[mapserver-dev] future developments regarding symbology and thematic mapping

[Steve Lime]

Hi Karsten: I see nobody responded to your request, probably because there's
not much definite to say at this point about any of your questions. I've added
comments inline. Other developer types, please feel free to correct me or add
to the discussion.

Steve

>>> On 3/10/2008 at 8:24 AM, in message
<000801c882b2$0479cd60$bb41ad83 at igfdomain.igf.uniosnabrueck.de>, "Karsten
Hoffmann" <khoffmann at igf.uni-osnabrueck.de> wrote:
> Hi developers,
> 
> at the German speaking FOSSGIS conference in April I'll give a 20 minutes
> presentation about map symbols, diagrams and cartographic design with
> MapServer 5.0. I would also like to tell a little bit about future steps. In
> my master thesis two years ago I already dealt with the construction of
> thematic maps with MapServer 4.6.
>  
> I read the release plan for the version 5.2
> http://mapserver.gis.umn.edu/development/release_plans/mapserver_5_2/release 
> _plan/ and I have some questions concerning future developments.

This document needs to be cleaned up a bit. It looks to be carrying on the wishlist
from 5.0, which is a good place to start but some other potential additions are 
definitely missing.

> 1.) Could you tell me a little bit more about range support for thematic
> mapping and auto classification? Will this be a feature where the user can
> choose from different classification methods (like equal intervals,
> quantiles or natural breaks)  to create different classes automatically?
> This would be a nice and really useful feature for thematic mapping. 

Auto classification goes beyond what is currently supported. I too think it would be
a useful addition but it's not trivial. To do this efficiently we'd probably need access
to some sort of a pre-computed stats file since you wouldn't want to do those
computations on-the-fly. Then MapServer could use that input to develop 
classes in much the same way the current support does it, but automatically. I've 
always wondered where such a tool, stats generation, belongs. GDAL/OGR might
be one place.

> 2.) Since version 4.6 there is the possibility to create a continuous range
> of colors to map attribute values
> (http://mapserver.gis.umn.edu/development/rfc/ms-rfc-6/). But it seems to be
> still experimental and it's not officially documented. What are the plans
> here? For thematic mapping it would be more useful to have an automatic
> creation of a color range for classified data, so that each class gets a
> color. 
>
> For creating thematic maps with Mapserver I use the ColorBrewer
> (www.colorbrewer.org <http://www.colorbrewer.org/> ) to find good color
> ranges for my classified data. I select a color range which corresponds to
> my classes and I write the RGB values from the ColorBrewer in my mapfile. 
> I'm not sure if it's possible but perhaps the color ranges of the
> colorbrewer could be implemented in mapserver so that a user can directly
> select a color range for his classes.

There were many opinions and RFC-6 and I think we scared the proposer away. It
has recently surfaced again when Thomas was talking about plans to support
within feature alpha channel gradients. I countered with ideas about using the
gradient object that exists now (perhaps with some extension) to configure
within feature (alpha/color gradients) and across feature (see your question 1)
gradients. I'm not quite sure where that stands.
  
> 3.) In version 5.2 it will be possible to bind the size of pie charts to an
> attribute (https://trac.osgeo.org/mapserver/ticket/2136). This would also be
> useful for point symbols as simple diagrams without slices or bars. The idea
> is to have a symbol, let's say a filled circle or square, which varies
> continuously in size. The size is bound to an attribute for example the
> absolute number of habitants of different countries.

Thomas would have to comment, but it would seem doable.
 
> 4.) Are there any other plans concerning the development of symbology or
> thematic mapping? What about legends? Regarding the charts (which vary
> continuously in size) the map reader also wants to know what the size of the
> bars or the diameter of the pie charts means (which height is how much?). In
> these charts you have two pieces of information, one piece of information is
> transferred by different colors and another by different heights. Attached
> you can find two pictures as an example (for pie and bar charts). I have
> drawn these legends on my own and use them in my Mapserver legend as a kind
> of keyimages. Concerning the information of the height you should visualize
> in the legend the minval and maxval and perhaps another value in between.

As far as I know there are no concrete plans to extend legend drawing like this. I
imagine it's possible but it isn't a high priority. Patches gladly accepted! ;-)
 
> Best regards,
> Karsten Hoffmann
>  
> PS: You can find more examples on thematic maps with mapserver (version 4.6)
> and a tutorial about how to create them here (only in German):
> http://www.geog.fu-berlin.de/de/Karto/umn_karten/main.shtml (You'll find the
> examples in the lower middle section under "Anwendungsbeispiele Berlin".)