[R-sig-Geo] reading a gps from R

[Roger Bivand]

Patrick:

A simplified version of your readGPS() interfacing gpsbabel is included in
maptools 0.6-9. It turned out that even using the NMEA interface to
different Garmin receivers seems to give different data in different
columns. In addition, my trials have shown that some data frame rows may
be corrupted, so at least the first time a given receiver is interfaced,
the data read will need to be looked at carefully. From there on, one can
write a wrapper with the general readGPS() function inside to check for
erroneous rows and select required columns, as sketched in the example for
my Garmin etrex Legend Cx. Under non-Windows OS, reading from USB or
serial may need special permissions too - see the gpsbabel website, and
Vista may also require special treatment (untried).

Thanks for a useful contribution!

Roger

On Sun, 18 Feb 2007, Patrick Giraudoux wrote:

> On the thread and in case it may help somebody, find below this function 
> that can read Garmin Etrex Summit from R (whatever the plateform - 
> tested on Windows XP and Ubuntu 6.10 Edgy eft) if gpsbabel is installed.
> 
> Packaged in pgirbric at http://perso.orange.fr/giraudoux/SiteGiraudoux.html.
> 
> readGPS<-function(type="w",wcom=4){
> 
>     if (!(type=="w" | type=="t")) stop("type must be 'w' (waypoints) or 
> 't' (tracks)")
> 
>     if (Sys.info()["sysname"]!="Windows") {# OK sous linux Ubuntu 6.10
>         gpsdata<-system(paste("gpsbabel -",type," -i garmin -f 
> /dev/ttyUSB0 -o tabsep -F -",sep=""), intern=TRUE)
>         if (any(grep("Can't init",gpsdata))) stop("Cannot read GPS: 
> check connexion")
>         }
>     else {# OK sous Windows
>         if (any(grep(wcom,0:9))){
>             gpsdata <- system(paste("gpsbabel -",type," -i garmin -f 
> com",wcom,": -o tabsep -F -",sep=""), intern=TRUE)
>             if (any(grep("Can't init",gpsdata))) stop("Cannot read GPS: 
> check connexion and com port number 'wcom'")
>             }
>         else stop("Windows com port number 'wcom' must be one of 0:9")
>     }
>     gpsdata<-read.table(textConnection(gpsdata), fill=TRUE)
>    
>    
>     if (type=="w"){
>         gpsdata<-gpsdata[,c(2,6,7,17)]
>         names(gpsdata)<-c("ident","long","lat","altitude")
>         
> gpsdata$altitude<-as.numeric(substr(as.character(gpsdata$altitude),1,nchar(as.character(gpsdata$altitude))-1))
>         attributes(gpsdata)$type<-"Waypoints"
>     }
>    
>     if (type=="t"){
>         gpsdata<-gpsdata[,c(3,4,14)]
>         names(gpsdata)<-c("long","lat","altitude")
>         ident<-1:length(gpsdata[,1])
>         gpsdata<-data.frame(ident=ident,gpsdata)
>         
> gpsdata$altitude<-as.numeric(substr(as.character(gpsdata$altitude),1,nchar(as.character(gpsdata$altitude))-1))
>         attributes(gpsdata)$type<-"Track"
>     }
>    
>     attributes(gpsdata)$sysTime<-Sys.time()
>     gpsdata
> }
> 
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-- 
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