Re: Weight
You are quite correct that the force is different in different places. It's standard practice to calibrate a scale in the location where it's installed, partly for this very reason. (Calibrated using kg weights, of course, so that the mass shown on the scale is accurate.)
The real challenge is getting force-weights (like a 50-Newton weight) calibrated properly at an off-site lab. You have to specify your local gravity so the lab adjusts the weight to be 50 N at YOUR facility, not theirs!