Page 159 - MATLAB Recipes for Earth Sciences

154 7 Spatial Data

'MapLonLimit',[40 50], ...
'Frame','on', ...
'MeridianLabel','on', ...
'ParallelLabel','on');
plotm(data(:,2),data(:,1),'k');
Note that the input for plotm is given in the order longitude, followed by the

latitude. The second column of the data matrix is entered first. In contrast,
the function plot requires an xy input. The fi rst column is entered fi rst. The
function axesm defi nes the map axis and sets various map properties such
as the map projection, the map limits and the axis labels.

7.3 The 2-Minute Gridded Global Elevation Data ETOPO2

ETOPO2 is a global data base of topography and bathymetry on a regular
2-minute grid. It is a compilation of data from a variety of sources. It can be
downloaded from the US National Geophysical Data Center (NGDC) web
page

http://www.ngdc.noaa.gov/mgg/fliers/01mgg04.html

From the menu bar Free online we select Make custom grids which is linked
to the GEODAS Grid Translator. First we choose a Grid ID (e.g., grid01), the
Grid Data Base (e.g., ETOPO2 2-minute Global Relief), our computer sys-
tem (e.g., Macintosh) and the Grid Format (e.g., ASCII for both the data and
the header). Next we defi ne the longitude and latitude bounds. For example,
the latitude (lat) from -20 to +20 degrees and a longitude (lon) between +30
and +60 degrees corresponds to the East African coast. The selected area can
be transformed into a digital elevation matrix by pressing Design–a–grid.
this matrix may be downloaded from the web page by pressing Download
your Grid Data, Compress and Retrieve and Retrieve compressed fi le in the
subsequent windows. Decompressing the fi le grid01.tgz creates a directory
grid01_data. This directory contains various data and help fi les. The sub-
directory grid01 contains the ASCII raster grid fi le grid01.asc that have the
following content:

NCOLS 901
NROWS 1201
XLLCORNER 30.00000
YLLCORNER -20.00000
CELLSIZE 0.03333333
NODATA_VALUE -32768
270 294 278 273 262 248 251 236 228 223 ...
280 278 278 264 254 253 240 234 225 205 ...