Geoscience Reference
In-Depth Information
viding a framework for assessment of the accuracy of distribution models
developed with
GIS
.
Source of errors
GIS data layers are traditionally classified according to their data structure,
either raster or vector. To a certain extent, both error sources and accuracy eval-
uation methods have been investigated following this traditional classification.
Raster images may be obtained from remote sensing instruments carried by
aircraft or spacecraft platforms, or by converting an existing line map (vector
data structure) to a raster data structure. Two types of error are inherent in
remotely sensed images: geometric and radiometric. These error sources are
addressed in detail in numerous monographs and papers, including Colwell
(1983) and Richards (1986).
A raster image is usually made up of a regular grid of adjacent rectangular
cells or pixels (i.e., a rectangular tessellation). Geometric error in a remotely
sensed image is caused by movement in the remote sensing platform; distor-
tion caused by the earth's curvature and terrain; different centrifugal forces
from earth affecting spacecraft movement; the earth's rotational skew; distor-
tions introduced by the remote sensing device itself, including systematic dis-
tortions caused by sampling sequentially from each detector and nonlinear
scanning (Adomeit et al. 1981); and errors introduced by the georeferencing
process. Geometric error causes a point on the remotely sensed image to occur
in the wrong position relative to other points in the image.
Correction of geometric errors in remotely sensed data is now a routine
aspect of their preprocessing. The map or image is usually rubber-sheeted to fit
it to an appropriate map projection. Corrected images with geometric errors of
less than 0.5 pixel are now obtainable and acceptable
6
(Ford and Zanelli 1985;
Ehlers and Welch 1987; Skidmore 1989b). However, the base maps from
which control point information is derived may be of poor quality. Bell (1986)
reported that maps used to geometrically correct images of the Great Barrier
Reef contained errors of up to 1 km. The accurate selection of control points
is crucial in obtaining acceptable results.
Points within a rubber-sheeted image are no longer on a regular grid
because they have been warped to fit into the projection defined by the ground
control points (
GCP
s). To obtain a regular grid, an interpolation method is used
to nominate a value for a regular grid point that falls between the points in the
rubber-sheeted image. Lam (1983) provides an excellent review of other inter-
polation methods, including splines, finite difference, and kriging.
Radiometric errors occur as a result of differential scattering of electromag-
