Maurice,
please don't find my post offensive... I just want to say that sometimes investing into abilities and knowledge is more important than getting a good tool
"Spatial comprehension" I understand something like this
http://www.fibonicci.com/images/spat...-10.png?7e35a6
Or first image below. Imagine you would get a task to draw section C-C.
I'm graduated engineering and during my studies the was a module called "Engineering Graphics" where we actually were learning so called by myself - spatial comprehension. There were different tasks (similar to second picture) or more theoretical such as to draw how would look like line in 3-rd projection (there are given 2 projections) that comes through surface. The surface also is tilted in space.