Question Regarding Relative Motion


I have my InSAR los velocity mm/year over my study region. I want to compare my InSAR results with GPS and model output but both results are in vertical velocity rate mm/year not los converted. From this form some post, I converted the vertical velocity to los . As we know InSAR measurement is relative with respect to reference point , my question is when I convert GPS and model vertical velocity to los should I take InSAR reference point motion for making GPS and model los velocity to relative or is there any other way to convert GPS and model absolute velocity to relative. Please help me in details.

Note : I know this is a basic question, sorry for that.


I think both are calculated from the same datum plane right (WGS84).