Hi Joe

The M'soft webcam I am using appears to have a sensor active area about 2.5mm wide That would help explain why my resolution results are a little more than 3 microns.

Noise. There is a discussion on the MEW site about astronomical photography and the need to reduce noise. Camera sensors for this application are sometimes super cooled to reduce it. No I am not suggesting this level of sophistication for a webcam. However it may partly explain why the measured value particularly the right of the decimal point is ever changing? the first two digits to the right are very important. If they are stable we get a 100 fold increase in accuracy, maybe impossible but worth striving for.

Catenary sag.
For a DIY person setting up say a router with 3 Metre rails roughly what is needed for 2.4 Metre travel, a stretched wire will sag due to gravity. Using your measured value table as a reference and given an input length the calculation could be applied and the true calculated value displayed. The weight of the wire is almost negligible the applied tension can known, say a barbell weight and needs to be input as a constant also. I am not sure how much work this would be to implement in the software, It would be very useful if it can be done.

One of the advantages of stretched wire is that it is very easy to align to high accuracy, 0.0001" positioning is fairly easy to achieve by placing a precision block at each end of the object being measured. By stretching the wire over the blocks you have very stable line parallel to your work piece.

I will run a set of tests tonight at .01mm increments. and check the residual chart. and work on the light source.

Regards
John