accuracy of the least accurate data layer. Frequently, the GIS user must decide on
the degree of inaccuracy acceptable for a project. For a project involving parcels
of land, high-orders of accuracy (to millimeters if possible) might be warranted.
For estimation of runoff from a large drainage basin, lesser accuracy might be
acceptable because the soils data layer includes boundaries that are lines attempting
to make sharp differentiation in what is usually a continuum of soil characteristics
transitioningover many meters. The production of spatially accurate geographic
data is not a trivial matter.
There are two types of geographically referenced data models attached to GIS
attribute data. They are raster and vector data. Raster data are conceived of as in
a regular grid of cells of definite size; sizes vary considerably among GIS projects.
The cells usually range from meters to many kilometers on a side. They are useful
with spatially continuous data such as elevation. For example, an elevation can be
assigned as a cell attribute (the elevation could be a spatial average of all eleva-
tions in the cell or the elevation at the center of the cell). Vector data are composed
of polygons, lines, and points that represent discrete objects and features. Today’s
vector data models include topology in which a polygon, line, or point is associ-
ated with its neighboring vectors. This allows spatial associations of adjacency
and connectivity to be maintained no matter which scales or projections are used.
In complex GIS work, the GIS analyst frequently makes conversions from raster to
vector and vice versa and this involves conscious trade-offs of characteristics to
obtain the necessary accuracy of results.Geomorphology
Geomorphology is the science of the description, analysis, and the specification of
processes causing landforms. The term was coined in the U.S. Geological Survey
in the 1880s, although its scientific roots were much older because previous scientists
had been studying the erosion of sculpting landscapes. The geographer William
Morris Davis was a forceful early proponent of geomorphology and his considerable
influence within geography helped it become the intellectual linchpin of the fledging
discipline of geography in the early 20th century. Academically, geomorphology
became part of both geography and geology curricula with some consternation
among geographers and geologists continuing to this day. If one considers the scope
of geography, geology, and geomorphology, it is clear that geomorphology occupies
the nexus between the disciplines and contributes to both.
Nascent geomorphology has been around for two millennia. Classical Greek,
Roman, and Arab scholars wrote numerous travel geographies that have survived.144 Geomorphology