First, I was wrong on the sea level compressibility effect on IAS/CAS, "rivet gun" is right.
For CAS/IAS (
I am putting these together as modern airliners usually have no mean to see the difference between them thanks to AD-computer) measurement is affected by by compressibility at Mach numbers above M0.7.
Here you find a formula for EAS-CAS conversion (scroll to bottom):
Equivalent airspeed - Wikipedia, the free encyclopedia
When you insert a pressure ratio of "1" (Sea Level) the conversion factor becomes "1". As I wrote the formula in that article (I didn't develop it, I just spread the word), I am a bit sorry for my previous post.