To obtain the TAS from the IAS, you need to go through all the corrections of each intermediate until reaching the TAS. This means you need to correct for position and instrument error to get the CAS. Then, you need to apply a compressibility correction to obtain the EAS. And finally, you will apply a density correction to the EAS to obtain the TAS.

The dynamic pressure is a function of airspeed and density of the air. The ASI being calibrated for ISA conditions, any other air density will indicate an erroneous airspeed on the ASI. For instance, density reduces with increasing altitude and the IAS, and thus EAS will become progressively lower than the TAS..