earth

[Oli63]

I intended to find the problem with the camera coordinates from our microdrone, which seem to be completely wrong, possibly due to a bug in our software.

For this I experimented with markers. I took their ground positions from Google Earth, which is certainly a rough measure, but to get an idea of how it works, it should be good enough.
I left away all camera coordinates and calculated the full 3d model in quality "high", which looks perfectly fine. Then I tried to set markers, following the guided approach. I know, according to the nice new "Tutorial for Beginners: Orthophoto & DEM Generation (with GCPs)" I could have used only the point cloud to set them, but this was a nightmare to find and set the markers.
When I changed the settings to WGS84 (which seems to be the coordinate system, that Google Earth uses) and klicked "Update", the "Error(pix)" jump to many 1000s of pixels. When I remove them, klicking "Update" says "Can't estimate transformation". All this seems to me being some sort of voodoo, although it should be a simple thing. Could anyone explain, what I am doing wrong?

What I wanted was to calculate back from an existing model and ground marker coordinates to camera positions and compare them with those from our drone.

Besides, what's the idea behind the removing of all marker data, just because I remove the Z/Altitude information?

Thank you for your help.
Oliver

[gEEvEE]

Could you share some screenshots? Sure that you are not inputting UTM coordinates into your data?
BTW: Google Earth is extremely inaccurate (errors of up to 10 m can easily be found) so it is NOT a tool for decent georeferencing.

Geert

[Oli63]

The coordinates from Google Earth look like 14.358476° X/East and 48.271676° Y/North. I am not an expert but I suppose this is WGS84.

No doubt, but for this simple test they should be sufficient, as the errors I get from camera positions are MUCH worse.

[Alexey Pasumansky]

Hello Oliver,

You could speed up marker placement if you build a geometry in Point Cloud quality (actually it is a Guided approach) - then, when you use Create marker feature in the photo view, the current marker will appear on other photos it is supposed to be. So you'll need only to check and refine (if needed) markers positions after all of the markers are placed.

After all the markers are correctly placed you need:
- firstly to import camera coordinates (to be able to check the errors inside PhotoScan),
- if needed convert camera coordinates into WGS84,
- uncheck all cameras in Ground Control pane,
- input marker coordinates from Google Earth in WGS84,
- set WGS84 coordinate system in Ground Control (also you can set markers accuracy to 1-10 m for example, since markers positions are not presicely teken from Google Earth),
- click Update.

[Oli63]

Thank you all for your quick answers.

Alexey, I did as you wrote. When clicking update, nothing changed. Altering marker accuracy to 10 m made phantasy numbers. With accuracy 2 m the worst "Error(m)" is still 277 m, the Error(pix) is more than 20000.
This doesn't seem plausible. For testing I only used 4 marker points. I have 168 cameras in 1 chunk. The Error(m) is in millions for each photo.
Strange. What's wrong?

[Alexey Pasumansky]

Hello Oliver,

Million meter errors usually appear when camera and marker coordinate systems mismatch.

And regarding errors for markers you have described, seems that they could be caused by mistakes in placement  - for example marker labels are confused for some photos.

[Oli63]

Ok. There seems to be my mistake.
I'm currently confused, how to tell PS, what for (markers or camera positions) I want to define (or convert) the coordinate system. There's 2 windows, one for marker and one for camera positions.
Is it controlled by clicking the Check On/Off square in the first column? What does the check do? The "Convert" button isn't always active. Why?

[Alexey Pasumansky]

Currently Convert button works both for cameras and markers, I think we will make an option to change only one type but for now they are converted at the same time, so maybe storing marker or camera coordinates in .txt file would be handy.
Checkboxes in Ground Control pane are not taken into account and coordinates are converted for all instances in pane.

Checkboxes indicates if the maker/camera is involved into georeferencing or optimization process.

Convert button is not active when Local coordinates are selected for current chunk.

[Oli63]

Just to let you all know: the confusion in my model with big errors came from a typing mistake I made when I transferred the Google Earth coordinates into PS. Alexey found it quickly. Very impressive. Your support is excellent. Many thanks!

Oliver