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.TH MAP 6
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.SH NAME
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map \- digitized map formats
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.SH DESCRIPTION
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Files used by
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.IR  map (7)
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are a sequence of structures of the form:
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.PP
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.EX
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struct {
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	signed char patchlatitude;
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	signed char patchlongitude;
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	short n;
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	union {
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		struct {
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			short latitude;
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			short longitude;
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		} point[n];
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		struct {
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			short latitude;
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			short longitude;
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			struct {
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				signed char latdiff;
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				signed char londiff;
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			} point[\-n];
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		} highres;
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	} segment;
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};
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.EE
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where
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.B short
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stands for 16-bit integers and there is no padding within or between
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.BR structs .
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Shorts are stored in little-endian order, low byte first.
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To assure portability,
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.I map
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accesses them bytewise.
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.PP
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Fields
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.L patchlatitude
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and
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.L patchlongitude
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tell to what
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10-degree by 10-degree
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patch of the earth's surface a segment belongs.
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Their values range from \-9 to 8 and from \-18 to 17,
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respectively, and indicate the coordinates of the
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southeast corner of the patch in units of 10 degrees.
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.PP
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Each segment of
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.RB | n |
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points is connected; consecutive segments
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are not necessarily related.
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Latitude and longitude
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are measured in units of 0.0001 radian.
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If
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.B n
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is negative, then
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differences to the first and succeeding points
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are measured in units of 0.00001 radian.
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Latitude is counted positive to the north and
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longitude positive to the west.
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.PP
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The patches are ordered lexicographically by
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.L patchlatitude
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then
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.LR patchlongitude .
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A printable
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index to the first segment of each patch
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in a file named
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.I data
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is kept in an associated file named
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.IB data .x\f1.
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Each line of an index file contains
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.L patchlatitude,
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.L patchlongitude
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and the byte position
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of the patch
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in the map file.
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Both the map file and the index file are ordered by
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patch latitude and longitude.
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.SH "SEE ALSO"
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.IR map (7)
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.br
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The data comes from the World Data Bank I and II and
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U.S. Government sources: the Census Bureau, Geological
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Survey, and CIA.