The last issue I want to address in my 3D series is the problems of perspective. I find this issue particulary challenging.
“Same with estimating sizes of oblique-viewed 3D domes for proportional symbols. The problem is further magnified when the data is re-projected to an Earth globe view making the task of estimating heights/sizes of the polygons even harder (since the user has to mentally compensate for the curvature of the earth). In short their concern is we are sacrificing accuracy for eye candy.” (Sean Gorman)
Yes, the use of proportional symbols on a 3D globe raises some serious questions. Here are my 3D Collada domes of world population:
At least, the dome shape makes it possible to calculate the volume of each object, as the volume should represent the statistical value. I'm not sure how to scale irregular objects properly, - like a 3D person.
The main issue, as stated by Sean above, is how the user are going to estimate the volume of the domes when seen in perspective. The size of the domes are determined by two factors: the size of the population and the "distance" from the point of view. This makes it hard to compare 3D objects.
One solution is to use a non-perspective projection (orthogonal projection) which makes it easier to make cross-scene comparsions (Shepherd, 2008). Using proportional images with the KML Icon element might be an option.
Download KMZ. These symbols keep their relative size when you spin the globe. But what if the user expects the symbols to be scaled as the domes? If I overlay the two symbols it looks like this:
The result is clearly different from a viewer's perspective! Is it possible to do proportional symbol mapping accurately on a 3D globe, or should it be avoided? I'm not sure.
Shepherd, I. D. H., 2008, “Travails in the Third Dimension: A Critical Evaluation of Three-dimensional Geographical Visualization”. Book chapter in "Geographic Visualization: Concepts, Tools Applications".