Hi Mike,
Yes on the plane it's easy. But in spherical geometry it's not more so when
the points may span the entire globe, I have a previous algorithm but just
wanted to know if there is something ready made already.
Once I implement that algorithm I will post a link to it on the list here so
that people interested in calculating centroids on the surface of earth
might use it.
Sure I will look at R.
You are really doing a wonderful job, I see you handle a lot of emails
everyday. :P
~Apurv Verma
B. Tech.(CSE)
IIT- Ropar
Mobile - 09256587545
On Mon, Jul 25, 2011 at 6:20 PM, Michael Bedward
<[email protected]>wrote:
> Hi Apurv,
>
> Thanks for that - I understand better now.
>
> I guess by multidimensional scaling you mean any heuristic that would
> give a "good enough" solution with some indication of goodness of fit,
> e.g. classic MDS but also simulated annealing etc. I don't think
> either GeoTools, or the JTS library which it uses for geometry
> operations, have what you want, though I'd be happy to be corrected by
> someone else here.
>
> In the plane I **think** that the coordinate that minimizes the sum of
> distances to all points in a set is the median, ie. centroid.x =
> median of point.xs, centroid.y = median of point.ys. If your points
> are often close together (local or regional scales) you could just
> work with Cartesian coordinates in some convenient map projection.
> However, you emphasize spheroidal distance in your post so I'm
> guessing that your point sets cover a large area. In that case I've no
> idea whether there is an easy solution, but once again it depends on
> how critical the minimum sum of distances criterion is for your
> application.
>
> GeoTools does have the GeodeticCalculator class which can compute
> distances between points on a spheroid. So you could use that in
> conjunction with an optimizing algorithm from another library.
>
> As an aside, why are you searching in GeoTools for this ? I would
> have thought the first stop would have been R (which has many packages
> for spatial analysis) or similar.
>
> Sorry I can't be more directly helpful but please let us know how you go.
>
> Michael
>
>
> On 25 July 2011 19:32, Apurv Verma <[email protected]> wrote:
> > Hi Michael,
> > I am extremely sorry that I did not cc my message to the list last time.
> > Thanks for providing me with the link.
> >
> > Well let me tell me you my problem in detail.
> > From the pdf I found that the minimum distance centroid would be the
> closest
> > that I would want. How can I compute it. However it would be better if
> > something of the following is provided.
> > I have a set of points on the surface of earth. I want to calculate a
> > central point such that the distance from it to the given points is in
> > required proportion with the additional requirement that this point
> should
> > be inside the polygon obtained by joining the points.
> > It's easy to see that a such a requirement cannot be perfectly met. Let
> me
> > give you an analogy in the euclidean geometry rather than spherical
> > geometry.
> > Suppose I have 4 points in the Euclidean plane. I want to calculate a
> point
> > which is equidistant/distances in equal proportions to the 4 points. Now
> > this is not always possible because if I choose a circle to pass through
> 3
> > points, it's not necessary that the fourth point too lies on the circle.
> > (because 3 points uniquely define a circle.)
> > So then we can do a kind of multidimensional scaling. Such that we try to
> > find a point which satisfies the criterion as best as it can.
> > While this is challenging enough to calculate on a euclidean plane.
> > Spherical geometry makes things even worse!!
> >
> > I hope I was able to explain myself. Please do ask me if I was not clear.
> >
> > --
> > thanks and regards,
> >
> > ~Apurv Verma
> > B. Tech.(CSE)
> > IIT- Ropar
> > Mobile - 09256587545
> >
> >
> >
> >
> >
> >
> > On Mon, Jul 25, 2011 at 1:10 PM, Michael Bedward <
> [email protected]>
> > wrote:
> >>
> >> Hello,
> >>
> >> Please reply via the list.
> >>
> >> So what sort of centroid do you want ? There are many in common use
> >> (see
> http://user.gs.rmit.edu.au/rod/files/publications/MSIA_Centroid.pdf
> >> for a comparison of some).
> >>
> >> Or is it enough to have any point that is inside some polygon formed
> >> by the data points e.g. the convex hull ?
> >>
> >> Michael
> >>
> >>
> >> On 25 July 2011 16:58, Apurv Verma <[email protected]> wrote:
> >> > Yes it is a type of centroid calculation only.
> >> > Given a polygon on the surface of earth, I need to calculate its
> >> > centroid.
> >> > Have a look at this page.
> >> > http://www.fmepedia.com/index.php/InsidePointReplacer
> >> >
> >> > thanks and regards,
> >> >
> >> > ~Apurv Verma
> >> > B. Tech.(CSE)
> >> > IIT- Ropar
> >> > Mobile - 09256587545
> >> >
> >> >
> >> >
> >> >
> >> >
> >> >
> >> > On Mon, Jul 25, 2011 at 10:37 AM, Michael Bedward
> >> > <[email protected]> wrote:
> >> >>
> >> >> Hello Apurv,
> >> >>
> >> >> > I am absolutely new to the geo tool library. In fact I want to use
> >> >> > it
> >> >> > for a
> >> >> > specific purpose in the open source project "phyloGeoRef".
> >> >>
> >> >> Great to see taxonomy / biogeography making an appearance here :)
> >> >>
> >> >> > Here is the
> >> >> > functionality that I want.
> >> >>
> >> >> > Given a set of (lat,long) pairs on the globe. I have to calculate
> the
> >> >> > inside
> >> >> > point replacer for these set of nodes. Is there some function in
> Geo
> >> >> > Tools
> >> >> > that provides this functionality. If not, Is there any other method
> >> >> > to
> >> >> > do it
> >> >> > ?
> >> >>
> >> >> Can you provide more detail about the required output. I'm not
> >> >> familiar with the term "inside point replacer" and Wikipedia doesn't
> >> >> seem to know it either. Is it a type of centroid calculation ?
> >> >>
> >> >> Michael
> >> >
> >> >
> >
> >
>
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