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GEOPHYSICS OF NORTH AMERICA - MAGSAT CONTOURS magsat.doc
This directory contains the Magsat contours.
In order to understand how the data are arranged, one must understand
something about the process of generating these data. The contouring was
done in 3 regions each 59 degrees wide. Each contiguous section of each
contour line is called a segment and each segment is made up of some number
of line fragments. Each segment starts with an implicit move command and
the fragments are the endpoints of a series of draw commands. The data
storage scheme therefore has three basic units: regions, segments, and
points (where points are the ends of fragments).
The regionalization of the data is expressed in the file structure. The
data are stored in 3 files, each called MAGSAT.NUM, where NUM is the region
number. Each data file has the longitude and latitude pairs of the
endpoints of the contour lines for all of the contours within the given
region. These values are stored in two 4-byte floating point numbers.
Parameters for MAGSAT contour Regions:
Region Minimum Minimum Maximum Maximum Number of Number of
Longitude Latitude Longitude Latitude Segments Points
0 170 0 229 50 98 12188
1 229 0 289 50 149 19737
2 289 0 349 50 121 14783
The index file (MAGSAT.INX) contains information about the segments. There
is one 20 byte record for each segment. Each record contains the contour
level for the segment (2 byte integer), the minimum longitude (degrees
east), minimum latitude, maximum longitude (degrees east), maximum latitude
of the box surrounding the segment (each a 4 byte floating point number),
and the number of points in the segment (2 byte integer). The relationship
between the index file and the data files is known through the byte offset
to the beginning of each data set in the index file. These byte offsets can
be calculated from the number of segments in each data set (20 * cumulative
number of segments).
The contour level is a relative value related to the actual data value and
contour interval as follows:
Contour interval = 1 nanoTessla
First contour level = 1 Contour data value = -10 nanoTesslas
Zero contour level = 11 Contour data value = 0 nanoTesslas
Maximum contour level = 25 Contour data value = 14 nanoTesslas
These are the offsets in bytes to the first segment of each region in the
MAGSAT contour index file
Region Offset Region Offset Region Offset
0 0 1 1960 2 4940
The region parameters and the byte offsets are contained in the file
contour.h which is on your program disk.
The programs listindx and listdata (on the accompanying diskette) read the
index and data files respectively and output the contents to standard output
as ASCII.