Dot density maps

[Stephen Woodbridge]

OK, that makes sense and I cold use something similar. My thought on
this and I'm not sure if this is what Sean Gillies was referring to in
his post. is that you could have the polygon layer represented twice in
the mapfile, once for each thematic layer< in the underlying layer you
would use straight color fill for example for the sales and then on the
upper layer (population) use a symbol where the dot density is
represented by the symbol.

You could have multiple symbols with varying dot densities or you could
use various hatching patterns/color and the underlying polygon color
would show through the hatching or dot patterns.

I am assuming that if the symbol image is defined with a transparent
background then the underlying color will in fact show through. Any
developer care to comment on this?

-Steve W.

Steve Hall wrote:
> 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
>
>
>