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Therefore, generalization is constrained by the topical theme, the map de-
sign, the features available for processing, and the target scale. And as
Mustière and Moulin (2002) note, generalization decisions ought to be in-
formed by the spatial context of the database such as the relations between
objects in a geographic region. The complexity of the generalization in
turn is directly related to how clearly the features to be generalized can be
separated from their surroundings (Heinzle and Anders 2007).
The context of this research project is situated in relation to an ongoing re-
search effort with the United States Geological Survey (USGS) Center of
Excellence for Geographic Information Science (CEGIS). CEGIS has been
collaborating with the University of Colorado research team, designing
tools for creating reduced scale versions of hydrographic data for The
National Map (http://nationalmap.gov). Research efforts have been directed
towards creating fully automated generalization solutions which preserve
differing hydrographic characteristics reflected in specific physiographic
landscapes (Buttenfield et al. 2010). Differentiated generalization routines
are designed to reflect generic physiographic contexts across the cotermi-
nous Unites States, using terrain and climate factors (Stanislawski et al.
2010, forthcoming).
3- Research Objectives
Given the size and extent of the United States, the NHD is a substantially
large and varied dataset. The USGS maintains and coordinates the NHD as
a vector database containing surface water features of the United States.
Several versions of this database include a high resolution compilation, for
use at 1:24,000 (24k); and a medium resolution version compiled for use at
1:100,000 (100k) (http://nhd.usgs.gov/data.html). A local resolution ver-
sion is currently under development for selected parts of the country which
densifies the features in the 24k database.
In best cartographic practice, topographic base maps will often delineate a
primary water channel or centerline. At present, the NHD schema does not
incorporate a centerline attribute. This is due to interrelated factors. First,
the 24k NHD is updated on an irregular cycle which can be very short, and
based on unexpected events (e.g., storms or floods) or on expected events
(e.g. impacts due to urbanization, mining, or human development). Essen-
tially, the database is updated more frequently in regions where changes to
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