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1989-03-24
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GEOPHYSICS OF NORTH AMERICA - ISOSTATIC GRAVITY CONTOURS igrav.doc
This directory contains the contours of the Isostatic Gravity data set for
the United States.
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 25 regions. 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 25 files, each called ISOGRAV.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 ISOGRAV Contour Regions:
Region Minimum Minimum Maximum Maximum Number of Number of
Longitude Latitude Longitude Latitude Segments Points
0 232.292 40.708 237.500 49.500 425 14377
1 233.333 32.000 237.500 40.708 230 5937
2 237.500 40.708 245.000 49.500 598 17606
3 237.500 31.917 245.000 40.708 1113 29288
4 237.583 31.500 243.625 31.917 79 1176
5 245.000 40.708 252.500 49.459 490 18669
6 245.000 31.917 252.500 40.708 714 21463
7 245.417 30.917 252.500 31.917 70 1211
8 252.500 40.708 260.000 49.459 306 13486
9 252.500 31.917 260.000 40.708 432 17108
10 252.500 27.291 260.000 31.917 114 3989
11 260.000 40.708 267.500 49.417 433 19839
12 260.000 31.917 267.500 40.708 247 13357
13 260.000 24.583 267.500 31.917 210 10700
14 267.500 40.708 275.000 48.875 329 16675
15 267.500 31.917 275.000 40.708 313 13678
16 267.500 23.999 275.000 31.917 277 12085
17 275.000 40.708 282.500 47.500 123 5092
18 275.000 31.917 282.500 40.708 338 16235
19 275.000 23.528 282.500 31.917 368 15939
20 282.500 40.708 290.000 47.208 180 7862
21 282.500 31.917 290.000 40.708 432 15010
22 282.500 23.500 288.584 31.917 497 16750
23 290.000 40.708 295.708 47.833 233 7726
24 290.000 33.500 295.626 40.708 211 4702
The index file (ISOGRAV.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 = 10 milligals
First contour level = 1 Contour data value = -220 milligals
Zero contour level = 23 Contour data value = 0 milligals
Maximum contour level = 33 Contour data value = 100 milligals
These are the offsets in bytes to the first segment of each region in the
ISOGRAV contour index file:
Reg Offset Reg Offset Reg Offset Reg Offset Reg Offset
0 0 1 8500 2 13100 3 25060 4 47320
5 48900 6 58700 7 72980 8 74380 9 80500
10 89140 11 91420 12 100080 13 105020 14 109220
15 115800 16 122060 17 127600 18 130060 19 136820
20 144180 21 147780 22 156420 23 166360 24 171020
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.