[postgis-users] Amoeba Hulls

[Paul Ramsey]

Indeed, nice to see that everything can be strung together.  I am trying 
to visualize what a "squishyhull" function would have to do to work 
though :)  It's a not uncommon request... the calculated equivalent of 
someone squinting and drawing a line around a group of points.

P

Mike Leahy wrote:
> 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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