Hey!
First of all I can recommend two tutorials about how to use PL/R in PostGIS
[1][2]. One possibility to interpolate splines with R is to use R's xspline
package [3]. Some theoretical background 'bout those x-splines you'll find in
the related paper [4]. I tried some other spline-methods in R but with dubious
results. xspline works fine for me. A wrapping PL/R-function could look like
this:
/**
* Interpolates a LineString using R's X-spline-method
*
* Parameters:
* 1st - A *single* PostGIS-geometry (POINT, LINESTRING or POLYGON) *as text*:
Use ST_AsEWKT() to pass a geometry as text.
* 2nd - Shape parameter used in R's X-spline-method [-1.0,...,1.0]. A shape
of 0.0 does not interpolate anything. Default: 0.5.
*
* Returns:
* A PostGIS-LINESTRING-geometry covering the interpolated points.
*
* Usage:
* > SELECT interpolateXSpline(ST_AsEWKT(geom), 1.0) FROM your_table
*/
create or replace function interpolateXSpline(text, float DEFAULT 0.5) returns
geometry as $$
# Select the points (also from line or polygon-boundary) into a data frame
points <- pg.spi.exec(
sprintf("WITH pts AS (SELECT (ST_DumpPoints(ST_GeomFromEWKT(%1$s))).geom AS
geom)
SELECT ST_X(geom) AS x, ST_Y(geom) AS y FROM pts;",
pg.quoteliteral(arg1)
)
)
# Interpolate spline
plot(points$x, points$y, type="n") # Dummy-call of plot() for xspline to work
spline_pts <- xspline(points$x, points$y, shape=arg2, draw=FALSE)
# Here you could simply output spline_pts and return.
# Otherwise proceed to build a WKT linestring as shown below.
# LineString-creation: Build beginning of WKT string for output to PostGIS
out_line = "LINESTRING("
for (i in 1:(length(spline_pts$x)-1)) {
out_line = sprintf(
"%1$s %2$f %3$f,",
out_line, spline_pts$x[i], spline_pts$y[i]
)
}
# LineString-creation: Append middle part
out_line =
sprintf("%1$s %2$f %3$f)",
out_line,
spline_pts$x[length(spline_pts$x)],
spline_pts$y[length(spline_pts$y)]
)
# LineString-creation: Finalise (close)
oline <- pg.spi.exec(
sprintf("SELECT ST_GeomFromText('%1$s', ST_SRID(ST_GeomFromEWKT(%2$s))) AS
ln;",
out_line,
pg.quoteliteral(arg1)
)
)
return(oline$ln[1])
$$ language 'plr' IMMUTABLE;
I tried to get it to work with the 1st parameter as bytea to be able to pass
binary geometry but got unserialize-errors in R. So for now you have to convert
your input geometry to EWKT first and pass it as text.
Have fun! Michi
[1] http://www.bostongis.com/?content_name=postgresql_plr_tut01#87
[2] http://www.bostongis.com/?content_name=postgresql_plr_tut02#98
[3] http://stat.ethz.ch/R-manual/R-devel/library/graphics/html/xspline.html
[4]
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.44.5770&rep=rep1&type=pdf
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