Dot density maps

[Steve Hall]

Hi Steve,

Thanks for this - I am familiar with thematic mapping and this is a specific
request for a dot-density map alas!

I don't like them that much myself by my "client" does...  They particularly
like the way that (in MapInfo anyway) you can overlay the dot-density
thematic on top of a "regular" thematic like the one you are talking about
here.  A popular use is to thematically map a sales count of their product
using your technique here (e.g the more "green" it is, the more sales they
have) but then put something like Population "on top" with a dot-density
layer so you can, in the same map, compare your product sales against
population.

I know this is a pretty specific requirement but it's part of a potential
plan to wean some clients off of MapInfo "desktop" and onto a server-based
platform (and hopefully MapServer based).  MapInfo seem to be amongst the
very few that provide dot-density mapping.  Not even ESRI's ArcIMS does
it...

Thanks anway,

Steve

----- Original Message -----
From: "Stephen Woodbridge" <woodbri at swoodbridge.com>
To: "Steve Hall" <steve.hall at mercatorgeosystems.co.uk>
Cc: <MAPSERVER-USERS at LISTS.UMN.EDU>
Sent: Friday, February 25, 2005 4:43 PM
Subject: Re: [UMN_MAPSERVER-USERS] Dot density maps


> 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