Re: [Geotools-gt2-users] position on LineString that is closest to Point

[Schuster S. <ste...@ge...>]

Hi Wouter,

thanks a lot for you're suggestion, but this works only for Lines that have
only two point, a startPoint and an endPoint.
In my case, I must also be able to handle LineStrings with more Coordinates.

Therefore, I have to figure between which two Coordinates of the LineString
the relevant Point could be.

I have to think about that, but thanks so far!

Stefan


-----Ursprüngliche Nachricht-----
Von: Wouter Schaubroeck [mailto:wou...@gm...] 
Gesendet: Donnerstag, 4. März 2010 14:32
An: Schuster Stefan
Cc: geo...@li...
Betreff: Re: [Geotools-gt2-users] position on LineString that is closest to
Point

Hi Stefan,

Perhaps you could use math to solve the problem (only if the points
are in a Euclidean plane):

You have the coordinate of the point, and you have the coordinates of
the endpoints of the line. As a third, you have the orthogonal
distance between the point and the line.

Next, you need to calculate the distance between one endpoint of the
line, and the point.
Mathworld:
http://mathworld.wolfram.com/Point-PointDistance2-Dimensional.html

Now you have 2 distances in the triangle, and you can use Pythagoras
to solve your missing distance (the distance between the endpoint, and
the orthogonal crossing point)

Hope this helps,

grtz, wouter
--
Wouter
http://blog.giswhat.be



On 4 March 2010 13:35, Schuster Stefan <ste...@ge...> wrote:
> Hi again,
>
>
>
> while trying to implement my suggested solution, I realised that it is not
> only ugly, but it also doesn’t work, as there is no “CIRCLE” Geometry I
> could use.
>
> Therefore, I really need help, thanks in advance!
>
>
>
> Stefan
>
>
>
>
>
> ________________________________
>
> Von: Schuster Stefan [mailto:ste...@ge...]
> Gesendet: Donnerstag, 4. März 2010 12:42
> An: geo...@li...
> Betreff: [Geotools-gt2-users] position on LineString that is closest to
> Point
>
>
>
> Hi,
>
>
>
> Assume I have a Point p and a  LineString ls.
>
>
>
> By the statement
>
> double dist = ls.distance(p)
>
> I easily can figure out the distance between them.
>
>
>
> But now I want to now the Point p_cut on the LineString that was used to
> calculate the distance, more accurate I want to know the length of the
> LineString from the beginning to the Point p_cut.
>
>
>
> Any suggestions?
>
>
>
> My first idea was to create a circle around Point p with a radius of
> distance, and the intersection between this circle and the LineString is
> point p_cut.
>
> To get the length of LineString from the beginning to this point the
> following algorithm could do it:
>
>
>
> Create new empty line String lsCopy
>
> Add first coordinate of the original LineString to lsCopy
>
> DO
>
>             Add next coordinate of the original LineString to lsCopy
>
>             IF NOT lsCopy intersects p_cut THEN
>
>                         Continue;
>
>             ELSE
>
>                         Remove last coordinate from ls_copy
>
>                         Add coordinates of p_cut to ls_copy
>
> WHILE original LineString has next coordinate
>
>
>
>
>
> I have not yet tried it, I guess it would work but this looks very ugly
and
> I wonder if there is a more beautiful solution?
>
>
>
> Thanks for help
>
>
>
> Stefan
>
>
>
>
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