Dot density maps

[Sean Gillies]

Or you could use a range of symbols to fill the polygons with
increasing size of the symbol envelope for decreasing density.
Theoretically, that is, I haven't tried it.

There is also an enhancement bug filed in Bugzilla for style-level
transparency which would let you use ranges of solid fill transparency
for your classes.  I've got a customer that will need this eventually,
so I might start on it this spring or summer.

cheers,
Sean

On Feb 25, 2005, at 9:43 AM, Stephen Woodbridge wrote:

> 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
>
>
--
Sean Gillies
sgillies at frii dot com
http://users.frii.com/sgillies