[R] passing of longitude and lattitude arguments to read URL in Google Maps and extract routes

[David Winsemius]

On Jul 2, 2013, at 1:59 AM, Franckx Laurent wrote:

> Dear all
> 
> I try to use Google Maps to calculate travel times per transit between an origin an destination.
> 
> The guidelines for the search can be found at: https://developers.google.com/maps/documentation/directions/#TravelModes
> 
> When I submit the latitude and the longitude of the origin and destination as literals, things work fine.
> 
> For instance, the following code executes correctly and we obtain the distance and trip duration (the output of the search is in JSON format and is converted to an R object with fromJSON)
> 
>        library(rjson)
>        library(gooJSON)
>        route <- url('http://maps.googleapis.com/maps/api/directions/json?                                      origin=51.13854,4.384575&destination=51.13156,4.387118&region=be&sensor=false&mode=transit&departure_time=1372665319')
>        route_file  <- file("route_file.json")
>        L <- readLines(route,-1)
>        writeLines(L, route_file)
>        close(route)
>        routesR_zone1_to_zone20 <- fromJSON( file = route_file )
>        routesR_zone1_to_zone20$routes[[1]][[3]][[1]]$distance$value/1000
>        routesR_zone1_to_zone20$routes[[1]][[3]][[1]]$duration$value/60
> 
> 
> However, what I am really interested in is to repeat this operation for thousands of origin-destination pairs. The longitude and the latitude of the origins and destinations then become variables.
> 
> For instance:
> 
>> lat_or
>        [1] 51.13854
>> long_or
>        [1] 4.384575
>> lat_des
>        [1] 51.13156
>> long_des
>        [1] 4.387118
>> route <- url('http://maps.googleapis.com/maps/api/directions/json?            origin=lat_or,long_or&destination=lat_des,long_des&region=be&sensor=false&mode=transit&departure_time=1372665319')
>> route_file  <- file("route_file.json")
>> L <- readLines(route,-1)
>> writeLines(L, route_file)
>> close(route)
>> routesR_zone1_to_zone20 <- fromJSON( file = route_file )
>> routesR_zone1_to_zone20
>        $routes
>        list()
> 
>        $status
>        [1] "NOT_FOUND"
> 
> Thus, although the coordinates are the same as in the previous example, this time, no route is found.
> 
> I suppose that the problem is that, when the url is accessed, lat_or etc are not "translated" in the corresponding numeric values, and that Google tries to calculate the route between the literals " lat_or,long_or" and " lat_des,long_des".

Yes. That seems likely. R would not interpret a text literal. I don't understand why you are not using the ordinary text handling function 'paste0'.

?paste0

> lat_or <- 51.13854
long_or <- 4.384575
lat_des <-  51.13156
long_des <- 4.387118

> paste0("origin=",lat_or,",",long_or,"&destination=",lat_des,",",long_des) 
[1] "origin=51.13854,4.384575&destination=51.13156,4.387118"


(Also tested on Mac with R 3.0.0 RC using Google JSON Data Interpreter for R, Version: 1.0.01.)

-- 
David.
> 
> Does anyone have a suggestion on how to circumvent the problem?
> 
> 
> 
> 
> Laurent Franckx, PhD
> VITO NV
> Boeretang 200, 2400 MOL, Belgium
> 
-- 

David Winsemius
Alameda, CA, USA