[R-sig-Geo] Distribute points along lines

[Roger Bivand]

On Sun, 5 Jul 2009, Agustin Lobo wrote:

> Roger,
>
> Roger Bivand wrote:
>
>> so a proportion in [0,1], but it seems to work backwards at present, with 
>> an offset of 0 anchoring to the final, not the first, point.
>
> Would it be too hard adding the offset to the first point? And actually
> ordering the points starting from the initial point of the line?

The problem was that sp:::sample.Line() was not handling a zero-length 
line segment (two identical consecutive coordinates in a Line object) 
properly, provoking an error when offset was set to 0.

I've committed changes to sourceforge:

cvs -z3 \
-d:pserver:anonymous at r-spatial.cvs.sourceforge.net:/cvsroot/r-spatial \
co -P sp

(all one line), then

R CMD INSTALL sp

to try out. I have also reversed the meaning of offset for lines, 0 is now 
the start coordinate of the Line object. If possible, could you please 
check out the revision, install it from source, and try it - it resolves 
the problem for your case as far as I can tell.

>
>> 
>>> I'm actually trying to set real field sampling sites, so this matters. The 
>>> best would be being able to set the first point in the line and then the 
>>> nb of points for random sites (as it is currently done) and the length 
>>> interval for regular sampling.
>>> 
>>> By now I can adjust the nb. of points for the interval I want if the 
>>> length of the line is known, but how can I calculate the length of the 
>>> lines? calcLength requires another type of spatial object.
>> 
>> calcLength() is in PBSmapping. Adding a length slot to Line objects is an 
>> option, or rather providing length methods which would be less invasive. 
>> I'll take a look.
>
> Thanks, please let me know. By now I measure the actual length in qgis.

There are LineLength(), LinesLength(), and SpatialLinesLengths() 
functions in sp, more used internally than exposed, but should be OK. I've 
converted LineLength() to use C code, so they can now also measure lengths 
in km even if the data are in geographical coordinates (assuming WGS84), 
as in spDistsN1().

Roger

>
> Agus
>
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no