[R-sig-Geo] Converting downloaded PNG images using RgoogleMaps to SpatialGridDataFrame class (proj4string?)

[Tomislav Hengl]

Hi all,

Thanks to Markus Loecher and colleagues we can now easily obtain background maps from Google Earth
and use it for plotting/interpretation of spatial data.  

This runs pretty smoothly (e.g. a map of the Netherlands):

> library(RgoogleMaps)
# obtain the API key and save into the home folder;
> MyMap <- GetMap.bbox(center=c(52.1551723,5.3872035), zoom=7, destfile="netherlands.png", maptype
="hybrid")
Read 1 item
[1] "http://maps.google.com/staticmap?center=52.1551723,5.3872035&zoom=7&size=640x640
+   &maptype=hybrid&format=png32&key=****&sensor=true"
trying URL 'http://maps.google.com/staticmap?center=52.1551723,5.3872035&zoom=7&size=640x640
+   &maptype=hybrid&format=png32&key=****=true'
Content type 'image/png' length 703541 bytes (687 Kb)
opened URL
downloaded 687 Kb

netherlands.png has GDAL driver PNG 
and has 640 rows and 640 columns
> PlotOnStaticMap(MyMap, lat=52.1551723, lon=5.3872035)

But how to convert this image into a SpatialGridDataFrame to allow overlays, export to GIS formats
etc?

I am not even sure what the correct proj4string for this image is?

> str(MyMap)  # this only gives coordinates of the centre
List of 4
 $ : num 52.2
 $ : num 5.39
 $ : num 7
 $ : num [1:640, 1:640] 187 60 66 1 66 202 151 1 214 220 ...
  ..- attr(*, "COL")= chr [1:256] "#88887C" "#887440" "#747838" "#0C1C20" ...
  ..- attr(*, "type")= chr "rgb"

> GDALinfo("netherlands.png")
# the coordinates are not embedded!


Thanks,

Tom Hengl
http://home.medewerker.uva.nl/t.hengl/