CBD(5) FILE FORMATS CBD(5)

CBD(5) FILE FORMATS CBD(5)

NAME
cbd - vector map data format for mapbrowser and mapwriter

DESCRIPTION
This is a file format for compressed binary map database
(*.cbd) files, designed to encode features from a vector map
database. Map data in this format can be used by the map-
browser and mapwriter programs to browse and render raster
map images from the vector data.

This format encodes data line graph (DLG) data in latitude-
longitude coordinates into a compressed format designed by
Brian Reid and slightly modified by Steve Putz. It was
designed to efficiently encode map data such as from the
World Data Bank II. The cbd files used by mapbrowser are
7-9 times smaller than original uncompressed data files.

World Bank Database
The CIA World Bank II database is available from the U.S.
Government and is in the public domain. The map database is
divided into continents (North America, South America,
Europe, Africa, and Asia), and within continents it is
divided into "Coastlines, Islands, and Lakes" (cil files),
"Rivers" (riv files), "International political boundaries"
(bdy files) and "National political boundaries" (pby files).
Each file is divided into several thousand "segments", and
each segment is divided into some number of "strokes".

The CIA World Bank II original data is a series of COBOL
records, specifying 5,719,617 individual vectors, which
occupies about 130 megabytes of disk space (This format is
apparently compatible with a package called GS-CAM). The
"cbd" files used by the mapbrowser program are compressed
binary encodings of it, that collectively occupy about 15
megabytes of disk space. The "cbd" files are produced from
the original data by the ciamap program.

U.S. GeoData
The Earth Science Information Centers (ESIC) distribute
digital cartographic/geographic data files produced by the
U.S. Geological Survey (USGS) as part of the National Map-
ping Program. 1:2,000,000-scale DLG data for the United
States is available on CD-ROM. The "Graphic" format on the
CD-ROM is the same GS-CAM format used by the World Bank
Database.

Data File Format
The .cbd encoding divides a database into several files. In
each file there is a header record, a series of segments and
strokes, and a segment index. A separate mapset file
describes the relationships of the files in a database.

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CBD(5) FILE FORMATS CBD(5)

File Header Format
The original format had a 40 byte header consisting of just
the first five integer values shown below plus 20 unused
bytes. The modified format used by mapbrowser adds eight
additional integer fields for a total of 52 bytes.

struct cbdhead {
long magic; /* Magic number */
long dictaddr; /* Offset of segment dictionary in file */
long segcount; /* Number of segments in file */
long segsize; /* Size of segment dictionary (bytes) */
long segmax; /* Size of largest segment's strokes, (bytes/2) */
/* the following apply to CBD_MAGIC2 only */
long maxlat,minlat,maxlong,minlong; /* Bounding box of map */
long features; /* bits indicate feature "ranks" present */
long scale_shift; /* bits to shift coordinate data */
long lat_offset; /* lattitude offset */
long lng_offset; /* longitude offset */
};
The first 4-bytes in Brian's original cbd files contain
0x20770002 (CBD_MAGIC). For the extended format, the first
word should contain 0x20770033 (CBD_MAGIC2).

#define CBD_MAGIC 0x20770002
#define CBD_HEADSIZE1 40 /* size of old header */
#define CBD_MAGIC2 0x20770033
#define CBD_HEADSIZE2 (sizeof(struct cbdhead))

In order to support efficient skipping of an entire file,
the extended format header includes coordinates defining the
bounding box of all vectors in the file and a bit mask indi-
cating which feature codes are present in the file. The
scale_shift value (if non-zero) is an exponent for scaling
the coordinate data by a power of two. A negative
scale_shift allows coordinates to be represented at a reso-
lution finer than integer seconds of latitude/longitude.
The latitude/longitude offsets are added to the vector data
after scaling. The scale and offsets are also applied to
the bounding box coordinates in the file header and segment
dictionary (I think).

Segment Header Format
Each segment begins with a segment header, followed by data
compressed strokes.

struct seghead {
BIT32 orgx,orgy; /* Origin of first stroke in segment */
BIT32 id; /* Segment identifier serial number */
BIT16 nstrokes; /* How many strokes in the segment follow */
};

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CBD(5) FILE FORMATS CBD(5)

The data-compression scheme uses integer seconds to
represent latitude/longitude coordinates (shifted and scaled
as indicated in the file header), and stores each stroke as
a [dx,dy] from the previous point. If dy will fit into 8
bits and dx into 7 bits, then the entire [dx,dy] is stored
in a 16-bit field with bit 0x4000 turned on as a flag. If
either value is too large for that scheme, then both are
stored as 32-bit values, with the 0x40000000 bit turned off
(even in negative numbers) in the first of them.

#define MAX8y ((short) 0x7F) /* largest signed 8-bit value */
#define MAX8x ((short) 0x3F) /* largest signed 8-bit value */
#define SHORTFLAG 0x4000 /* flag saying this is a short stroke */
#define SHORTBYTE 0x40 /* flag saying this is a short stroke */

Segment Dictionary Format
The segment dictionary sits on the end of the file, is
pointed to by the dictaddr value in the file header, and
points to the segment headers via the absaddr values.

struct segdict {
BIT32 segid; /* Segment identifier serial number */
BIT32 maxlat,minlat,maxlong,minlong; /* Bounding box of strokes */
BIT32 absaddr; /* Address in file of segment header */
BIT16 nbytes; /* # bytes of strokes that follow */
BIT16 rank; /* Type of feature this segment draws */
};
The bounding box information in each segment dictionary
entry allows clipped segments to be quickly skipped. The
rank value is a positive integer feature code indicating
which map feature type is represented by the segment. The
feature codes are normally defined in the mapset file for
the database.

Feature Codes
The feature code (rank) is an indication of what each seg-
ment depicts. The assignments of codes can be different for
different databases but must be consistent among cbd files
within a database. The following are the feature codes used
in the World Bank II database:

In "Boundary" files:
01 Demarcated or delimited boundary
02 Indefinite or in Dispute
03 Other line of separation of soverignity on land

In "Coast, Islands and Lakes" files:
01 Coast, islands and lakes that appear on all maps
02 Additional major islands and lakes
03 Intermediate islands and lakes
04 Minor islands and lakes
06 Intermittent major lakes

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CBD(5) FILE FORMATS CBD(5)

07 Intermittent minor lakes
08 Reefs
09 Salt pans -- major
10 Salt pans -- minor
13 Ice Shelves -- major
14 Ice Shelves -- minor
15 Glaciers

In "Rivers" files:
01 Permanent major rivers
02 Additional major rivers
03 Additional rivers
04 Minor rivers
05 Double lined rivers
06 Intermittent rivers -- major
07 Intermittent rivers -- additional
08 Intermittent rivers -- minor
10 Major canals
11 Canals of lesser importance
12 Canals -- irrigation type

FILES
cbdmap.h Header file for compressed binary map data-
base (*.cbd) files.

ciamap.c Program for converting from GS-CAM format to
cbd format.

/import/mapbrowser
Location of mapbrowser software and data at
Xerox PARC.

BUGS
The format described here includes extensions of Brian
Reid's original cbd format necessary to support features of
the mapbrowser program. Files in this format are unfor-
tunately not compatible with Brian's netmap program.

AUTHOR
Original format by Brian Reid <[email protected]>
Modified by Steve Putz <[email protected]>

SEE ALSO
mapset(5), mapbrowser(1), mapwriter(1)

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