Thus, you will find the base of a CTA is often 1200 AGL, so that an aircraft can fly 500' AGL minimum and still be 700' clear below the CTA. This is not limited to US.
In this case, what is the CTA lower limit marked on charts? 1200FT AGL or 1500FT AMSL? Assuming in this case the terrain height 1200 AGL = 1500 AMSL of course.