gmap: Get a Google map

Description

Retrieve a 'Google Map' that can be used as a background for plotting points and other spatial data. You should check the terms of service before you use these maps: http://code.google.com/apis/maps/terms.html The projection of the returned maps is "Mercator" and your data may need to be transformed before you can plot on top of these maps. You can use the Mercator function for that, and to transform map coordinates back to longitude/latitude. This function uses the Google static maps web-service, and is based on functions by Markus Loecher for the RgoogleMaps package. The purpose of using this function should be to display the map in a browser. You should check the Google terms of use http://code.google.com/apis/maps/terms.html to see if your usage of this function (and the underlying Google API) is permitted.

Usage

gmap(x, exp=1, type='terrain', filename='', style, ...) 
Mercator(p, inverse = FALSE)

Arguments

A textual locality description, or an Extent object or an object that can be coerced to one (e.g. a Raster* or Spatial* object)

exp

An expansion factor to enlarge (by multiplication) the extent specified by x

type

Character. Choose from 'roadmap', 'satellite', 'hybrid', 'terrain'

filename

Character. Filename (optional). You can open the resulting file in a GIS program

style

Character. Additional style arguments. See http://code.google.com/apis/maps/documentation/staticmaps/#StyledMapFeatures. Note that certain style features do not work in combination with (the default) type='terrain'

...

Graphical parameters

Points. A two-column matrix, or a SpatialPoints object

inverse

Should the inverse projection be done (i.e. from Mercator to longitude/latitude

Value

A RasterLayer

Details

If argument x is a textual locatlity description, the geocode function is used to retrieve the extent that should be mapped. Change the type to 'roadmap' if the map returned says "sorry we have no imagery here"; or use a larger extent. The returned RasterLayer has a Mercator projection. To plot points (or lines or polygons) on top of it, these need to be transformed first. A matrix of lon/lat data can be transformed with the Mercator function used in the example below. 'Spatial*' objects can be transformed with spTransform p <- spTransform(x, "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs")

Examples

Run this code

if(require(rgdal)){
# get a map using names
g = gmap('Australia')
plot(g, inter=TRUE)
gs = gmap('Sydney, New South Wales, Australia', type='satellite')
plot(gs, inter=TRUE)
gs = gmap('Sydney, Australia', type='satellite', exp=3)
plot(gs, inter=TRUE)
gs = gmap('Sydney, Australia', type='hybrid', exp=8)
plot(gs, inter=TRUE)

# from a maxtrix with lon/lat points
x = runif(30)*10 + 40
y = runif(30)*10 - 20
xy = cbind(x, y)
g = gmap(xy, type='hybrid')
plot(g, inter=TRUE)
points(Mercator(xy) , col='red', pch=20)

# or from an Extent object
e = extent( -121.9531 , -120.3897 , 35.36 , 36.61956 )
# you can also get an Extent object by clicking on the map twice after using:
# drawExtent()
r = gmap(e)
plot(r, interpolate=TRUE)

# transform points to Mercator for plotting on top of map:
pt <- matrix(c(-121, 36), ncol=2)
ptm <- Mercator(pt)
points(ptm, cex=3, pch=20, col='blue')
Mercator(ptm, inverse=TRUE)

# transform Spatial objects to Mercator for plotting on top of map
# here for points, but particularly relevant for lines and polygons
pt <- data.frame(pt)
coordinates(pt) <- ~X1 + X2
proj4string(pt) <-"+proj=longlat +datum=WGS84 +ellps=WGS84"
ptm2 <- spTransform(pt, CRS("+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs"))
points(ptm, col='red', pch='x', cex=3)

# styles:
\dontrun{
d = gmap("Brooklyn", type='roadmap',  style="feature:road.local|element:geometry|hue:0x00ff00|saturation:100&style=feature:landscape|element:geometry|lightness:-100")
}
}

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