One of the problems with perspective is foreshortening - the inability to compare heights at different distances.
This reminds me of one of my favourite Father Ted sketches...
"ok one last time.." (showing Father Dougal some toy cows)
"these are small, but the ones out there are far away."
This is why using 3d pie charts are a bad idea - foreshortening distorts the angles, which are difficult enough to compare to begin with.
There are 3 types of 3d.
Perspective : given two objects of the same height, the further away one looks smaller.
Reverse Perspective: given two objects of the same height, the one closer to you looks smaller. This looks really confusing and surreal.
Orthographic: if two objects are the same height, they appear to have the same height irrespective of distance. This happens because the light travels in parallel lines, rather than radiating out from the observer point - you're seeing the view from an infinite distance away.
The advantage of the Orthographic Camera
Blender is capable of rendering using an orthographic camera, and these allow us to compare the heights of mountains. We don't need to concern ourself with how far away something is; this preserving of height at any distance means we can easily compare heights of two separate maps.
This profile comparison compares the heights of the Ben Nevis ridge and the terrain around Edinburgh. The high point in Edinburgh is Arthurs' Seat - to its left is Castle Rock and Corstorphine Hill. Behind it in White is the Ben Nevis ridge, looking from the NW.
The image below shows how Blender has been set up (the view is from above). The triangle on the left shows the camera's field of view. The green mesh was taken from around Edinburgh (North to the top, so North is left on the image above). The taller, white mesh is the Ben Nevis ridge.
This property is not limited to side-on views, as shown here. It also works when looking at an angle. So if you want to do a 3d rendering of a surface, orthographic view gives equal optical weight across the whole mesh.
I got an interesting challenge from Donald Noble via Twitter...
What would Ben Nevis look like if you were standing in Musselburgh, looking towards Arthurs Seat, but using normal perspective?
To answer this question, I had move both meshes so that the camera was positioned in Musselburgh (for the Lothian mesh) and Caol (for the Ben Nevis mesh). The meshes were rotated so that the camera pointed roughly in the direction of the highest point.
I used perspective view, with a FOV of 50mm. I had to make the Ben Nevis mesh slightly transparent for this to work... and here's the result...
Ben Nevis is about 5 times as high as Arthurs Seat; in perspective, only about twice as high. But interesting to compare!
Used OS Open Terrain data, crown copyright and database right 2015.