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I need some advice about how to improve the visualization of cartographic information.

User can select different species and the webmapping app shows its geographical distribution (polygonal degree cells), each specie with a range of color (e.g darker orange color where we find more info, lighter orange where less info).

The problem is when more than one specie overlaps. What I am currently doing is just to calculate the additive color mix of two colors using http://www.xarg.org/project/jquery-color-plugin-xcolor/

As you can see in this image, the resulting color where two species overlap (mixed blue and yellow) is not intuitive at all.

enter image description here

Someone has any idea or knows similar tools where to get inspiration? for creating the polygons I use d3.js, so if more complex SVG features have to be created I can give a try.

Some ideas I had are...

1) The more data on a polygon, the thicker the border (or each part of the border with its corresponding color)

2) add a label at the center of polygon saying how many species overlap.

3) Divide polygon in different parts, each one with corresponding species color.

thanks in advance, Pere

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Do you know the maximum number of species that may overlap a square? –  Baltok Nov 20 '12 at 20:32
    
noops... to keep map (and tooltips) readable i could restrict to 10 or 15 different species. –  user1249791 Nov 20 '12 at 20:41
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With so many potential overlaps in one square, I think your idea of displaying the number of overlapping species may be the cleanest approach. Alternatively, color distinctions may work if you assign a specific color for the number of overlaps and have a legend. –  Baltok Nov 20 '12 at 20:45
    
How many different species are there total in the data? –  Dan C Nov 20 '12 at 21:03
    
no, i don't know how many different species are there total in the data. User can choose from different hundreds of species, which can overlay (or not) –  user1249791 Nov 21 '12 at 7:36
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3 Answers

For multiple species, pie chart polygon symbology could work, but these kinds of displays can get pretty busy.

I noticed that the mapping center has 77 posts on the topic of overlapping polygon displays:

http://search.esri.com/results/index.cfm?do=mappingcenter&h=10&ho=0&q=overlapping+polygons

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It sounds like your programming skills are accute enough that you might use something like.
Calculate the number of species and either devise a symbology for that or use hatch overlay which might be the angle of the hatch depicts the species number.
for instance angle 10 means 2 species, 20 means 3, and so on.

The label could be used with coding such as 24 means species 2 and species 4, 248 = species 2, 4 and 8. These examples don't work exaclty like I was thinking but I know I have seen it where the number can only mean one set of values.

These are some ideas I have just quickly for "inspiration". There was a question on gis stackexchange which is one of the top viewed questions and might provide more reading and insight.

That is if reading (time) is an option.
What is on your bookshelf
And then there is the hope that whuber will help with an answer.
Or you could just search for answers by whuber and get quite a bit of inspiration.

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I like hatching when I make maps for my enemies.

Other than that, I once worked on a project where we had the following problem to solve in under an hour:

  • We needed to display overlapping polygons with several bit (Y/N) attributes.
  • We wanted to avoid obscuring other coincident data in the map, i.e. Points, labels
  • We needed the end user of the printed exhibit to be able to look at the map and quickly identify whether the area was confirmed (Y) for a myriad of attributes (in this case, 4) without having to search for a complicated interpretation of symbology.

Legend

The solution we came up with worked rather well considering the time constraints on our cartographic design. We relied on the most natural thematic renderer: color. The user could immediately identify a polygon as a positive or negative for each of the four values (green, yellow, orange, red).

Overlapping polygons

The way we accomplished this was by using alternating dashed outlines with a negative offset and a hollow fill.

Green: Offset: -4, Cartographic Line Template: xxxxoooo
Yellow: Offset: -4, Cartographic Line Template: ooooxxxx
Orange: Offset: -2, Cartographic Line Template: xxxxoooo
Red: Offset: -2, Cartographic Line Template: ooooxxxx
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Great answer! I edited your post to improve readability and removed your signature, per the help. –  Paul Nov 7 '13 at 18:27
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