The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Voi XXXVII. Part B4. Beijing 2008
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3.3 Construction of the Derived Single-line river network
Topology
After extracting of the wide rivers and lakes centerlines we
obtain a tree-like single-line network, but the arcs have no
relationship yet. We use the POLYVRT model to construct the
topology structure of the river network. The POLYVRT
topology structure, which is based on nodes and arcs, is
developed by American Laboratory for Computer Graphics and
Spatial Analysis. In this structure, topology relationships are
apparently kept in the Feature Attribute Table, so spatial
analysis can be carried out relatively faster. POLYVRT
structure is efficient for linear analysis.
With the help of the software ARC/INFO, we construct the
river network topology structure. After calculating topology
relationship using CLEAN command, two fields FNODE and
TNODE are added to the attribute table. Now the attribute table
contains a complete network topology relationship, it records
all the nodes in the network and their connection relationship.
FNODE and TNODE represent the start and end node of a arc,
the arcs that share a common node are connected. The
following figure shows the attribute table after calculating the
topology structure.
River-ID
Shape
FNODE
TNODE
1
Polyline
1
2
2
Polyline
2
3
3
Polyline
2
5
4
Polyline
4
5
5
Polyline
5
6
Figure5. The attribute table constructed by calculating topology structure
4 FINDING THE MAINSTREAM AND CALCULATING
RIVER LENGTH
4.1 The Three Principles for Identifying Mainstream
The exact length of a river is determined by the headwater and
embouchure. Unlike embouchure which is easy to be located in
the topographic data, the headwaters is difficult to be found, for
large rivers usually have a great many of tributaries (every
tributary has its own headwaters). So we have to select which is
the appropriate headwater that defines the source of a river.
Many geographers have made great efforts to trace the sources
of rivers, and they summarize three important principles to be
accorded to select appropriate mainstream. 1) Length: The
headwater stream from which the river has the longest length
can be the mainstream; 2) Orientation: The headwater stream
which accord to the main orientation of the whole river can be
the mainstream; 3) Amount of water: The headwater stream
which has the largest amount of water can be the mainstream.
Now, 1 will illustrate what the tree principle mean and how they
are presented and calculated by the computer.
4.1.1 Length:
The length of the mainstream is the sum length of the arcs that
constitute the mainstream. When selecting mainstream for the
purpose of calculating the river length, apparently this principle
is the most important factor among the three factors. But if we
adopt this principle alone to select mainstream, this often can’t
lead to satisfactory results.
As illustrated by figure 6, Point A is the embouchure of the river,
points B, C, D are three choices that can be the headwaters and
the mainstream is the river between the point A and the
headwaters. If identifying mainstream using the length principle