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

[Edzer Pebesma]

EPSG:4326, or WGS84, is only one way to refer to places on the earth by
longitudes and latitudes; it seems that google maps come in a Mercator
projection, EPSG:3857, read e.g. this:

http://docs.openlayers.org/library/spherical_mercator.html

or

https://developers.google.com/maps/documentation/javascript/maptypes#MapCoordinates

Iliffe and Lott: Datums and Map Projections: For Remote Sensing, GIS and
Surveying

provide an excellent explanation on what goes on here, including what
google maps does.

On 04/24/2014 09:56 AM, Agustin Lobo wrote:
> (follow up of previous message, I pressed enter as if I were in the R
> session and sent the message...)
> 
> On Wed, Apr 23, 2014 at 4:37 PM, Barry Rowlingson
> <b.rowlingson at lancaster.ac.uk> wrote:
>>  - I imagine the web services expect EPSG:4326.
> 
> What they expect, yes. But what about what we retrieve? The bbox
> coordinates might be in geographic coordinates (epsg:4326) but
> the actual projection of the matrix could be any other. Actually, I
> think that no geographic object without CRS information should be
> admitted in R.
> 
> In order to investigate this further, I've done the following:
> 
> mibb <- matrix(c(-69.88634, -48.78450, -34.05533, -18.63424),byrow=TRUE,nrow=2)
> delme <- get_map(location = mibb, maptype = "hybrid", source=
> "google", crop = FALSE, zoom = 5)
> 
> mgmap <- as.matrix(gmap)
> vgmap <- as.vector(mgmap)
> vgmaprgb <- col2rgb(vgmap)
> gmapr <- matrix(vgmaprgb[1,],ncol=ncol(
> mgmap),nrow=nrow(mgmap))
> gmapg <- matrix(vgmaprgb[2,],ncol=ncol(mgmap),nrow=nrow(mgmap))
> gmapb <- matrix(vgmaprgb[3,],ncol=ncol(mgmap),nrow=nrow(mgmap))
> rgmaprgb <- brick(raster(gmapr),raster(gmapg),raster(gmapb))
> projection(rgmaprgb) <- CRS("+init=epsg:4326")
> extent(rgmaprgb) <- unlist(attr(gmap,which="bb"))[c(2,4,1,3)]
> rgmaprgb
> plotRGB(rgmaprgb)
> 
> This looks good, but when I save as GTiff
> writeRaster(rgmaprgb,file="rgmaprgb",format="GTiff",overwrite=TRUE,datatype="INT1U")
> 
> and check with QGIS, there is a very significant shift versus the GM
> layer that is downladed by plugin OpenLayers.
> 
> Thus I worry that my overlay using rasterVis:levelplot and ggmap:
> layer could actually be wrong as well.
> 
> I'm going to test on a more local area and let you know.
> 
> Agus
> 
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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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