Dot density maps

[Stephen Woodbridge]

Steve,

The goal of this algorithm is to in effect shade the polygon so that it
is darker if you have more hits in it. AKA: thematic mapping. You can
achieve the same idea by coloring the polygon based on a color
ramp/steps where the color assigned is based on the number of hits the
polygon gets.

So if you had an attribute with the number of hits assign to each
polygon, you could create a mapfile to color the polygons based on:
LAYER
   TYPE POLYGON
   ...
   CLASS
     EXPRESSION ([attribute] < value1 && [attribute] >= value2)
     COLOR R G B
     ...
   END
   CLASS
     EXPRESSION ([attribute] < value2 && [attribute] >= value3)
     COLOR R2 G2 B2
     ...
   END
   ...
END

If you want to do this with a JOIN then you should look at OGR ODBC
connection or using PostGIS.

-Steve W.


Steve Hall wrote:
> Hi all,
>
> Sorry about the delay in getting this out - I don't have access to
> MapInfo all the time....
>
> Attached is a sample image of the kind of thing I mean.  This is a
> map of Total Population by Postcode for Melbourne, Aus.  Each dot
> represents 25 people.  The dot placement within postcode boundary
> (the red line) is random (i.e the dots location within the polygon is
> meaningless) but number of dots is highly relevant.
>
> The maps can be useful in this kind of context to give an impression
> of "density".
>
> Thanks
>
> Steve
>
>
> ----- Original Message ----- From: "Steve Lime"
> <steve.lime at dnr.state.mn.us> To: <MAPSERVER-USERS at lists.umn.edu>;
> <steve.hall at MERCATORGEOSYSTEMS.CO.UK> Sent: Thursday, February 10,
> 2005 2:51 AM Subject: Re: [UMN_MAPSERVER-USERS] Dot density maps
>
>
>> I'm not familiar with the term, however if you post a sample map
>> there may well be a way...
>>
>> Steve
>>
>>>>> Steve Hall <steve.hall at MERCATORGEOSYSTEMS.CO.UK> 02/09/05
>>>>> 2:11 PM
>>>>>
>> Hi all,
>>
>> Has anyone ever done any dot-density mapping using MapServer?  For
>> those familiar with MapInfo i'm looking to replicate something like
>> the dot-density maps found in that product.  I can't seem to find
>> any reference to it in the docs....
>>
>> Many thanks,
>>
>> Steve Hall