Combining data layers - Spatial Data Analysis: An Introduction for GIS Users - page 65

5

Combining data layers

Introduction

5.1

h is chapter is concerned with the nature of connections between objects in terms of
overlap or proximity between dif erent objects. Operators that deal with overlaps
between objects require information on where specii c features overlap in space—the
overlap of lines is termed the 'intersection. Such operators provide the basis of multi-
criteria decision analysis (MCDA, see Section 5.3) whereby information in multiple
data layers is taken into account. Overlay operators are a key component of GIS sot -
ware packages and they enable the identii cation of features that share the same geo-
graphical space (whether points, lines, or areas). h e main focus in this chapter is on
vector data; Section 10.2 deals briel y with raster overlay.
Exploring connections between objects may provide the basis of many dif erent
kinds of analyses. At a simple level, for example, there may be a need to identify which
areas border another area (i.e. which areas are contiguous). Using topological struc-
tures, as detailed in Section 2.2.3, this kind of information may be obtained directly.
Ot en the concern will be to identify which objects contain other objects or are con-
tained by other objects (e.g. which local government boundary contains which houses).
In many cases, connectivity between dif erent kinds of objects is of interest. Finding
the common areas from two sets of polygon features (e.g. highly populated areas and
highly polluted areas) is one kind of application that is frequently encountered in GIS
contexts. h is chapter is concerned with overlaps between features in dif erent data
layers.

Multiple features: overlays

5.2

Overlay operators combine information in various ways from two or more sets
of spatial data. Such operators are concerned with inclusion and with overlap or