[postgis-users] Dot Density idea

Andrew Sheedy drew at olvec.net
Sun May 9 21:27:17 PDT 2010 

That is so cool

On Sat, May 8, 2010 at 11:30 PM, Jan Hartmann <j.l.h.hartmann at uva.nl> wrote:

> How about this site?
>
>
> http://www.johndcook.com/blog/2009/03/16/quasi-random-sequences-in-art-and-integration/
>
> Jan
>
>
> On 05/07/10 03:59, Ben Madin wrote:
>
>> Martin,
>>
>> Assuming you wanted feedback - from an epidemiologists perspective, I like
>> the first one - Complete Spatial Randomness is an important concept to avoid
>> engendering a perception of order or clustering, and I'm thinking as long as
>> the relative 'dot-density' is correct for each polygon, the CSR approach is
>> less like to lead people to think the dots are located on significant points
>> - in the context of obscuring actual locations etc this is important.
>>
>> If you want it more regular, then maybe you don't need to randomise it -
>> or maybe this just means the function needs a user parameter to be able to
>> set the level of randomisation - From 0 (completely ordered) to 1
>> (completely Random) (... to 2.5 completely clustered?)
>>
>> Just my thoughts - also for my money, I wouldn't change dot sizes. very
>> confusing.
>>
>> cheers
>>
>> Ben
>>
>>
>>
>> On 07/05/2010, at 6:11 , Martin Davis wrote:
>>
>>
>>
>>> Check out this blog post for some images of different kinds of random
>>> point fields:
>>>
>>>
>>> http://lin-ear-th-inking.blogspot.com/2010/05/more-random-points-in-jts.html
>>>
>>> Martin Davis wrote:
>>>
>>>
>>>> Sounds like it could work - with maybe a bit of fiddling to deal with
>>>> cases where the grid cells overlapped the polygon only slightly?
>>>> Random perturbation by cell radius can still result in some points being
>>>> very close together.  (And I think this would also be an issue where only a
>>>> small part of each grid cell overlapped the polygon).  This may or may not
>>>> be desirable.  Perhaps a further check could be made to reduce the radius
>>>> for points where this occurs.   Or maybe some sort of simulated annealing
>>>> process could be use to push the points into a more even distribution.
>>>>
>>>> M
>>>>
>>>> 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
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>> instead.  I
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> 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
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> postgis-users mailing list
>>>>>>>> postgis-users at postgis.refractions.net
>>>>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> ------------------------------------------------------------------------
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> 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
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>> _______________________________________________
>>>>> postgis-users mailing list
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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
>>>
>>>
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