68
N.M. Mattikalli and E.T. Engman
Commonly employed GISs (e.g., Arc/Info) mainly use vector format to store
data. In this format, data are collected as points, lines, and polygons, where each
structure holds information for a specific region (Fig. 4.3). The standard approach
is to represent polygon boundaries as lines (normally a sequence of lines that
make up the polygon boundary). Lines are represented as a sequence of very short,
straight line segments which are, in tum, represented by an ordered sequence of
points representing the end points of short line segments. Thus, digital representation of line and polygon information generally consists of an ordered sequence of
[x, y] coordinate pairs. Point, line and polygon features are associated with unique
identifiers that are assigned with descriptive attributes. Both, the vector and raster
structures have advantages and difficulties which are well described in the literature (e.g., Peuquet, 1984; Burrough, 1990), yet their fundamental differences, as
described below, can make data conversion between them a complicated task
(Piwowar et aI., 1990).
In the recent past, many commercial GISs have been adapted to offer raster image display and handling capabilities (e.g., Arc/Info Version 6.0 or later), and
several others offer both raster and vector capabilities (e.g., GRASS) (see Chap.
3). Integration of remotely sensed data with GIS data occurs naturally in a raster
GIS because data structures are approximately the same for both sources. In a
vector system, the integration requires more effort, and several technical problems
need to be overcome for the true integration. Important problems in the integration
are the raster/ vector dichotomy, generalization and accuracy of digital information (Piwowar et aI., 1990; Lunetta et aI., 1991). In addition, there are two difficulties to be addressed. First, there may be a necessity for reduction or reclassification of categories based on a range of attribute values, such as slope,
cover type, cover density etc. For example, a digital map layer in a GIS can have
thousands of classes (e.g., a forest cover map can have more than 1800 different
classes, Goodenough, 1988). But, many remote sensing products have less than
Origin!
35 35 3<; 36 36
135 3<; 36 36 36
37 ~ 36 36 3jl
37 37 37 G8 38
37 37 37 18 38
37 37 l1J 39 ~
39 39 39 3.9 40
39 39 39 40 40
(a)
c33. 113
\35 35
1.35 35
3~ ,;35
38 38
~ "41
40 ~I
40 40
33
I\t
35
35
~
41
41
f-41
tL--...L..._--->.......J
Origin(b)
Fig. 4.3. Representation of spatial data in a GIS: (a) raster formatted data consists of a sequence
of orderly placed pixels (or picture elements); (b) vector formatted data consists of polygon
entities to represent features
N.M. Mattikalli and E.T. Engman
Commonly employed GISs (e.g., Arc/Info) mainly use vector format to store
data. In this format, data are collected as points, lines, and polygons, where each
structure holds information for a specific region (Fig. 4.3). The standard approach
is to represent polygon boundaries as lines (normally a sequence of lines that
make up the polygon boundary). Lines are represented as a sequence of very short,
straight line segments which are, in tum, represented by an ordered sequence of
points representing the end points of short line segments. Thus, digital representation of line and polygon information generally consists of an ordered sequence of
[x, y] coordinate pairs. Point, line and polygon features are associated with unique
identifiers that are assigned with descriptive attributes. Both, the vector and raster
structures have advantages and difficulties which are well described in the literature (e.g., Peuquet, 1984; Burrough, 1990), yet their fundamental differences, as
described below, can make data conversion between them a complicated task
(Piwowar et aI., 1990).
In the recent past, many commercial GISs have been adapted to offer raster image display and handling capabilities (e.g., Arc/Info Version 6.0 or later), and
several others offer both raster and vector capabilities (e.g., GRASS) (see Chap.
3). Integration of remotely sensed data with GIS data occurs naturally in a raster
GIS because data structures are approximately the same for both sources. In a
vector system, the integration requires more effort, and several technical problems
need to be overcome for the true integration. Important problems in the integration
are the raster/ vector dichotomy, generalization and accuracy of digital information (Piwowar et aI., 1990; Lunetta et aI., 1991). In addition, there are two difficulties to be addressed. First, there may be a necessity for reduction or reclassification of categories based on a range of attribute values, such as slope,
cover type, cover density etc. For example, a digital map layer in a GIS can have
thousands of classes (e.g., a forest cover map can have more than 1800 different
classes, Goodenough, 1988). But, many remote sensing products have less than
Origin!
35 35 3<; 36 36
135 3<; 36 36 36
37 ~ 36 36 3jl
37 37 37 G8 38
37 37 37 18 38
37 37 l1J 39 ~
39 39 39 3.9 40
39 39 39 40 40
(a)
c33. 113
\35 35
1.35 35
3~ ,;35
38 38
~ "41
40 ~I
40 40
33
I\t
35
35
~
41
41
f-41
tL--...L..._--->.......J
Origin(b)
Fig. 4.3. Representation of spatial data in a GIS: (a) raster formatted data consists of a sequence
of orderly placed pixels (or picture elements); (b) vector formatted data consists of polygon
entities to represent features