R help - Aug 2004 - latitude longitude data

Dear R-helpers,

I get GPS readings with bug counts (bugs meaning
insects in this case) made along rows in crop fields
and use these to make maps of bug distribution.  The
GPS readings are not quite accurate enough for my
purpose, so since I know what row each reading is made
in, I adjust the latitudinal coordinate using:

    grd<-lm(lat~lon+Row,data)
    data$lat<-predict(grd[,c("lon","Row")])

which adjusts the latitude pretty well when the rows
run East-West, but not at all when the rows run
North-South, and it doesn't adjust the longitude at
all.

Is there a better approach I could use to adjust both
longitude and latitude onto the nearest point in the
row, whatever the direction of the rows? In other
words, move the point onto the row in a direction that
is perpendicular to the row?

cheers,
Mikkel

Mikkel Grum <mi2kelgrum <at> yahoo.com> writes:

: 
: Dear R-helpers,
: 
: I get GPS readings with bug counts (bugs meaning
: insects in this case) made along rows in crop fields
: and use these to make maps of bug distribution.  The
: GPS readings are not quite accurate enough for my
: purpose, so since I know what row each reading is made
: in, I adjust the latitudinal coordinate using:
: 
:     grd<-lm(lat~lon+Row,data)
:     data$lat<-predict(grd[,c("lon","Row")])
: 
: which adjusts the latitude pretty well when the rows
: run East-West, but not at all when the rows run
: North-South, and it doesn't adjust the longitude at
: all.
: 
: Is there a better approach I could use to adjust both
: longitude and latitude onto the nearest point in the
: row, whatever the direction of the rows? In other
: words, move the point onto the row in a direction that
: is perpendicular to the row?

If you have the lonlats spaced along the row in a matrix called actual
and the gps lonlats in matrix called gps then its just a matter of finding
the nearest actual row to each gps row.  rdist.earth from fields will do the 
lonlat distance calculation:

require(fields)

# test data
gps <- ozone$lon.lat[seq(3),]
actual <- ozone$lon.lat[-seq(3),]

# use rdist.earth from fields to get distance from each gps to each actual
# and find least
f <- function(i) {
	dst <- rdist.earth(gps[i,,drop=F], actual)
	actual[order(dst)[1],]
}
t(sapply(1:nrow(gps), f))