[postgis-users] Amoeba Hulls

[Mike Leahy]

Hey there,

If you can determine the closest 80% of points by whatever criteria, 
wouldn't you be able to use the convexhull() function?  I just tried 
this, and it looks okay to me:

testdb=# select astext(convexhull('MULTIPOINT((0 1),(0 0),(1 0),(1 
1))'::geometry));
              astext
--------------------------------
  POLYGON((0 0,0 1,1 1,1 0,0 0))
(1 row)

I did the same thing after creating a points table with the separate in 
individual records with the same overall coordinates in the multipoint 
example above, and it worked okay too:

testdb=# select astext(convexhull(collect(p))) from testpoint;
              astext
--------------------------------
  POLYGON((0 0,0 1,1 1,1 0,0 0))
(1 row)

It wouldn't be too hard to modify this to work using where condition 
that filters out the records of interest...or maybe on a saved view.

The only problem is that it wouldn't produce a nice curvy polygon 
outline like in the sample William provided.

Regards,
Mike

Paul Ramsey wrote:
> So my guess is that you use drive-time to segment your population of 
> customers relative to the store into the "nearest X%" and then draw a 
> "shape" around that cloud of points.  And drawing the shape is the "fun" 
> part.
> 
> William Andersen wrote:
>> It appears to be the same as this functionality in Business Analyst
>>
>> http://www.esri.com/software/arcgis/extensions/businessanalyst/about/customer-market.html 
>> <http://www.esri.com/software/arcgis/extensions/businessanalyst/about/customer-market.html> 
>>
>>
>> I thought this was done using some older version of Arcview, but i'm 
>> not very familiar with esri's offerings.
>>
>> Will
>>
>> On 7/5/06, *Paul Ramsey * <pramsey at refractions.net 
>> <mailto:pramsey at refractions.net>> wrote:
>>
>>     This wouldn't be in vanilla arcview, was it in Network Analyst? 
>> The top
>>     80% of points by drive distance might yield this shape.  Finding the
>>     points would be straightforward, and then the hull building would 
>> be the
>>     hand-waving part.
>>
>>     P
>>
>>     William Andersen wrote:
>>      > Paul, Steve,
>>      >
>>      > Thanks for the quick replies, unfortunately it's pretty hard to 
>> tell
>>      > from those images if they match.
>>      >
>>      > I've done some more digging and it turns out that these shapes 
>> were
>>      > created in Arcview 3.x. The notes I have say...
>>      >
>>      >  > This approach selects a number of the outliers and joins the
>>     extreme
>>      > points using elliptical arcs.
>>      >  > The arcs are all created in a direction moving out from the
>>     store.
>>      >
>>      > However, I dont see customer points at the discontinuities in the
>>     hulls,
>>      > so it appears that the "extreme points" are perhaps interpolated.
>>      >
>>      >
>>      > Will
>>      >
>>      > On 7/5/06, *Paul Ramsey* < pramsey at refractions.net
>>     <mailto:pramsey at refractions.net>
>>      > <mailto:pramsey at refractions.net
>>     <mailto:pramsey at refractions.net>>> wrote:
>>      >
>>      >     William,
>>      >
>>      >     It doesn't look like this is a standard algorithm, but more
>>     likely a
>>      >     particular empirical technique provided by the particular
>>     software you
>>      >     were using.  So substituting some other technique might 
>> yield a
>>      >     different shape entirely... do any of the techniques
>>     mentioned here
>>      >        
>> <http://www.geospatial-online.com/geospatialsolutions/article/articleDetail.jsp?id=1348 
>>
>>      >     <
>>     
>> http://www.geospatial-online.com/geospatialsolutions/article/articleDetail.jsp?id=1348>> 
>>
>>      >     sound like what was done to your data?
>>      >
>>      >     Paul
>>      >
>>      >     William Andersen wrote:
>>      >      >
>>      >      > I'm fairly new to postgis, and working to automate a 
>> number of
>>      >      > processes.
>>      >      >
>>      >      > We are trying to compute market area polygons that look
>>     like the
>>      >      > attached image. These were created by some older software.
>>      >      >
>>      >      >
>>      >      > They are referred to as Amoeba Hulls, and they contain 80%
>>     of a
>>      >      > store's customers. However I can't find any solid
>>     documentation that
>>      >      > would allow me to reproduce them.
>>      >      >
>>      >      > Does anyone have any ideas how these shapes are created 
>> or an
>>      >      > alternate name that I might be able to google?
>>     Additionally, we may
>>      >      > be in a position to finance the development of this 
>> feature.
>>      >      >
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