Difference between Ground Control Points vs Check Points ?

[PROBERT1968]

I am having a really hard time to grasp a concept about the difference in between Ground Control Points vs Check Points ? I re-read the Metashape manual 1.5.5 and still don't understand. I have google it to find some examples or pictures but still not understood it.  Is this the standard term for people in Photogrammetry world ?

Would anyone mind explain what it is and what does the difference mean ?

How do I turn it on or off and why some turn on and some off ?

Thank you

[SAV]

Hi Probert1968,

There are quite a few terms used, such as ground control points (GCPs), ground markers, control points, check points, markers .... and more. It can get quite confusing. Let's try to clarify a few things.

In Metashape, so called markers are used to georeference and scale photogrammetric models and for camera calibration purposes. Sometimes markers are also used to help the Structure-from-Motion algorithm to align cameras, but that's a special case.

Ground control points are points that you have placed/marked in the field and surveyed before acquiring the (aerial) imagery. The coordinates of these ground control points are later imported into Metashape in order to link them with the pictures. As soon as you import ground control points into Metashape they become markers.
Metashape lists all markers in the reference pane. By default they are all checked/enabled (there is a checkbox on the left next to each marker), which means they are used as reference information to control the model and camera calibration. That's when a marker becomes a check point.
If you uncheck a marker (no tick in the check box next to each marker), then this marker/control point becomes a check point. This means that it is not directly used in the photogrammetric modelling process anymore. But it's still available to check the accuracy of the model that you've generated. Basically you compare X/Y/Z values from your model to the X/Y/Z values of your measured ground control points (which are now a check points). You check the error between the model and the real world measurement, hence the name check point.

Note that if you have surveyed your GCPs with high/cm level accuracy, then you can expect to get a total error of about 1-3 times your ground sampling distance (GSD).


I hope this helps. All the best.

Regards,
SAV

[PROBERT1968]

Thank you , SAV.

The only things is HOW do I know if I need to turn on or off each marker ? I am not doing surveyor. I am doing old old old aerial photographs of the forest I am working on.

Have you been in that position before ?