Graphics Programs Reference
In-Depth Information
7.4 The 30-Arc Seconds Elevation Model GTOPO30
The 30 arc second (approximately 1 km) global digital elevation data set
GTOPO30 only contains elevation data, not bathymetry. The data set has
been developed by the Earth Resources Observation System Data Center
and is available from the web page
http://edcdaac.usgs.gov/gtopo30/gtopo30.html
The model uses a variety of international data sources. However, it is based
on raster data from the Digital Terrain Elevation Model (DTEM) and vec-
tor data from the Digital Chart of the World (DCW). The GTOPO30 data
set has been divided into 33 pieces or tiles. The tile names refer to the lon-
gitude and latitude of the upper-left (northwest) corner of the tile. The tile
name
e020n40
refers to the upper-left corner of the tile. In our example, the
coordinates of the upper-left corner are 20 degrees eastern longitude and
40 degrees northern latitude. As example, we select and download the tile
e020n40
provided as a 24.9 MB compressed
tar
fi le. After decompressing
the
tar
fi le, we obtain eight fi les containing the raw data and header fi les in
various formats. Moreover, the fi le provides a GIF image of a shaded relief
display of the data.
Importing the GTOPO30 data into the workspace is simple. The Mapping
Toolbox provides an import routine
gtopo30
that reads the data and stores
it onto a regular data grid. We import only a subset of the original matrix:
latlim = [-5 5]; lonlim = [30 40];
GTOPO30 = gtopo30('E020N40',1,latlim,lonlim);
This script reads the data from the tile
e020n40
(without fi le extension) in
full resolution (scale factor = 1) into the matrix
GTOPO30
. The coordinate
system is defi ned by using the
lon/lat
limits as listed above. The resolution
is 30 arc seconds corresponding to 1/120 degrees.
[LON,LAT] = meshgrid(30:1/120:40,-5:1/120:5);
A grayscale image can be generated from the elevation data by using the
function
surf
. The fourth power of the colormap
gray
is used for darken-
ing the map at higher levels of elevation. Subsequently, the colormap is
fl ipped vertically in order to obtain dark colors for high elevations and light
colors for low elevations.
figure
surf(LON,LAT,GTOPO30)
shading interp
