[R-sig-Geo] random sampling along contour

[Seth Bigelow]

Edzer, totally cool script. Fixed distance from the transect rather than
along contour is absolutely fine. Works like a charm -- thanks a million!

--Seth


-----Original Message-----
From: r-sig-geo-bounces at r-project.org
[mailto:r-sig-geo-bounces at r-project.org] On Behalf Of Edzer Pebesma
Sent: Wednesday, October 09, 2013 10:28 AM
To: r-sig-geo at r-project.org
Subject: Re: [R-sig-Geo] random sampling along contour

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Seth, after running your script, I tried:

b = gBuffer(t.sl, width = .1, byid=FALSE) plot(b, col='#ff000088', add=TRUE)
i = gIntersection(b, v.sldf, byid = TRUE) plot(i, col = 'blue', add=TRUE)
pts = spsample(i[1], n=1, type='random') l = lapply(1:length(i), function(x)
spsample(i[x], n=1, type='random')) npts = do.call(rbind, l[!sapply(l,
is.null)]) plot(npts, add=TRUE, col = 'green')

which goes a fixed distance from the transect, but not a fixed distance
along each contour. Note that you get one point per unique contour level.


On 10/09/2013 03:01 PM, Seth Bigelow wrote:
> Barry,
> 
> I do mean exactly what you have stated, i.e., "plus or minus up to 50m 
> either side of the points created by your plot, but along the contour 
> line? In other words, I go to each of those points on your plot, 
> choose a runif(1,-50,50) and traverse that distance on the contour 
> line to get my sampling point?"
> 
> In my real-world application, there do not seem to be either of the 
> 'edge' issues you raise, since the flank of the mountain I am sampling 
> is not highly dissected. But if the contour line is a loop, it would 
> be fine to 'go around', or just throw an 'NA'.
> Similarly, if the contour line ends, an 'NA' would be ok.
> 
> Thanks for thinking about this!
> 
> -Seth Bigelow
> 
> 
> -----Original Message----- From: b.rowlingson at gmail.com 
> [mailto:b.rowlingson at gmail.com] On Behalf Of Barry Rowlingson Sent:
> Wednesday, October 09, 2013 8:18 AM To: Seth Bigelow Cc:
> r-sig-geo at r-project.org Subject: Re: [R-sig-Geo] random sampling along 
> contour
> 
> On Wed, Oct 9, 2013 at 12:32 PM, Seth Bigelow <seth at swbigelow.net>
> wrote:
>> Hello all:
>> 
>> 
>> 
>> I wish to place sampling plots at regular elevation intervals up a
> mountain.
>> Plot centers should be placed at random within a 100 m wide transect 
>> along the contour. Can anyone suggest an efficient way of doing this? 
>> Code below places plots on a single transect on the volcano dataset, 
>> but does not do the 'along the contour'
>> randomization.
> 
> I'm not clear what you mean here, the plot is a big help though and +1 
> for that.
> 
> By "a 100m wide transect along the contour" do you mean plus or minus 
> up to 50m either side of the points created by your plot, but along 
> the contour line? In other words, I go to each of those points on your 
> plot, choose a runif(1,-50,50) and traverse that distance on the 
> contour line to get my sampling point?
> 
> What if the contour line at that point is a loop less than 100m long? 
> Do I go around? The other edge case is literally an edge case
> - when the contour line hits the edge...
> 
> Barry
> 
> _______________________________________________ R-sig-Geo mailing list 
> R-sig-Geo at r-project.org 
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> 

- --
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster Heisenbergstraße
2, 48149 Münster, Germany. Phone: +49 251
83 33081 http://ifgi.uni-muenster.de GPG key ID 0xAD3A77F1 -----BEGIN PGP
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