Message-ID: <XFMail.090529131430.Ted.Harding@manchester.ac.uk>
Date: 2009-05-29T12:14:30Z
From: (Ted Harding)
Subject: Regular point pattern on multi-segment line
In-Reply-To: <4A1FC5A3.1000308@ifremer.fr>
On 29-May-09 11:23:15, Pascal LAFFARGUE wrote:
> I need your help for the following purpose :
>
> I would like to create regularly spaced points on a multisegment
> line (a 'psp class' object).
>
> A function of the spatstat library( = pointsOnLines() ) is dedicated
> to that objective but the problem is that the probability of falling
> on a particular segment is proportional to the length of the segment.
>
> What about a solution using the whole multi-segment line and not
> individual segments for points distribution ?
>
> Many thanks in advance for your help !
>
> Plaff
If I understand you right, one fairly obvious approach would be the
following.
1. Let L be a vector of lengths of segments: L[i] is the length
of segment i (say there are K segments).
2. Use X <- sort(sum(L)*runif(N)) to place N points uniformly
on (0,sum(L))
3. Let Lcum <- c(0,cumsum(L))
4. whichsegs <- cut(X,breaks=Lcum,labels=FALSE)
Then whichsegs is a vector of N integers, from 1:K, which give the
segments into whch the N points in X fall.
So, for(i in (1:K)), extract the values of X[whichsegs==i],
subtract Lcum[i] from each, and then place the points at the
resulting distances along segment i.
I think this should work (not tested in detail)!
Ted.
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Date: 29-May-09 Time: 13:14:27
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