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Plan 9 - man page for map (plan9 section 6)

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MAP(6)											   MAP(6)

       map - digitized map formats

       Files used by map(7) are a sequence of structures of the form:

       struct {
	    signed char patchlatitude;
	    signed char patchlongitude;
	    short n;
	    union {
		 struct {
		      short latitude;
		      short longitude;
		 } point[n];
		 struct {
		      short latitude;
		      short longitude;
		      struct {
			   signed char latdiff;
			   signed char londiff;
		      } point[-n];
		 } highres;
	    } segment;
       where  short stands for 16-bit integers and there is no padding within or between structs.
       Shorts are stored in little-endian order, low byte  first.   To	assure	portability,  map
       accesses them bytewise.

       Fields  and  tell  to  what  10-degree by 10-degree patch of the earth's surface a segment
       belongs.  Their values range from -9 to 8 and from -18 to 17, respectively,  and  indicate
       the coordinates of the southeast corner of the patch in units of 10 degrees.

       Each segment of |n| points is connected; consecutive segments are not necessarily related.
       Latitude and longitude are measured in units of 0.0001 radian.  If  n  is  negative,  then
       differences  to	the  first and succeeding points are measured in units of 0.00001 radian.
       Latitude is counted positive to the north and longitude positive to the west.

       The patches are ordered lexicographically by then A printable index to the  first  segment
       of  each patch in a file named data is kept in an associated file named data.x.	Each line
       of an index file contains and the byte position of the patch in the map	file.	Both  the
       map file and the index file are ordered by patch latitude and longitude.

       The  data  comes from the World Data Bank I and II and U.S. Government sources: the Census
       Bureau, Geological Survey, and CIA.

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