I'm not an expert. I only do Architecture for my personal Project. So I try to understand what a real Architect want to achieve. So please excuse when I say something totally silly throughout this thread
Lets say I have imported some Site data into FreeCAD like this (Not a real import. only create a quick Draft wire for the showcase)

- Import_Houseplacement.png (11.38 KiB) Viewed 1050 times
The red rectangle might be the Position of the house and the Compass Points to the true North. Meaning when you look on the site on a map, this is where North is on the map. (I assume that model North is the positive Y Axis. Is this correct?)
Now the house is not ideal to draw because I can not use the grid or any horizontal constraints in a Sketch because the whole house is rotated against the global coordinate System.
So I would like to rotate the whole site, so that the house can be build easily.

- Site_Rotated.png (14.19 KiB) Viewed 1050 times
Now we can see that the real North also rotated about 7 Degrees in this example. And the angle between the Positive Y axis and the Compass North is the declination angle.
So every time i rotate the site, I have to adjust the declination angle manually.
@yorik for me it would make sense to calculate the declination property based on the site Rotation and Compass roation properties. So I have to set the Compass Rotation once when I Import the Site geometry and when I rotate the whole Site the property will adjust accordingly. (But maybe I am wrong with my previous assumptions, then this might be complete nonsense

)
regis wrote: ↑Tue Mar 05, 2019 9:28 pm
And when it's time to print sheet, we assign it back to the true north which takes the real coordinates of the site and makes the drawing ready for print.
Maybe you can give me a example of what should happen when one switches between true north and model north
