Retrieve a 'Google Map' that can be used as a background for plotting points and other spatial data.
The projection of the returned Raster object is "Mercator" (unless you use lonlat=TRUE), and other spatial data may need to be transformed before it can be plotted on top of the Google map. You can use the Mercator function to transform points from longitude/latitude to Mercator. For SpatialLines and SpatialPolygons objects, use spTransform in the rgdal package.
This function uses the Google static maps web-service, and is based on functions by Markus Loecher for the RgoogleMaps package.
gmap(x, exp=1, type='terrain', filename='', style=NULL, scale=1, zoom=NULL,
size=c(640, 640), rgb=FALSE, lonlat=FALSE, map_key, geocode_key, ...)Mercator(p, inverse = FALSE)
RasterLayer
a textual locality description, or an Extent object (with longitude/latitude coordinates), or an object that can be coerced to one (such as a Raster* or Spatial* object), in any (known) coordinate system
numeric. An expansion factor to enlarge (by multiplication) the extent specified by x
character. Choose from 'roadmap', 'satellite', 'hybrid', 'terrain'
character. Filename (optional). You can open the resulting file in a GIS program
character. Additional style arguments. See https://developers.google.com/maps/documentation/maps-static/overview?csw=1#StyledMapFeatures.
Note that certain style features do not work in combination with (the default) type='terrain'
1 or 2. Using 2 doubles the number of pixels returned (and thus gives you better image quality if you need a large image)
integer between 0 (the whole world) to 21 (very small area), centered on the center of the extent
vector of two integers indicating the number of columns and rows that is requested (what is returned depends on other factors as well). Maximum values are c(640, 640), so you can only select a smaller area than the default. Note that the number of pixels returned can be doubled by using scale=2
logical. If TRUE, a RasterBrick is returned with three layers (red, green, blue). This can be plotted with plotRGB
logical. If TRUE the Raster object returned has a longitude/latitude CRS instead of Mercator
character. Your Google API key for mapping (and billing). See https://developers.google.com/maps/documentation/javascript/get-api-key)
character. Your Google API key for geocoding (and billing). Only relevant if x is a textual locality description. See https://developers.google.com/maps/documentation/javascript/get-api-key)
additional parameters
Points. A two-column matrix, or a SpatialPoints object
Should the inverse projection be done (from Mercator to longitude/latitude?)
Robert Hijmans and Sebastien Rochette, based on code by Markus Loecher, Sense Networks <markus at sensenetworks.com> in the RgoogleMaps package
If argument x is a textual locality 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 longitude/latitude 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")
if (FALSE) {
mymapkey = "pk-tHVbDiymfUL"
mygeokey = "Skxe99-adfKeax"
library(rgdal)
# 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', map_key=mymapkey)
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, map_key=mymapkey)
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)
# get a map using names
g = gmap('Australia', map_key=mymapkey, geocode_key=mygeokey)
plot(g, inter=TRUE)
gs = gmap('Sydney, New South Wales, Australia', type='satellite',
map_key=mymapkey, geocode_key=mygeokey)
plot(gs, inter=TRUE)
gs = gmap('Sydney, Australia', type='satellite', exp=3,
map_key=mymapkey, geocode_key=mygeokey)
plot(gs, inter=TRUE)
gs = gmap('Sydney, Australia', type='hybrid', zoom=10, scale=2,
map_key=mymapkey, geocode_key=mygeokey)
plot(gs, inter=TRUE)
# styles:
g <- gmap("Brooklyn", style="feature:road.local|element:geometry|hue:0x00ff00|saturation:100
&style=feature:landscape|element:geometry|lightness:-100", type='roadmap',
map_key=mymapkey, geocode_key=mygeokey)
plot(g)
}
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