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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
The second stage of the process handles the situation where a
contour line's all neighbor contour lines are on the same
elevation. As in figure 5 contour lines a, b and e are adjusted by
the first stage, c and d are passed to and processed in the second
stage.
c
3. AUTOMATIC CONTOUR LABELLING
Based on above relation model and the inherent properties of
contour lines it is possible to propagate the known local
information (elevation) to entire contour map. Elaboration in
last section is based on the assumption that all involved contour
lines have set the right elevation. It is better to extend the above
definition at elevation monotone region of the map.
Lemma if Dc,;c; » D, and c; is on the left (right) of c;, then
Ni(cj)9e» (N.(c;)7c;). and further H(c;)* Zl H-«H(c;Y( H(cj)- À
H-H(c;). Where Dc;c; is the degree of proximity of contour
pair (¢;, c») and D, is the threshold value. |
The process of automatic labelling elevation value to contour
line is as following.
1. Construct CDT on the contour map and compute the
degree of proximity
2. Interactively labelling two contour lines c; and c»
with the right elevation value. Saddle in the contour map
produces ambiguous when propagate elevation using
above rule. Thus the two setting contour lines must be
lower than the lowest saddle. Adjust the c; and c; to the
right direction as mentioned in section 2.3.
3. Every adjusted contour line is used as a referent
contour line once. Get all contour lines that have relation
of l-order proximity with the referent contour line a, label
the left ones and the right ones whose degree of proximity
with the referent contour line is great than D, with H(a)*
AH and H(a)- AH respectively. If à is a closed contour
line and the adjusting ones on the right side are closed too,
label them with H(a). Adjust the direction of them.
Reiterate this step until all contour lines is labelled with
elevation.
Figure 6 Ambiguous situation when automatically labelling
contour lines
At some situation it may not be enough by interactively setting
only two contour lines as local known information. As in figure
6, assuming contour line a is a reference and ^ and c are on its
left, then ^ and c will be labelled as H(a)* AH, but it is wrong
if a and b are one contour line broken by the bound actually. So
if we label some contour line, e.g. f; as the highest contour, then
after the automatic process reaches fa reversing process can be
triggered to check if there is wrong.
There is other situation like local pits that cannot get the right
result when using above approach because the above approach
takes that the whole map has not negative region. This problem
can be dealt with the strategy as mentioned last paragraph by
specifying the contour line at which elevation increment is
becoming negative.
4. CONNECTING BROKEN PARTS OF CONTOUR
LINES
There are broken contour lines in typical topographic maps
caused by restricted space in map and the intersections with
other features and annotations. The broken contour lines would
become even more after the process of scanning and automatic
recognition. Connecting the broken parts into one contour
manually is evidently not an elegant solution. The automatic
approach of this task includes two steps: identify the broken
parts of one contour line first and connect them in sequential
order secondly. Based on the topological relation the 7 parts 1a;
| i = 1, n} that are bona-fide one contour line satisfy three
conditions.
|. All parts have equal elevation.
2. Every part at most has one end node at the boundary
of the contour map. And of all parts, the count of end
nodes at the boundary of contour map does not
exceed two.
3. All parts have same topological relation (Ni, N. or N,)
with one contour line or multiple contour lines who
are 0-order proximal (Np).
Figure 7 The connection of broken contour lines
The second step is the connection of broken parts of one
contour line, which is performed in CDT. There may be various
situations of the set of the broken parts. Figure 7 illustrates one
of them and the connection process is designed as below.
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