Geoscience Reference
In-Depth Information
stream networks occur (http://nhd.usgs.gov/nhd_faq.html). Second, the
sheer volume of NHD stream data precludes computing a new hierarchy of
centerlines for all basins and subbasins nationally, for a database undergo-
ing frequent updates. In response to the cartographic demand for centerline
inclusion on topographic maps, the CEGIS and CU research teams have
designed a tool to identify a centerline automatically during generalization
processing, but to do so it is necessary to enrich the high resolution NHD
with additional attributes. Once delineated at the largest (compiled) scale,
centerlines can be simplified for smaller mapping scales, albeit using toler-
ance parameters which differ from those applied to other flowlines.
Previous approaches to centerline delineation are also known as methods
for finding the “main channel” or the “primary channel”. Central to these
many approaches is the idea that the generalization process should pre-
serve enough structural detail to maintain the continuity of all features, and
their relative position in a geographical network (Thomson and Brooks
2007). Marr (1982) refers to the natural acuity of humans to see continua-
tion along a network; and Thomson and Richardson (1999) cite visual con-
tinuity of geographic phenomena as crucial for the quality of communica-
tion and interpretation of maps.
A potential proxy for subbasin centerlines is maintained by the NHD as
a feature type named "artificial path”. Artificial paths delineate the flow
of water through polygonal features, such as a large river
(http://nhd.usgs.gov/nhd_faq.html), and establish continuity of the stream
network through lakes, ponds, and other polygonal water features. How-
basin in Missouri (NHD subbasin # 10290107), while the set of artificial
paths present numerous segments contributing to a centerline feature, the
set is often not continuous.
