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some other NHD subbasins. This limitation will be discussed in more
detail later in the paper.
Following centerline delineation, a post-processing step is required to
actually enrich the database with centerline attribution. First a field is
added to the original NHDFlowline table to store a flag for every feature
which forms a part of the centerline. Post-processing selects all original
NHDFlowlines that match part or all of a reach in the master centerline
list, and adds a flag for the selected flowlines. This attribute field is not
part of the original NHD schema, but enriches the NHD file and can be
accessed subsequently for cartographic base mapping, or for generalization.
5- Results
The delineation algorithm was implemented as a tool using Python 2.5
coding and geoprocessing functions available in ArcGIS 9.3.1.
Figure 5
shows results for delineating the centerline of the Pomme de Terre sub-
basin. The total length of all artificial paths (
Figure 5
, left) is 808 km,
comprised of 2,901 reaches. Following the preprocessing steps of the algo-
rithm which intersect all reaches with water polygons, the total length is
reduced to 462 km comprised of 1,083 reaches. In comparison to the
method of simply selecting artificial paths, the intersection method elimi-
nates more than 1,800 reaches which could not reasonably become a part
of the centerline, thus reducing by more than half the set of reaches which
the algorithm must search. Following complete delineation (
Figure 5
,
right) the centerline length grows to 488 km with 1,152 reaches. The
algorithm fills a total of 26 km of gaps, delineating an additional 69
reaches.
6- Discussion
The algorithm presented in this research presents an automated method for
centerline delineation of NHD subbasin data. Development of this algo-
rithm is one tool in an ongoing project to build a geoprocessing toolbox
containing routines for data enrichment, pruning, simplification and vali-
dation, which will be disseminated for use with
The National Map
.
