# [postgis-users] Dot Density idea

Jan Hartmann j.l.h.hartmann at uva.nl
Thu May 6 10:39:36 PDT 2010

```I did this long ago. Regular points won't work, you'll get all kinds of
moire patterns. I just computed random x and y values from the origin of
the grid cell within the x- and y grid-size. Can't remember having any
problems with overlapping points.

Jan

On 05/06/10 19:35, Paul Ramsey wrote:
> Even-yet-random :) nice requirement. How about just starting with a
> regular grid and then perturbing the elements randomly with a radius
> of a cell size? You can use the area of the polygon and number of
> needed points to calculate the appropriate cell size and go from
> there.
>
> P
>
> On Thu, May 6, 2010 at 10:28 AM, 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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>>>
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>>>
>> --
>> Martin Davis
>> Senior Technical Architect
>> Refractions Research, Inc.
>> (250) 383-3022
>>
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>>
>>
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```