Canada 50k Digital Elevation Model
Matt Wilkie, maphew@gmail.com

http://sydney.freeearthfoundation.com/mattwilkie/canada_50k_dem

2007 September 07

This project was in preparation of sending a Canada-wide elevation model to Google Earth and NASA World Wind. It was completed a couple of years ago, although the data is not yet generally available through those tools -- thus the reason for hosting it here (thank you Adam Nowacki and the Free Earth Foundation!).

The data are marked draft because only a very preliminary review of the resulting mosaick has been done. There is no guarantee the data is complete or faithfully represents it's source. I am interested feedback on the product and it's utility but I make no promises that any problems identified will ever be rectified. Of course I will happily post your fixes here, should there be any!

Process used to create this version of the Canada 50k DEM:

1. Download 1:50,000 Digital Elevation Models for all of Canada from http://geobase.ca/. Some of that process is described here: http://yukongis.ca/bin/view/Main/GeoBase, basically a lot of:

	wget --continue --recursive --level=2 --no-host-directories --cut-dirs=3 \
	ftp://ftp.cits.rncan.gc.ca/pub/geobase/1129608060/cded1/

2. Unpack the dems and mosaick into larger more manageable chunks (unzip_dems.bat, dem2tif.bat). 
Essentially:
	
	:: Unpack
	for %%a in (%tiles%) do unzip -d %%a %%a/\*.zip \*.dem )
	
	:: Mosaick
	gdalwarp -srcnodata -32767 -dstnodata -32767 \
	-co compress=lzw -wm 1024 -rc -wo init_dest=no_data -multi \
	-t_srs nad83 %srcdir%\*.dem  output.tif
	
3. Using gdalwarp like this builds a much larger file than needed, so we compact down to more optimal size with:

	for /r %%a in (*.tif) gdal_translate -co compress=lzw %%a d:\packed\%%~nxa


And that's about it. Some fifteen thousand ittty bitty elevation models are now in only 71 larger files.