If the data points span most of the globe then the definition of inside-ness becomes a little difficult :)
A link to your algorithm posted to the list would be much appreciated. It could well be useful for other applications. Good luck with it. Michael On 26 July 2011 00:44, Apurv Verma <[email protected]> wrote: > 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 >> >> > >> >> > >> > >> > > > ------------------------------------------------------------------------------ Magic Quadrant for Content-Aware Data Loss Prevention Research study explores the data loss prevention market. Includes in-depth analysis on the changes within the DLP market, and the criteria used to evaluate the strengths and weaknesses of these DLP solutions. http://www.accelacomm.com/jaw/sfnl/114/51385063/ _______________________________________________ Geotools-gt2-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/geotools-gt2-users
