[R-sig-Geo] projection of ggmap:get_map() output

[Edzer Pebesma]

Nice, thank you!

Markus Loecher, author of RgoogleMaps, explained that ggmap uses

https://github.com/dkahle/ggmap/blob/master/R/LonLat2XY.R

to go from long/lat to XY (pixels in google map tiles), which was taken
from the RgoogleMaps one:

https://github.com/markusloecher/RgoogleMaps/blob/master/R/LatLon2XY.R

which points to code by someone called Neil Young in a google groups:

https://groups.google.com/forum/?hl=en#!topic/Google-Maps-API/0hA6wp6VaW8

I would expect that your cylindrical, equidistant projection (linear in
long/lat) must collide more with the Mercator used by google when the
plotted area becomes larger.

On 04/28/2014 11:58 AM, Oscar Perpiñan wrote:
> Hello,
> 
> I couldn't run your code because I am not able to read the file.
> Anyway, some comments about your last email:
> 
> - "layer" is a function of the latticeExtra package. This function
> modifies the panel function of a "trellis" graphic produced with a
> function of the lattice package. This function, together with
> "+.trellis" (of the same package) allows for overlaying layers. It is
> important to note that it does not carry out any projection. It only
> displays the graphical result of another function (for example,
> sp.polygons or whatever).
> 
> - You create the canvas with "levelplot" displaying the ggmap raster,
> and then overlay the data with layer. This is the same approach that
> ggmap uses. Instead, I prefer to rely on the main data to be displayed
> to configure the graphical window instead of using an auxiliary image
> as the starting point.
> 
> - I'm not a fan of the ggmap approach. I agree with you that the
> resulting object should include the projection information and even
> adhere to the sp classes. However, I think that their results are
> useful if managed with care.
> 
> In order to show that this is not an issue strictly related to
> rasterVis::levelplot, I have written some code with spplot. I think
> that the concordance between the ggmap raster and the shapefile is
> aceptable.
> 
> Best,
> 
> Oscar.
> 
> ##########################################################
> library(sp)
> library(ggmap)
> ## latticeExtra must be loaded after ggmap because both ggplot2 and
> ## latticeExtra define a 'layer' function. We need the definition from
> ## latticeExtra.
> library(latticeExtra)
> 
> ## Shape example
> library(maptools)
> owd <- getwd()
> setwd(system.file("shapes", package = "maptools"))
> sc90 <- readShapeSpatial("co45_d90")
> proj4string(sc90) <- CRS("+proj=longlat +datum=NAD27")
> setwd(owd)
> 
> ## Get the stamen map
> bbPoly <- bbox(sc90)
> gmap <- get_map(c(bbPoly), maptype='toner', source='stamen', crop=FALSE)
> bbMap <- attr(gmap, 'bb')
> latCenter <- with(bbMap, ll.lat + ur.lat)/2
> lonCenter <- with(bbMap, ll.lon + ur.lon)/2
> height <- with(bbMap, ur.lat - ll.lat)
> width <- with(bbMap, ur.lon - ll.lon)
> 
> ## Use latticeExtra::+.trellis and latticeExtra::layer
> spplot(sc90['AREA'], fill='transparent', col='indianred1') +
>                   layer(grid.raster(gmap,
>                                     x=lonCenter, y=latCenter,
>                                     width=width, height=height,
>                                     default.units='native'),
>                         under=TRUE)
> 
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> 

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Heisenbergstraße 2, 48149 Münster, Germany. Phone: +49 251
83 33081 http://ifgi.uni-muenster.de GPG key ID 0xAC227795

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