(i.e., they have a common coordinate system). Each layer
is partitioned into zones (regions), where the zones are sets
of locations with a common attribute value. Examples of
layers are the land-use layer, which is divided into land-use
zones (e.g., wetland, river, desert, city, park and agricultural
zones) and the road network layer, which contains the roads
that pass through the portion of space that is covered by the
layer.
Data interpretation operations available in GISs char-
acterize [Aronoff, 1989, Berry, 1987, Samet et al., 1995,
Tomlin, 1990]:
e individual locations,
e locations within neighborhoods, and
e locations within zones,
and constitute respectively the three classes of operations,
i.e., local, focal and zonal operations. Notice that all data
interpretation is done in a layer-by-layer basis. That is, each
operation accepts one or more existing layers as input (the
operants) and generates a new layer as output (the product).
The first class of data-interpreting operations (local oper-
ations) includes those that compute a new value for each
location on a layer as a function of existing data explicitly
associated with that location. The data to be processed by
these operations may include the zonal values associated with
each location on one or more layers. Local operations include:
e Search operations, i.e., retrieval of information associ-
ated with individual locations on a layer.
e Classification and recoding operations, i.e., assignment
of new attribute values to individual locations on a
layer.
e Generalization operations, i.e., reduction of detail as-
sociated with individual locations on a layer.
e Overlay operations, i.e., assignment of new attribute
values to individual locations resulting from the com-
bination of two or more layers.
Focal operations compute new values for every location as a
function of its neighborhood. A neighborhood is defined as
any set of one or more locations that bear a specified distance
and/or directional relationship to a particular location, the
neighborhood focus. Focal operations include:
e Search operations, i.e., retrieval of information charac-
terizing the immediate or extended vicinity (the region
of interest) of individual locations on a layer.
e Proximity operations, i.e., assignment of new attribute
values to individual locations on a layer, which depict
their distance or direction in a neighborhood with re-
spect to the neighborhood focus.
e interpolation operations, i.e., assignment of new at-
tribute values to individual locations on a layer derived
by averaging sets of two or more target values asso-
ciated to selected locations in their immediate or ex-
tended vicinity.
e Surfacial operations, i.e., assignment of new attribute
values to individual locations on a layer indicating their
surfacial characteristics (slope, aspect, volume, etc.).
830
e Connectivity operations, i.e., assignment of new at-
tribute values to individual locations on a layer derived
from a running total of the results being retained in
a quantitative or qualitative step-by-step fashion and
considering the values associated to locations in the
immediate or extended vicinity (optimum path finding,
etc.).
The third and final class of data-interpreting operations
(zonal operations) includes those that compute a new value
for each location as a function of existing values associated
with a zone containing that location. Zonal operations in-
clude:
e Search operations, i.e., retrieval of information char-
acterizing individual locations on a layer that coincide
with the zones of another layer.
e Measurement operations, i.e., assignment of new at-
tribute values to individual locations on a layer that
correspond to a measurement (e.g., area, length) char-
acterizing their zones.
3 SITE SELECTION BASED ON A SEQUENCE OF
GIS OPERATIONS
The purpose of this section is to present a sequence of data-
interpretation operations which may compose one or more
procedures! to accomplish the task of site selection for a res-
idential housing development. The basic approach to this is
to create a set of constraints, which restrict the planned ac-
tivity, and a set of opportunities, which are conducive to the
activity. The combination of the two is considered in order
to find the best locations.
In the simplified situation that follows the set of constraints
and opportunities consists of?:
e vacant area (i.e., no development),
e dry land,
e level and smooth site (e.g., slope « 10%),
e nearness to the existing road network, and
e south-facing slope.
In addition all candidate sites should have an adequate size
to satisfy the needs of the planning activity (e.g., between 1
and 1.5 sq km).
The whole task requires as input three layers of the region
under examination:
e hypsography layer: the three-dimensional surface of
the region (altitude values),
e development layer: it depicts the existing infrastructure
of the region (e.g., roads, buildings, etc.), and
e moisture layer: it depicts the soil moisture of the region
(e.g., lakes, wet-lands, dry-lands, etc.).
The procedure of site selection, based on the sets of con-
straints and opportunities determined above, may consist of
the following sequence of operations?:
!A procedure is any finite sequence of one or more operations that are
applied to meaningful data with deliberate intent [Tomlin, 1990].
2a wider set could be taken into account, but this subset is enough to
illustrate some basic data-interpreting operations available in GISs.
3the syntax adopted for the operations is:
new-layer = Operation-class(operation-subclass) of existing-layer and ...
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
