[postgis-users] Dot Density idea

Ben Madin lists at remoteinformation.com.au
Thu May 6 18:59:08 PDT 2010 

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
>>>> 
>>>> _______________________________________________
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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
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