Thanks for all the suggestions, they are giving me a lot to think about.<br><br>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<br>
<br><a href="http://www.esri.com/software/arcgis/extensions/businessanalyst/graphics/market-boundaries-lg.jpg">http://www.esri.com/software/arcgis/extensions/businessanalyst/graphics/market-boundaries-lg.jpg</a><br><br>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.
<br><br>If i can create something that meets the same rules and has a similar "look" then i'm sure it would suffice.<br><br>Graham<br><br><div><span class="gmail_quote">On 7/5/06, <b class="gmail_sendername">Mike Leahy
</b> <<a href="mailto:mgleahy@alumni.uwaterloo.ca">mgleahy@alumni.uwaterloo.ca</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Hey there,<br><br>If you can determine the closest 80% of points by whatever criteria,<br>wouldn't you be able to use the convexhull() function? I just tried<br>this, and it looks okay to me:<br><br>testdb=# select astext(convexhull('MULTIPOINT((0 1),(0 0),(1 0),(1
<br>1))'::geometry));<br> astext<br>--------------------------------<br> POLYGON((0 0,0 1,1 1,1 0,0 0))<br>(1 row)<br><br>I did the same thing after creating a points table with the separate in<br>individual records with the same overall coordinates in the multipoint
<br>example above, and it worked okay too:<br><br>testdb=# select astext(convexhull(collect(p))) from testpoint;<br> astext<br>--------------------------------<br> POLYGON((0 0,0 1,1 1,1 0,0 0))<br>(1 row)<br>
<br>It wouldn't be too hard to modify this to work using where condition<br>that filters out the records of interest...or maybe on a saved view.<br><br>The only problem is that it wouldn't produce a nice curvy polygon<br>
outline like in the sample William provided.<br><br>Regards,<br>Mike<br><br>Paul Ramsey wrote:<br>> So my guess is that you use drive-time to segment your population of<br>> customers relative to the store into the "nearest X%" and then draw a
<br>> "shape" around that cloud of points. And drawing the shape is the "fun"<br>> part.<br>><br>> William Andersen wrote:<br>>> It appears to be the same as this functionality in Business Analyst
<br>>><br>>> <a href="http://www.esri.com/software/arcgis/extensions/businessanalyst/about/customer-market.html">http://www.esri.com/software/arcgis/extensions/businessanalyst/about/customer-market.html</a><br>
>> <<a href="http://www.esri.com/software/arcgis/extensions/businessanalyst/about/customer-market.html">http://www.esri.com/software/arcgis/extensions/businessanalyst/about/customer-market.html</a>><br>>>
<br>>><br>>> I thought this was done using some older version of Arcview, but i'm<br>>> not very familiar with esri's offerings.<br>>><br>>> Will<br>>><br>>> On 7/5/06, *Paul Ramsey * <
<a href="mailto:pramsey@refractions.net">pramsey@refractions.net</a><br>>> <mailto:<a href="mailto:pramsey@refractions.net">pramsey@refractions.net</a>>> wrote:<br>>><br>>> This wouldn't be in vanilla arcview, was it in Network Analyst?
<br>>> The top<br>>> 80% of points by drive distance might yield this shape. Finding the<br>>> points would be straightforward, and then the hull building would<br>>> be the<br>>> hand-waving part.
<br>>><br>>> P<br>>><br>>> William Andersen wrote:<br>>> > Paul, Steve,<br>>> ><br>>> > Thanks for the quick replies, unfortunately it's pretty hard to
<br>>> tell<br>>> > from those images if they match.<br>>> ><br>>> > I've done some more digging and it turns out that these shapes<br>>> were<br>>> > created in Arcview
3.x. The notes I have say...<br>>> ><br>>> > > This approach selects a number of the outliers and joins the<br>>> extreme<br>>> > points using elliptical arcs.<br>>> > > The arcs are all created in a direction moving out from the
<br>>> store.<br>>> ><br>>> > However, I dont see customer points at the discontinuities in the<br>>> hulls,<br>>> > so it appears that the "extreme points" are perhaps interpolated.
<br>>> ><br>>> ><br>>> > Will<br>>> ><br>>> > On 7/5/06, *Paul Ramsey* < <a href="mailto:pramsey@refractions.net">pramsey@refractions.net</a><br>>> <mailto:
<a href="mailto:pramsey@refractions.net">pramsey@refractions.net</a>><br>>> > <mailto:<a href="mailto:pramsey@refractions.net">pramsey@refractions.net</a><br>>> <mailto:<a href="mailto:pramsey@refractions.net">
pramsey@refractions.net</a>>>> wrote:<br>>> ><br>>> > William,<br>>> ><br>>> > It doesn't look like this is a standard algorithm, but more<br>>> likely a
<br>>> > particular empirical technique provided by the particular<br>>> software you<br>>> > were using. So substituting some other technique might<br>>> yield a<br>>> > different shape entirely... do any of the techniques
<br>>> mentioned here<br>>> ><br>>> <<a href="http://www.geospatial-online.com/geospatialsolutions/article/articleDetail.jsp?id=1348">http://www.geospatial-online.com/geospatialsolutions/article/articleDetail.jsp?id=1348
</a><br>>><br>>> > <<br>>><br>>> <a href="http://www.geospatial-online.com/geospatialsolutions/article/articleDetail.jsp?id=1348">http://www.geospatial-online.com/geospatialsolutions/article/articleDetail.jsp?id=1348
</a>>><br>>><br>>> > sound like what was done to your data?<br>>> ><br>>> > Paul<br>>> ><br>>> > William Andersen wrote:<br>>> > >
<br>>> > > I'm fairly new to postgis, and working to automate a<br>>> number of<br>>> > > processes.<br>>> > ><br>>> > > We are trying to compute market area polygons that look
<br>>> like the<br>>> > > attached image. These were created by some older software.<br>>> > ><br>>> > ><br>>> > > They are referred to as Amoeba Hulls, and they contain 80%
<br>>> of a<br>>> > > store's customers. However I can't find any solid<br>>> documentation that<br>>> > > would allow me to reproduce them.<br>>> > >
<br>>> > > Does anyone have any ideas how these shapes are created<br>>> or an<br>>> > > alternate name that I might be able to google?<br>>> Additionally, we may
<br>>> > > be in a position to finance the development of this<br>>> feature.<br>>> > ><br>>> > _______________________________________________<br>>> > postgis-users mailing list
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