I really like this idea!
Yes, I want to know about those orc population densities.
No, the voices in my head say they have nothing to do with psi.
Pretty much all the maps created by the guild are some form of Reference map, displaying a broad range of general information. In real life, the vast majority of maps created are highly specific Thematic maps which display one or more specific data sets, usually in order to fulfill some particular purpose (A map of the distribution of an invasive species in order to convince someone to take action to contain it.)
So, my suggestion for a challenge is to create a thematic map. It might be a map of Orc Population Density, A comparison of psionic background levels against instances of schizophrenia, or a map of magic levels, transportation systems, and local legal status of mages as part of the process for selecting the location for a new magic academy.
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*Scratches head and mumbles* "What........
Even if I am correct, I would still advise you to follow your heart. This whole Cartographers Guild thing, it seems to me, is more about passion and skill and learning new techniques than it is about definitions and rules.
Kingdom Of Shendenflar Campaign Setting (WIP)
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Great idea. Since a bunch of us don't do a lot of them, "any thematic map" might be a good enough focus. But even the Wikipedia explanation leaves a lot of room for judgment. I mean, were the April 2010 contest entries thematic? Worldwide distribution of beers.... When I think of thematic maps there's something more than just spatial distribution - else a regular political map would be "thematic, showing distribution of population"....
The dasymetric map shown as one of five types of thematic maps in that Wikipedia article looks a lot like a climate zone map, but it shows values along a range - plant hardiness being a sort of linear progression. So would a typical climate map count?
Well as with so many things, Thematic and Reference maps can get a bit "fuzzy" around the edges. For one thing, many Thematic maps have a Reference map as a "base map" underneath.
Dasymetic maps take a bit of thinking you wrap your head around. They are sort of recursive in that you are defining regions to aggregate data based on the data you are aggregating. That's what makes them difficult and computationally intensive to produce, but it's also what makes them so good at representing a dataset.
A biome map is more of an abstract categorization based on a wide range of features rather than an aggregation of a numerical data set which is what a Dasymetric map is doing. Think of a dasymetric map as being like a census map where you redefine the census boundaries to try to minimize the variation in population density within a region, for that specific census after gathering the data.
A categorization map is certainly be a thematic map, but they are comparatively simple and so don't have a fancy name. Not that it's easy to make a good categorization map if you have a large and complex set of categories but it's not so much a technical problem as a design one. Geological maps for instance are infamous for their dense categorizations and arcane symbology.