Interpolating between two graphs

[justmcd]

Hi

I have scatter graphs created from reading taken from one off the books used to design crotch plates ( see attached). I have managed to do interpolation for various dt values at angles given by each graph. However I have a problem interpolating for an angle other than the one given a different degrees (eg. 53 degrees). The ultimate solution that I am looking for is to get dt value for any degree between 30 and 90 degrees.

Regards
Justmcd

[Andy Pope]

You need to do the same thing with the angles as you have done with the db values.

I extended/repeated the last column in the lookup table to avoid any errors in formula.