The last correction to be applied is parallax.  So far the observer has been able to find the heavenly body’s centre above the horizontal plane.  All that is left is to allow for the earth’s radius.  In the drawing parallax is the angle OXC.  As you can see the true altitude (angle XCR) is equal to the sum of the apparent altitude (angle XOH) and parallax (angle CXO).

Correction for parallax.