Theoretical Background - Response of Flood Events to Land Use and Climate Change - page 13

The terrain is simulated as a series of planar triangular facets produced by joining
all the adjacent points. The assumption that the surfaces are planar is adequate if the
points have been chosen at changes in grade in the terrain. The accuracy of the TIN
model can be improved by the addition of break lines. These lines represent dis-
continuities in the terrain surface such as cliffs, ridges, and streams and they indicate
where interpolation between adjacent points is invalid [ 3 ]. b. In a gridded DEM, the
elevation points are spaced at a regular interval to create a grid or lattice [ 3 ]. DEM
represents a surface as a regular grid of locations with sampled or interpolated values.
By incorporating stream lines and cliff lines, a gridded DEM can represent all
the discontinuities that can be represented by a TIN and has the significant
advantage of being directly compatible with other sources of natural resource data
in grid form. It is also readily used by many grid-based applications [ 3 ].
Where the gridded DEM has been derived from a primary data source such as
contours or a TIN, then the direct relationship between the elevation value and the
actual value on the ground, at the respective location, is dependent on the
algorithm used to interpolate the grid and the resolution of the grid itself.
The DEM, as its name indicates, is a model of the elevation surface. However,
the DEM is often not treated as a model, but is commonly accepted as a ''correct''
representation of the earth's surface. But, it should not be neglected that DEM
data, like other spatial data sets are subject to error. The estimation of errors in a
DEM is often not evaluated by DEM users and applicants [ 8 ]. Where the DEM is
directly observed from aerial photogrammetry or a field survey, the elevation value
is truly representative of the value that is found on the ground at the location of
that point, provided there is no significant measurement error.
Significant measurement errors can arise due to ground cover by vegetation and
buildings, especially when elevations are measured by aerial and space-borne
platforms. These measurements are also affected by complex terrain [ 9 ]. This is
necessary if the DEM were to be used for aircraft flight planning where the
minimum flying height of the aircraft is critical. This could also aid the sitting of
signal transmitters and receivers, although high points could then have errors in
horizontal position by up to half of one grid interval [ 3 ]. In areas away from peaks,
these values are approximately the same. In general, Burrough [ 10 ] and Wise [ 23 ]
listed the possible errors in DEM data sets as the following:
• Data errors due to the age of data, incomplete density of observations, or results
of spatial sampling.
• Measurement errors such as positional inaccuracy, data entry faults.
• Processing errors such as numerical errors in the computer, interpolation errors,
or classification and generalization problems.
DEM is usually produced form sampled or observed data points that are used as
its source. The contour lines themselves may represent a model of terrain. It can
also be acquired directly, for instance photo grammatically from a stereo model or
indirectly, from analog cartographic data, satellite images or by field surveying,
and so on. Ideally, the data sources are applied without application of interpolation
techniques. Application of interpolation techniques is not necessary if the data