382
Chapter 13 Digital Landscaping
topographic maps in all of their incarnations can represent terrain with contours, there are more detailed methods of digital terrain modeling available
through geospatial technology.
How Can Geospatial Technology
Represent Terrain?
In geospatial technology, terrain and landscape features can be represented
by more than just contour lines on a map. A Digital Terrain Model (DTM)
is the name given to a model of the landscape that is used in conjunction
with GIS or remotely sensed imagery. The function of a DTM is to accurately
represent the features of the landscape and be useful for analysis of the terrain itself. The key to a DTM is to properly represent a z-value for x and y
locations. With a z-value, the model can be shown in a perspective view to
demonstrate the appearance of the terrain, but this doesn’t necessary make
it a three-dimensional model. In fact, a DTM is usually best described as a
two-and-a-half dimensional model. In a two-dimensional (2D) model, all
coordinates are measured with x and y values without a number for z at
these coordinates. In a two-and-a-half-dimensional (2.5D) model, a single z value can be assigned to each x/y coordinate as a measure of elevation
at that location. In a full three-dimensional (3D) model, multiple z-values can be assigned to each x/y coordinate. Most DTMs have one elevation
value measured for the terrain height at each location, making them 2.5D
models (Figure 13.5).
An example of a type of DTM that’s used in terrain modeling is a TIN (Triangulated Irregular Network), in which selected elevation points (those that
the system deems the “most important”) of the terrain are used in constructing the model. Points are joined together to form non-overlapping triangles,
representing the terrain surfaces (see Figure 13.6). While TINs are often
used in terrain modeling in GIS, there’s another very common type of digital
terrain model called the DEM.
A similar question was raised back in Chapter 2: If all
of the latest USGS topographic maps are available
digitally and in easily accessible formats, is there
still a need to have printed copies (at a variety of
map scales) on hand? For instance, do surveyors,
geologists, botanists, or archeologists (or any other
professional who requires topographic information
while in the field) need to carry printed topo maps
with them? Is there a need for geographers to have
printed versions of several quads when seamless
digital copies (which can be examined next to one
another) can be easily (and freely) obtained? Are
there situations where a printed topo map is a necessary item given the readily available digital data?
If Everything’s Digital, Do We Still Need Printed Topo Maps?
Thinking Critically with Geospatial Technology 13.1
DTM a representation
of a terrain surface
calculated by
measuring elevation
values at a series of
locations.
two-and-a-halfdimensional (2.5D)
model a model of the
terrain that allows for
a single z-value to be
assigned to each x/y
coordinate location.
three-dimensional
(3D) model a model of
the terrain that allows
for multiple z-values to
be assigned to each x/y
coordinate location.
TIN Triangulated
Irregular Network. A
terrain model that
allows for non-equally
spaced elevation points
to be used in the
creation of the surface.
Chapter 13 Digital Landscaping
topographic maps in all of their incarnations can represent terrain with contours, there are more detailed methods of digital terrain modeling available
through geospatial technology.
How Can Geospatial Technology
Represent Terrain?
In geospatial technology, terrain and landscape features can be represented
by more than just contour lines on a map. A Digital Terrain Model (DTM)
is the name given to a model of the landscape that is used in conjunction
with GIS or remotely sensed imagery. The function of a DTM is to accurately
represent the features of the landscape and be useful for analysis of the terrain itself. The key to a DTM is to properly represent a z-value for x and y
locations. With a z-value, the model can be shown in a perspective view to
demonstrate the appearance of the terrain, but this doesn’t necessary make
it a three-dimensional model. In fact, a DTM is usually best described as a
two-and-a-half dimensional model. In a two-dimensional (2D) model, all
coordinates are measured with x and y values without a number for z at
these coordinates. In a two-and-a-half-dimensional (2.5D) model, a single z value can be assigned to each x/y coordinate as a measure of elevation
at that location. In a full three-dimensional (3D) model, multiple z-values can be assigned to each x/y coordinate. Most DTMs have one elevation
value measured for the terrain height at each location, making them 2.5D
models (Figure 13.5).
An example of a type of DTM that’s used in terrain modeling is a TIN (Triangulated Irregular Network), in which selected elevation points (those that
the system deems the “most important”) of the terrain are used in constructing the model. Points are joined together to form non-overlapping triangles,
representing the terrain surfaces (see Figure 13.6). While TINs are often
used in terrain modeling in GIS, there’s another very common type of digital
terrain model called the DEM.
A similar question was raised back in Chapter 2: If all
of the latest USGS topographic maps are available
digitally and in easily accessible formats, is there
still a need to have printed copies (at a variety of
map scales) on hand? For instance, do surveyors,
geologists, botanists, or archeologists (or any other
professional who requires topographic information
while in the field) need to carry printed topo maps
with them? Is there a need for geographers to have
printed versions of several quads when seamless
digital copies (which can be examined next to one
another) can be easily (and freely) obtained? Are
there situations where a printed topo map is a necessary item given the readily available digital data?
If Everything’s Digital, Do We Still Need Printed Topo Maps?
Thinking Critically with Geospatial Technology 13.1
DTM a representation
of a terrain surface
calculated by
measuring elevation
values at a series of
locations.
two-and-a-halfdimensional (2.5D)
model a model of the
terrain that allows for
a single z-value to be
assigned to each x/y
coordinate location.
three-dimensional
(3D) model a model of
the terrain that allows
for multiple z-values to
be assigned to each x/y
coordinate location.
TIN Triangulated
Irregular Network. A
terrain model that
allows for non-equally
spaced elevation points
to be used in the
creation of the surface.