# [postgis-users] Dot Density idea

George Silva georger.silva at gmail.com
Thu May 6 10:33:27 PDT 2010

```Using St_SnapToGrid is a way to do it. A simple algorithm could calculate
the ideal distance between points depending on the number of resulting
points and then just place them in correct place.

The way to do it is to inform attribute and the value of each dot. On the
same note, a proportional symbols representation could be done, but that can
be very tricky, since building the actual circles could give you extensive
overlap, and they should change according to scale, so I guess that could be
done by software. PostGIS could generate a view using the centroid for
proportional symbols using a point a softwares need to take care of the
representation.

George

On Thu, May 6, 2010 at 2:28 PM, Martin Davis <mbdavis at refractions.net>wrote:

> Good point about the need for even distribution of the points. That seems
> like a whole lot harder to code than simply randomly placing points in a
> polygon.  Does anyone have any pointers to algorithms for producing this
> effect?
>
> George Silva wrote:
>
>> The really big problem with dot density is that dots can overlap
>> themselves,
>> masking the real number, so if anything will be developed in this area,
>> the
>> points should be
>>
>> A) evenly distributed
>> or
>> B) randomly distributed, but with some sort of "colision" tests, so there
>> is
>> no or little overlap.
>>
>> This is a interesting idea, especially if we could make a materialized
>> view
>> with those points, which could be added to GIS software for presentation.
>>
>> George
>>
>> On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley <
>> ssufficool at rov.sbcounty.gov> wrote:
>>
>>
>>
>>> Looks nasty, but it might work:
>>>
>>> select
>>> st_line_interpolate_point(
>>>       st_intersection(
>>>               the_geom,
>>>               st_makeline(
>>>                       st_pointn(st_exteriorring(the_geom), (rand1.rand *
>>> st_npoints(st_exteriorring(the_geom)))::int),
>>>                       st_pointn(st_exteriorring(the_geom), (rand2.rand *
>>> st_npoints(st_exteriorring(the_geom)))::int)
>>>               )
>>>       )
>>>       ,rand3.rand
>>> )
>>> from insert_your_table_name_here,
>>> (select random() as rand, generate_series(1,1000) as point_number) as
>>> rand1
>>> JOIN (select random() as rand, generate_series(1,1000) as point_number)
>>> as
>>> rand2
>>>       ON rand1.point_number = rand2.point_number
>>> JOIN (select random() as rand, generate_series(1,1000) as point_number)
>>> as
>>> rand3
>>>       ON rand2.point_number = rand3.point_number
>>> WHERE st_geometrytype(
>>>       st_intersection(
>>>               the_geom,
>>>               st_makeline(
>>>                       st_pointn(st_exteriorring(the_geom), (rand1.rand *
>>> st_npoints(st_exteriorring(the_geom)))::int),
>>>                       st_pointn(st_exteriorring(the_geom), (rand2.rand *
>>> st_npoints(st_exteriorring(the_geom)))::int)
>>>               )
>>>       )
>>> ) = 'ST_LineString'
>>> AND oid = 5030 /* Enter your own OID here */
>>> limit 100
>>>
>>>
>>>
>>>
>>>
>>>> -----Original Message-----
>>>> From: postgis-users-bounces at postgis.refractions.net
>>>> [mailto:postgis-users-bounces at postgis.refractions.net] On
>>>> Behalf Of Martin Davis
>>>> Sent: Thursday, May 06, 2010 8:56 AM
>>>> To: John Abraham; postgis-users at postgis.refractions.net; Martin Davis
>>>> Subject: Re: [postgis-users] Dot Density idea
>>>>
>>>>
>>>> I was thinking the same thing!
>>>>
>>>> strk wrote:
>>>>
>>>>
>>>>> ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting
>>>>> function indeed. Sounds like a good job for GEOS/JTS.
>>>>>
>>>>> --strk;
>>>>>
>>>>> On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote:
>>>>>
>>>>>
>>>>>
>>>>>> One of the things I miss about using ESRI's GIS is the
>>>>>>
>>>>>>
>>>>> ability to do
>>>>
>>>>
>>>>> dot-density maps.  Within a polygon, the number of dots is
>>>>>>
>>>>>>
>>>>> proportional to a value, and the dots are randomly placed.  I
>>>> find it useful to be able to present several data values at
>>>> once (e.g. blue dots for population, red dots for employment).
>>>>
>>>>
>>>>> I also find that it is a more intuitive way of scaling for
>>>>>>
>>>>>>
>>>>> zone size
>>>>
>>>>
>>>>> than dividing the value by the area of the zone.  That is,
>>>>>>
>>>>>>
>>>>> the count
>>>>
>>>>
>>>>> of the dots represents the actual number, but the density
>>>>>>
>>>>>>
>>>>> of the dots
>>>>
>>>>
>>>>> represents the density of the number.  So I don't have to decide
>>>>>> whether to divide the value by the area of the polygon to plot
>>>>>> density: both the absolute number and the density are
>>>>>>
>>>>>>
>>>>> easily visible.
>>>>
>>>>
>>>>> Since my open-source GIS viewing systems (mostly QGIS and
>>>>>>
>>>>>>
>>>>> Mapserver)
>>>>
>>>>
>>>>> won't plot dot-density, I've done without.
>>>>>>
>>>>>> But today I realized that I can build these on the server
>>>>>>
>>>>>>
>>>>
>>>>
>>>>> can generate random points within the bounding-box of the polygon,
>>>>>> throwing out those that aren't contained within the polygon,
>>>>>> repeating until I have enough.  Then I can save these points as a
>>>>>> separate layer, and display this layer using almost any desktop or
>>>>>> web based viewer!
>>>>>>
>>>>>> Has anyone done this?  Can I do it in SQL or do I need to write
>>>>>> something in PL/pgsql?
>>>>>>
>>>>>> --
>>>>>> John Abraham
>>>>>>
>>>>>> PS I just bought the Postgis In Action book; enjoying it so far.
>>>>>> _______________________________________________
>>>>>> postgis-users mailing list postgis-users at postgis.refractions.net
>>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>> --
>>>> Martin Davis
>>>> Senior Technical Architect
>>>> Refractions Research, Inc.
>>>> (250) 383-3022
>>>>
>>>> _______________________________________________
>>>> postgis-users mailing list
>>>> postgis-users at postgis.refractions.net
>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>
>>>>
>>>>
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>>>
>>
>>
>>
>>  ------------------------------------------------------------------------
>>
>>
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>>
>
> --
> Martin Davis
> Senior Technical Architect
> Refractions Research, Inc.
> (250) 383-3022
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>

--
George R. C. Silva

Desenvolvimento em GIS
http://blog.geoprocessamento.net
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