[R-sig-Geo] Again, coordinate interpolation with NAs

[Edzer Pebesma]

On 08/29/2012 07:04 PM, Karl Ove Hufthammer wrote:
> Mario A. Pardo skreiv:
> 
>> I need a new set of coordinates interpolated at a given resolution,
>> let's say one data point each xx kilometers or degrees, or at a given
>> proportion of the total length, etc.
>>
>> Think of this coordinates as transect points, and I need to keep the
>> NAs, since they mark cut offs in my transect.
> 
> I would recommend using SpatialLines objects instead, which can 
> easily hold polylines and multiple lines. Example:
> 
> library(sp)
> library(geosphere)
> 
> lon=c(-115.86, -115.83, NA, -115.78, -115.78, -115.75, NA, -115.68, -115.66)
> lat=c(  29.76,   29.61, NA,   29.55,   29.53,   29.41, NA, 29.13, 29.09)
> d=data.frame(lon,lat)        # Convert to data frame
> d$group=cumsum(is.na(d$lon)) # Add line ID
> d=d[!is.na(d$lon),]          # Remove NA rows
> 
> d.lines=split(d,d$group)    # Split by line ID
> 
> # Convert into SpatialLines object
> d.sp=SpatialLines(lapply(d.lines, function(df) Lines(Line(df[c("lon","lat")]), ID=df$group[1])),
>                   proj4string=CRS("+proj=longlat"))
> 
> # Add intermediate points
> d.sp.int=makeLine(d.sp, interval=1000)
> 
> 
> # The actual coordinates are available in this list
> # (though I don’t know why there are duplicates –
> # a small bug in makeLine, perhaps?):
> lapply(d.sp.int at lines, function(x) x at Lines[[1]]@coords)
> 

I was thinking along the same line. I'm curious, btw, whether matlab's
interpm also interpolates along great circle lines, as makeLine does.

An alternative that interpolates along straight lines, building on your
script, is:

x = spsample(d.sp, type = "regular", n = 100)
plot(x, cex=.2, axes=TRUE)

but I'm not sure whether the first and last points are always included.

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de