Graphics Reference
In-Depth Information
Note
You will need to register to download the file, but the registration process only
takes a few seconds, and it doesn't cost anything.
The resulting download should be about 2.8 GB, just under the 3 GB limit for file down-
loads from this site. If it's too big, you'll have to select a smaller area to download.
Once you've downloaded the file, you will end up with a ZIP archive that contains a num-
ber of TIFF format raster image files. Next, we need to combine these images into a single
.tif
file for our basemap. To do this, we'll use the
gdal_merge.py
command that
comes with GDAL:
gdal_merge.py -o /dst/path/basemap.tif *.tif
Choose an appropriate destination for the
basemap.tif
file (by replacing
/dst/path
with a sensible location, for example, the path to your desktop). If the current directory is
not set to the folder that contains the downloaded
.tif
files, you'll need to also specify
the source path in the command.
It will take a while for this command to stitch together the various images, but the result
should be a single large file named
basemap.tif
. This is a TIFF-format raster image
that contains the aerial photograph you selected, geo referenced to the appropriate portion
of the Earth's surface.
Unfortunately, we can't use this file directly. To see why, run the
gdalinfo
command on
the downloaded file:
gdalinfo basemap.tif
Among other things, this tells us which coordinate reference system the file is using:
Coordinate System is:
PROJCS["NZGD2000 / New Zealand Transverse Mercator
2000",
GEOGCS["NZGD2000",
DATUM["New_Zealand_Geodetic_Datum_2000",
SPHEROID["GRS
1980",6378137,298.2572221010002,
AUTHORITY["EPSG","7019"]],
