The altitude of the sun is the angle between the horizontal line passing at yours eyes level and the light ray line from the sun.
At sea, this horizontal line can be shown by the horizon line, but in this case and because you are upper the sea level, this “horizontal” line is not really horizontal and you must to take in account the distance between the sea level and your eyes. This is the “depression” error.
When you use an artificial horizon, the real horizontal line pass exactly at yours eyes level, and you have no depression error to take in account.
Remember one thing : all the rays from the celestial body are parallels, and all the horizontal lines are parallels too. So the angle between these parallels is always the same, no matter your own altitude above the ground.