CALCULATING RIVER LENGTH BASED ON TOPOGRAPHIC DATA
ZHANG Lijuan 1 ' 2 HUANG Wei 2 JIANG Jie 2
’China University of Mining and technology, Xueyuan road, Haidian district,Beijing,China
2 National Geomatics Center of China, Baishengcun, Zizhuyuan, Beijing, China -
zhanglijuan_ngcc@ 163 .com
KEYWORDS: River Length, Topographic Data, Identifying Mainstream, Centerline, Multiple Criteria Decision
ABSTRACT:
River length has been the particular interest of people. Topographic data, which are obtained from field survey or digitizing from
topographies, can serve as ideal data source for river length calculating. The topographic data are vector-based data, which represent
river entities as polygonal line. Depending on data scale, rivers are represented in topographic data as single-line (narrow) or
double-line (wide). Unlike single-line rivers, double-line rivers can’t be used to computing length directly. So we constructed a
tree-like single-line river network by replacing double-line rivers and lakes with their centerlines. The exact river length is
determined by the position of the headwater and the position of the embouchure. The headwater is difficult to be located in the
topographic data, for relatively large rivers usually have numerous tributaries. There are presently three principles to accord when
determining the mainstream and the main headwater: a) Length; b) Orientation; c) Amount of water. We use the multiple criteria
decision approach, which is a well-known general approach, to determine the best one from many choices based on the three factors
and their weights.
1. INTRODUCTION
River length has been the particular interest of people, for
rivers are the ecological chain on which human and other life
rely and they play an important role in human civilization
development. Considerable work has been done by geographers
tring to find out the exact river length. The traditional way is
surveying the river length from the topographies. The
topographies are derived through field survey by surveyors.
This method is time and labor wasting, and often fails to get
precision result. Since topographic data are obtained from field
survey or digitizing from topographies, they can serve as ideal
data source for river length calculating.
The start and end point of a river are referred to headwater and
embouchure. Headwater is the water from which a river rises,
and it can be fountain, lake or glacier. The headwater differs
due to variable situations. Embouchure is the end of a river
where it flows into the sea, other river (tributary flows into
mainstream), lake, etc. The exact river length is determined by
the following factors: the position of the headwater; the
position of the embouchure and the source data used. Unlike
the embouchure, which can be easily found out, the headwater
is difficult to be made certain in the topographic data.
Generally speaking, relatively large rivers usually have more
than one headwater stream, so we have to select an appropriate
headwater stream as the mainstream while leave others as
tributaries. There are presently three principles to accord when
determining the mainstream and the main headwater: a) Length:
The headwater stream from which the river has the longest
length can be the mainstream; b) Orientation: The headwaters
stream which accord to the main orientation of the whole river
can be the mainstream; c) Amount of water: The headwater
stream which has the largest amount of water can be the
mainstream. The multiple criteria decision approach is a
well-known general framework to determine the best one from
many choices based on multiple factors and their weights. This
optimization technique is well founded in mathematics,
operations research, and in decision making. This general
concept allows for the solution of an overall, complex problem.
This paper proposes to use multiple criteria decision approach
for mainstream selection based on the three factors discussed
above.
The topographic data are vector-based data, which represent
river entities as polygonal line. Depending on data scale, rivers
are classified as single-line (narrow) or double-line (wide).
Unlike single-line rivers, the double-line rivers can’t be used to
computing length directly. So a single-line river network and its
topology structure must be constructed before performing
further analysis.
The paper is organized as follows: after a review of related
work, the construction of single-line river network using the
centerline replacing the double-line river is presented, together
with some examples showing the possibilities of wide rivers
and lakes. Then the approach for mainstream selection based on
multiple criteria decision is shown, giving the theoretical
background. Finally, a summary concludes the paper.
2. RELATED WORK
Centerlines are traditionally used to generalize river and road
systems (Nickerson, 1998), and it is well studied both in image
analysis (Duda and Hart 1973) and computational geometry
(Aggarwal et al. 1989). It can be extracted by computing
Voronoi diagrams (Boissonnat et al. 1993; Fortune 1987) or
constrained Delaunay triangulations (chew 1989). McAllister
and Snoeyink present the medial axis generalization of river
networks to get the benefit of calculating surface area.
There are mainly three factors affecting the identifying of the
mainstream (LIU 2001; QU 2004). Qu (2004) studies the
XIUHE upriver situation and compares three large tributaries
using the three factors to determine the headwaters of XIUHE
river. WU (1995) present an approach that constructs the river
system and selects the mainstream of a river mainly based on
the length principle. GUO (2003) organizes river data as graph
structure and determines the mainstream using its depth and
orientation in the graph.