Geology Reference
In-Depth Information
work on landform elements and their descriptors as
prosecuted by the morphological mappers.
DEMs are, therefore, a subset of DTMs. Topographic
elements of a landscape can be computed directly from a
DEM (p. 170). Further details of DEMs and their appli-
cations are given in several recent topics (e.g. Wilson and
Gallant 2000; Huggett and Cheesman 2002).
Geomorphometry
A branch of geomorphology -
landform morphometry
or
geomorphometry
- studies quantitatively the form
of the land surface. Geomorphometry in the modern
era is traceable to the work of Alexander von Humboldt
and Carl Ritter in the early and mid-nineteenth cen-
tury (see Pike 1999). It had a strong post-war tradition
in North America and the UK, and it has been 'rein-
vented' with the advent of remotely sensed images and
Geographical Information Systems (GIS) software. The
contributions of geomorphometry to geomorphology
and cognate fields are legion. Geomorphometry is an
important component of terrain analysis and surface
modelling. Its specific applications include measuring the
morphometry of continental ice surfaces, characterizing
glacial troughs, mapping sea-floor terrain types, guiding
missiles, assessing soil erosion, analysing wildfire prop-
agation, and mapping ecoregions (Pike 1995, 1999).
It also contributes to engineering, transportation, public
works, and military operations.
PROCESS
Geomorphic systems
Process geomorphologists commonly adopt a
systems
approach
to their subject. To illustrate what this
approach entails, take the example of a hillslope system.
A hillslope extends from an interfluve crest, along a val-
ley side, to a sloping valley floor. It is a system insofar as
it consists of things (rock waste, organic matter, and so
forth) arranged in a particular way. The arrangement is
seemingly meaningful, rather than haphazard, because it
is explicable in terms of physical processes (Figure 1.7).
The 'things' of which a hillslope is composed may be
described by such variables as particle size, soil moisture
content, vegetation cover, and slope angle. These vari-
ables, and many others, interact to form a regular and
connected whole: a hillslope, and the mantle of debris
on it, records a propensity towards reciprocal adjustment
among a complex set of variables. The complex set of
variables include rock type, which influences weathering
rates, the geotechnical properties of the soil, and rates
of infiltration; climate, which influences slope hydrology
and so the routing of water over and through the hills-
lope mantle; tectonic activity, which may alter baselevel;
and the geometry of the hillslope, which, acting mainly
through slope angle and distance from the divide, influ-
ences the rates of processes such as landsliding, creep,
solifluction, and wash. Change in any of the variables
will tend to cause a readjustment of hillslope form and
process.
Digital elevation models
The resurgence of geomorphometry since the 1970s is
in large measure due to two developments. First is the
light-speed development and use of
GIS
, which allow
input, storage, and manipulation of digital data repre-
senting spatial and aspatial features of the Earth's surface.
Second is the development of
Electronic Distance
Measurement
(
EDM
) in surveying and, more recently,
the
Global Positioning System
(
GPS
), which made the
very time-consuming process of making large-scale maps
much quicker and more fun. The spatial form of surface
topography is modelled in several ways. Digital repre-
sentations are referred to as either
Digital Elevation
Models
(
DEMs
)or
Digital Terrain Models
(
DTMs
).
A DEM is 'an ordered array of numbers that represent
the spatial distribution of elevations above some arbitrary
datum in a landscape' (Moore
et al
. 1991, 4). DTMs are
'ordered arrays of numbers that represent the spatial dis-
tribution of terrain attributes' (Moore
et al
. 1991, 4).
Isolated, open, and closed systems
Systems of all kinds are open, closed, or isolated accord-
ing to how they interact, or do not interact, with their
surroundings (Huggett 1985, 5-7). Traditionally, an
iso-
lated system
is a system that is completely cut off from
its surroundings and that cannot therefore import or
