@thomas.engel - My explanation was somewhat confusing. I’ll try to clarify it.
I’m not sure what boundaries you’ve finally chosen, so I tried to explain everything using the 2720920,1256200 to 2721920,1257200 corners that you mentioned a while back. I used these numbers to merge the two XYZ files into a 499 x 499 grid (1000m x 1000m with gridpoint every two meters). As you know, this created the grid in SketchUp at 2720921m in X and 1256201 in Y.
I selected the grid and used the move tool to move it a few meters to the left (West or -X) and then immediately typed 2720920 and hit enter. This jumped it in X the distance required. Then I moved it a few meters down (South or -Y) and typed in 1256200 and hit enter. This positioned the lower left corner of the terrain just off the origin at 1m in X and 1m in Y.
Your XYZ data is in CH1903+ x/y coordinates which is in meters. The KML locations for each building are expressed in degrees latitude and longitude. I used this converter to calculate the origin (CH1903+ 2720920, 1256200) to degrees:
This gave me:
These are the values I present when you run the plugin. This is the origin of your SketchUp model in degrees. They are subtracted from the KML locations (also in degrees) and then multiplied by the lat/long scale factors to locate the DAE files in the model.
As you probably know, the number of meters in a degree varies with your location. At the equator, traveling 0.01 degree north/south is about the same distance as east/west travel. However, as you move north or south away from the equator, the distance you travel east/west becomes shorter and shorter while the north/south distances remain about the same. I used this calculator to come up with a scale factor for north/south and one for east/west measurements:
Using these initial values resulted in the buildings being in the right neighborhood, but clearly out of place. After a couple of iterations, I settled on the values in the plugin as a suitable “fudge” factor. You can tweak these to control how accurately the KML import overlays your terrain import.