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