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surfPPC Tutorial No. 6, The Earth
GTOPO30 DEMs
UseExpander© to uncompress and untar the .tar.gz files
Use surfPPC© to
open the .HDR and then the .DEM file.
Examples
Use a browser to download a GTOPO30 DEM file(s) of your choice, then uncompress
it. If you have a access to a easy Mac utility such as Expander, by all means,
use it on .gz or .tar.gz files.
Otherwise a UNIX or LINUX gunzip program should suffice (UNIX: gunzip myDEM.gz)
The utility should convert a .gz file to a larger file without the .gz extension.
GTOPO30 files are usually also in tar format, which Expander handles too - you
get a folder containing the tar archive (UNIX: gunzip myDEM.gz; tar xvf myDEM
)
Notes to the curious
The GTOPO30 .DEM file is pure 16-bit binary data,
and the .HDR file is ASCII text. Other files in the folder(s) are not required by surfPPC.
Open surfPPC
Type b to set background to black or open preferences with o
Open .HDR file with o
Open .DEM file with o
Space bar to render on a plane,
3 to render isometric view.
1
Rendered on the plane, planar view, grids ON.
2
Detail
A Little Story
I had wanted to download and reconstruct the entire earth from this DEM data.
The strategy that finally worked was to determine a fixed scale factor for all
GTOPO30 DEM files, 10km. Where the oceans are zero in the file they
are converted to a large negative number so the (positive) land really pops
up. Once rendered with a linear map, the DEM images were composited in PhotoShop©.
It's still, I think, my largest single image, 32768x16384 pixels.
This was prior to the surfPPC capability to join images, which would have preserved
all the precision in the original files, so in the future I would be using the
surfPPC joining technique. The rendering step I did for each file before was
grinding my 10km into 256 steps but joining does not, until the very end.