IAS Vs CAS
The IAS is not the actual speed through the air even when the aircraft is at sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity). The IAS needs to be corrected for known instrument and position errors to show true airspeed under those specific atmospheric conditions, and this is the CAS. Despite this the pilot's primary airspeed reference, the ASI, shows IAS (by definition). The relationship between CAS and IAS is known and documented for each aircraft type and model.
Read more about this topic: Indicated Airspeed