Key concepts 1 GIS - Spatial Data Analysis: An Introduction for GIS Users - page 13

h e datum is dei ned such that it has the best i t to the surface of the Earth over the
area of interest.
Any projection entails making compromises, for example some projections distort
areas while others distort distances. Conformal projections preserve shape while equal
area projections preserve areas (as their name suggests) and not shapes—clearly
a projection cannot preserve both properties. A common example of a conformal pro-
jection is the Mercator projection and an example of an equal area projection is the
Albers equal area projection. Conformal projections like the Mercator projection pre-
serve angles locally and thus the Mercator projection is well-suited to the purpose of
navigation, for which it was developed. Projections also exist which preserve distances
along one or several lines (Clarke, 1999). An awareness of projection systems is impor-
tant in working with spatial data. If, for example, the concern is with areas of countries
then an equal area projection must be used. h e impact of choice of projection is a
function of the spatial scale of the map. In short, the larger the area of concern, the
greater the impact of selection of an appropriate projection will be (Clarke, 1999) and
the impact of poor choice of projection when using a map with a representative frac-
tion (i.e. ratio of distances on a map to distances in the same units in the real world) of
1:1,000,000 will be greater than when using a 1:10,000 map. A brief introduction to
projections is provided by Longley et al. (2005a) while a more detailed account is
given by Seeger (2005). Scale is discussed in Section 2.7.

Georeferencing

2.5

To make spatial data useable, they must be linked to some kind of spatial referencing
system, as detailed above. h e process of attaching spatial information to data is called
georeferencing. An example of georeferencing is the use of coordinates obtained using
a global positioning system (GPS) receiver to link positions on a remotely sensed
image to positions that have been surveyed, and the linked survey points are called
ground control points (GCPs). h e process of transforming an image, whereby the
transformed image i ts well to the GCPs and the image is then in coordinate space, is
termed 'georectii cation. Georectii ed images can be overlaid or combined with other
data that are georeferenced using the same system. h e term 'geocoding' is generally
used in relation to the determination of geographic coordinates from an address or
related data (McDonnell and Kemp, 1995) and this is outlined next.

Geocoding

2.6

An example of geocoding is the conversion of addresses into geographic coordinates.
A variety of databases and sot ware environments exist to facilitate links between
names, addresses, postcodes or zip codes, and geographic coordinates. Such data