have subsequently changed through erosion, deposition, and other geomorphic processes.
Despite the power of GIS, reliance on old data may unknowingly skew, bias, or negate results.

b. Areal Cover

Data on a give area may be completely lacking, or only partial levels of information may be
available for use in a GIS project. For example, vegetation or soils maps may be incomplete
at borders and transition zones and fail to accurately portray reality. Another example is the
lack of remote sensing data in certain parts of the world due to almost continuous cloud cover.
Uniform, accurate coverage may not be available, and the user must decide what level of
generalization is necessary, or whether further collection of data is required. ८. Map Scale

The ability to show detail in a map is determined by its scale. A map with a scale of 1:1000
can illustrate much finer points of data than a smaller scale map of 1:250000. Scale restricts
type, quantity, and quality of data (Star and Estes 1990). One must match the appropriate scale
to the level of detail required in the project. Enlarging a small scale map does not increase its
level of accuracy or detail.

d. Density of Observations

The number of observations within an area is a guide to data reliability and should be known
by the map user. An insufficient number of observations may not provide the level of
resolution required to adequately perform spatial analysis and determine the patterns GIS
projects seek to resolve or define. A case in point, if the contour line interval on a map is 40
feet, resolution below this level is not accurately possible. Lines on a map are a generalization
based on the interval of recorded data, thus the closer the sampling interval, the more accurate
the portrayed data.

e. Relevance

Quite often the desired data regarding a site or area may not exist, and "surrogate" data may
have to be used instead. A valid relationship must exist between the surrogate and the
phenomenon it is used to study but, even then, error may creep in because the phenomenon is
not being measured directly. A local example of the use of surrogate data are habitat studies
of the golden-cheeked warblers in the Hill Country. It is very costly (and disturbing to the
birds) to inventory these habitats through direct field observation. But the warblers prefer to
live in stands of old growth cedar Juniperus ashei. These stands can be identified from aerial
photographs. The density of Juniperus ashei can be used as surrogate measure of the density
of warbler habitat. But, of course, some areas of cedar may uninhabited or inhibited to a very
high density. These areas will be missed when aerial photographs are used to tabulate habitats.

f. Format

Methods of formatting digital information for transmission, storage, and processing may
introduce error in the data. Conversion of scale, projection, changing from raster to vector