1 .TH MAP 7
2 .SH NAME
3 map \- digitized map formats
4 .SH DESCRIPTION
5 Files used by
6 .IR  map (7)
7 are a sequence of structures of the form:
8 .PP
9 .EX
10 struct {
11 	signed char patchlatitude;
12 	signed char patchlongitude;
13 	short n;
14 	union {
15 		struct {
16 			short latitude;
17 			short longitude;
18 		} point[n];
19 		struct {
20 			short latitude;
21 			short longitude;
22 			struct {
23 				signed char latdiff;
24 				signed char londiff;
25 			} point[\-n];
26 		} highres;
27 	} segment;
28 };
29 .EE
30 where
31 .B short
32 stands for 16-bit integers and there is no padding within or between
33 .BR structs .
34 Shorts are stored in little-endian order, low byte first.
35 To assure portability,
36 .I map
37 accesses them bytewise.
38 .PP
39 Fields
40 .L patchlatitude
41 and
42 .L patchlongitude
43 tell to what
44 10-degree by 10-degree
45 patch of the earth's surface a segment belongs.
46 Their values range from \-9 to 8 and from \-18 to 17,
47 respectively, and indicate the coordinates of the
48 southeast corner of the patch in units of 10 degrees.
49 .PP
50 Each segment of
51 .RB | n |
52 points is connected; consecutive segments
53 are not necessarily related.
54 Latitude and longitude
55 are measured in units of 0.0001 radian.
56 If
57 .B n
58 is negative, then
59 differences to the first and succeeding points
60 are measured in units of 0.00001 radian.
61 Latitude is counted positive to the north and
62 longitude positive to the west.
63 .PP
64 The patches are ordered lexicographically by
65 .L patchlatitude
66 then
67 .LR patchlongitude .
68 A printable
69 index to the first segment of each patch
70 in a file named
71 .I data
72 is kept in an associated file named
73 .IB data .x\f1.
74 Each line of an index file contains
75 .L patchlatitude,
76 .L patchlongitude
77 and the byte position
78 of the patch
79 in the map file.
80 Both the map file and the index file are ordered by
81 patch latitude and longitude.
82 .SH "SEE ALSO"
83 .MR map (7)
84 .br
85 The data comes from the World Data Bank I and II and
86 U.S. Government sources: the Census Bureau, Geological
87 Survey, and CIA.