Interpolation is need if a Gauge is non Linear....
E.g. Flaps Gauge in B737....
The way from 0-5 on the gauge is way more like the way from 25-40 .... So here you need a Interpolation to define each point.
Lets say the Raw Value is 0-40 ..... The half would be 20 .... But in Half of the Gauge Moving range there is just 10. So its NOT Linear.... We need to calculate the single points!
We normaly use the Compass Logic here and define the SIM Value as 3600 ( 0,1° Steps)
To find out you use the TEST Mode and say for example "100" Test ( Turn by 10°) Is this the Flaps 1 Point ?
If to faar try 80 ... If not enough try 120 or so.... Simply figure out what "Output Value" You need for the Motor to hit exactly the "Flaps 1" Point on the Gauge.....
Do the same for each Position and note it an a paper.
Then you check the Sim Value.... Move Flaps in Sim to Pos 1 .... And check how the Sim Value looks like.
Lets expect Sim Value is 0 for Flaps 0 ..... 100 for Flaps 1 .... 200 for Flaps 2 and so on.
Then you use interpolation and say....
0=0 .... 100= The Value you figure out for POS1 ..... 200= Value you find out for POS2 and so on.
The interpolation finaly use the predefined Points you enter and calculate all points between.
In easy words... You say for multiple Raw Sim Values a "new" Value you like to have.
The 3600 Methode we prefer cause you can easy check the angle of the gauge with a geometric triangle (or with eyes) and you have a nerly good value in first try.
E.g. in the Picture above the Flaps15 Point is nerly 1800 ..... Cause its maybe 180° out of Flpas 0.