[postgis-users] Amoeba Hulls
William Andersen
wkandersen at gmail.com
Wed Jul 5 13:29:05 PDT 2006
Jessica,
That's definitely an interesting algorithm.
Unfortunately I believe that for a given alpha there wouldn't be a way to
assert that exactly 80% of the points fell within the final shape. In
essence, the scoop might be too large to create the detail needed to keep
those 20% of points outside of the polygon.
Will
On 7/5/06, Jessica M Salmon <jmsalmon at fs.fed.us> wrote:
>
> William,
>
> We group points into polygons to generate fire perimeters from discrete
> active fire detections (from satellites images). To do this, we use an
> algorithm known as Alpha Shapes.
> http://n.ethz.ch/student/fischerk/alphashapes/as/
>
> The picture you reference below looks similar to these Alpha Shapes.
>
> I have not yet implemented this processing in our postgis database. If I
> do, I will post the function to the mailing list, but honestly it's not
> very high priority ATM.
>
> Cheers,
> Meghan
>
>
>
> "William
> Andersen"
> <wkandersen at gmail To
> .com> "PostGIS Users Discussion"
> Sent by: <postgis-users at postgis.refractions.
> postgis-users-bou net>
> nces at postgis.refr cc
> actions.net
> Subject
> Re: [postgis-users] Amoeba Hulls
> 07/05/2006 01:40
> PM
>
>
> Please respond to
> PostGIS Users
> Discussion
> <postgis-users at po
> stgis.refractions
> .net>
>
>
>
>
>
>
> Thanks for all the suggestions, they are giving me a lot to think about.
>
> Unfortunately it's not always a convex hull. More complex patterns appear
> to have wedges taken out of them, like you can see in this image
>
>
> http://www.esri.com/software/arcgis/extensions/businessanalyst/graphics/market-boundaries-lg.jpg
>
>
> It appears that, in our case, the hulls contain the 80% of closest members
> as the crow flies (although some of the examples i've seen online seem to
> use drivetimes). Then they somehow construct a polygon that contains
> exactly those 80% and none of the remaining 20% - are there any standard
> routines for that? Presumably once they've got that polygon they replace
> the line segments with curves.
>
> If i can create something that meets the same rules and has a similar
> "look" then i'm sure it would suffice.
>
> Graham
>
> On 7/5/06, Mike Leahy <mgleahy at alumni.uwaterloo.ca> 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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> >> >
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> >> >
> >> >
> >> >
> >> >
> >>
> >>
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