Mostly I agree with Mark, he mentions a number of issues about the use of 3D thematic maps that I've already written about:
- You can't see all the countries on a virtual globe at once - a traditional projection maps is better
- The prism for once polygon can obscure another
- Values of 3D polygons are difficult to read off
A partial defense of 3D thematic maps: Since I first commented on the use of 3D in thematic maps I've warmed a little to to the idea. If you are dealing with a country that is able to be viewed all at once (like the UK) you could represent values for counties as fixed width columns. This would reduce the occlusion problem and would also stop people mixing up volume/height of the prism with the variable being displayed: its clearly related to height. As for reading off exact values, you can set up polygons on the ground which pop up a balloon with the value when rolled over with the mouse. Also, you could color the columns to relate to a color key which would allow quick 'actual value' reading without needing mouse movement. Having done some background reading on vision recently I suspect that this sort of display may be more memorable to non-expert map readers especially if there is some sort of large outlier value (e.g. if the variable is 'number of millionaires' in each county in the UK then there would be far more millionaires in greater London county than anywhere else).
Test it!: However, the above is only a suggestion, overall I agree with Mark that we have a workable system for displaying thematic maps now (2D color) and I whole heartedly endorse his point:
"...most importantly, do some user testing before presenting a new technique as the best thing ever: It’s how research works and why it is important."That being said. I do think that he's missed a point with the title of his post:
"Virtual Globes are a seriously bad idea for thematic mapping"and his last paragraph:
"So what things are Google Earth (and other Virtual Globes) good for? The consensus around here is (1) to engender, quite powerfully at times, a qualitative “sense of place” or “immersion”; (2) for virtual tourism (e.g., sit on top of Mt Everest) or virtual architecture/planning; and (3) to perform a kind of viewshed analysis and see what can and cannot be seen from locations (line-of-sight). All of those are inherently 3D-map reading tasks in which the immersive, 3D nature of the map is important. By comparison, population data (one number per country) is NOT inherently 3-dimensional and is only made to suffer when dressed-up in prism maps and 3D figurines."Thematic maps in Google Earth: I agree that representing population data on a country scale is not well represented in a virtual globe but I think that other traditional color based thematic maps can be usefully inserted into virtual globes for several reasons:
- For a small scale thematic map a virtual flight from altitude into the study area conveys information about scale, orientation, location and possibly altitude with the minimum of cognitive load.
- Draping a thematic map over topography can be a useful visualisation e.g. polygons showing erosion rate draped over a range of mountains, by eye you could then relate erosion to slope.
- Users can toggle layers on and off to see how geographical elements below relate to a thematic map and fly into areas of interest. E.g. Using a thematic map showing rock temperature at a depth of 10km the user identifies hot areas and then flies in to see how these areas relate to volcanoes in the region.