{ :-
Message Nature FAR Encoding process chart word |Length
(0-60) e
[p
$, 000 0.40 1 | I
32 ool 0.18 00] 3
9s oto 0-10 oil 3
$9, oll 0-10 0000 4
js 100 0.07 0100 4
9% iol 0.06 0101 4
97 Io 0.05 90010 S
98 H1 0:04 00011 5
Fig. 4 An example of Huffman encoding
small,
(2) Assign the digits O and 1 to the two mersages
with the smallest probabilities respectively, then
calculate the probability summation of that two mes-
sages.
(3) Take the above-mentioned sum of probabilities as
the probability of a new message, put it in the rest
original messages and rearrange the reduced set in
accordance with a sequence of declining probabili-
ties.
(4) Repeat the previous steps (2) and (3) till the
summation of two probabilities is equal to unit at
last.
(5) Set out the latest assignment of digits, trace
the way adverse to the encoding progresa of indivi-
dual messages and pick the allocated binary digits
at all steps in eucceesion. Finally the digital
etrings of O and 1 placed in such a way are taken
as the code words corresponding to the individual
messages respectively. Figure 4 illustrates an ex-
amnle of Huffman coding.
The principles of optimum coding which digital
image has to observe are: (1) the uniqueness of
decoding, and that any short code word does not
form a prefix of the longer one; (2) the mersage
with larger probability deserves a shorter code
word, and the message with smaller probability a
larger code word. Huffman coding meets the above-
mentioned elements at all. It is preferable to uti-
lize run length coding in cooperation with Huffman
coding. Generally run length coding ies conducted
rrior to the implementation of Huffman coding so
that one can reach the coding efficiency as high
as possible,
434
5. THE COMMUNICATION MODEL OF DIGITAL
MAPPING
In Figure 5 there is showed a model of communica-
tion system that is discussed in Information theory
. It ie also appropriate for dealing with the other
information transmiesion systems, so the diagram in
Figure 5 is a general information processing model.
We use the common flow scheme of digital mapping as
an example to explain the actual meaning in each
block of that diagram.
In the information source of digital mapping there
are unrectified airphotos, specification parameters
of camera, coordinates of ground controls and other
initial data. Aerial imagery and geodetic coordina-
tes belong to the different record types of geogra-
rhic information. The image in airphotos is merely
some approximate impression of the real rcenery ap-
rearance of the earth surface. The original airpho-
tos often involve various deformations due to both
geometric and optical errors in the aerial photo-
grarhic process. In those errors there are not only
the systematic but also the accidental ones. This
shows the information source consists of signals
and noises from the beginning. One can not adopt a
simple communication mode before data preprocessing.
Information destination is the output results of
digital mapping. They usually include the topogra-
phic and thematic maps, the orthophotomaps and the
other documentation of iconic represetation which
are rendered to urers. These products contain the
reliable records of the geographic information re-
lated to the aerial survey area. They may become a
significant input part of the regional geographic
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
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