MIL-PRF-89020B
primary file. It consists of four-byte records which are 32-bit integers
(high byte first). It is based on the first three or four characters of the
names in the primary file. Three characters are used for gazetteers with less
than 50,000 entries, and four are used for larger ones. The binary integer in
bytes 1-4 of the hash file tells how many characters were used. Characters
considered for the computation are given in the following table along with the
hexadecimal ASCII value and the decimal hash value. All other characters take
the value of the space.
ASCII
Character
Hex
Hash
space
20
0
`
27
1
,
2C
2
-
2D
3
.
2E
4
/
2F
5
A-Z
41-5A
6-31
grave
60
32
tilde ~
7E
33
3.9.6.4
GAZETTE.KEY. The key file is a dictionary of the 4-letter
designation codes. It is a text file of variable length records with
designation code in bytes 1-4, space in 5, variable length explanation, and CR
LF.
3.9.6.5
GAZETTE.DIR. This text file gives the abbreviated country
names, and expands them to their full length.
3.10
Logical file characteristics .
3.10.1 Data file structure. Arranged in 1° by 1° geographic areas.
Each data file shall contain data falling within a single one degree cell.
The reference origin for all data files shall be the southwest corner of the
one degree cell. Multiple data files shall be arranged primarily by ascending
latitude bands (90° South to 89° North), secondarily by ascending longitude
(180° West to 179° East).
3.10.2 Files extent. To provide overlap between adjacent data files,
the degree cell coverage in this standard includes the integer degree values
on all sides of the area. Each data record has one point of overlap with the
cell above and one with the cell below (if the record extends to the degree
cell limits). Entire data records lying on integer degree longitude values
shall exist also in the adjacent degree cell. Data files will not cross
integer degree latitude or longitude lines. Adjacent data files shall not
have gaps between them and the only overlap that exists is along adjacent
boundaries. All data files derived from coincident boundaries of adjacent
cells shall be comprised of duplicate data records.
3.10.3 Terrain elevation intervals . The latitude and longitude grid
spacing will be in whole second intervals. Latitude spacing will always be
consistent based upon the level of the data. Longitude spacing is dependent
upon the level of the data and the geographic zone. (See TABLES I, II and
III.)
3.10.4 Data value sequence. The elevations with a data record have a
constant longitude value. The first data value is the southernmost known
elevation and the last data value is the northernmost. No two data records
shall have the same longitude value.
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