GIS专业英语lesson 22(Data Analysis

Lesson 22 data analysis

II

Integrated Analytical Functions in a GIS

Most

GIS's

provide

the

capability

to

build

complex

models

by

combining

primitive analytical functions. Systems vary as to the complexity provided for spatial

modelling,

and

the

specific

functions

that

are

available.

However,

most

systems

provide a standard set of primitive analytical functions that are accessible to the user

in some logical manner. Aronoff identifies four categories of GIS analysis functions.

These are :

??

Retrieval, Reclassification, and Generalization;

??

Overlay Techniques;

??

Neighbourhood Operations; and

??

Connectivity Functions.

The range of analysis techniques in these categories is very large. Accordingly,

this section of the book focuses on providing an overview of the fundamental primitive

functions that are most often utilized in spatial analyses.

Retrieval, Reclassification and Generalization

Perhaps the initial GIS analysis that any user undertakes is the

retrieval

and/or

reclassification

of data. Retrieval operations occur on both spatial and attribute data.

Often

data

is

selected

by

an

attribute

subset

and

viewed

graphically.

Retrieval

involves

the

selective

search,

manipulation,

and

output

of

data

without

the

requirement to modify the geographic location of the features involved.

Reclassification

involves

the

selection

and

presentation

of

a

selected

layer

of

data

based

on

the

classes

or

values

of

a

specific

attribute,

e.g.

cover

group.

It

involves looking at an attribute, or a series of attributes, for a single data layer and

classifying the data layer based on the range of values of the attribute.

Accordingly, features adjacent to one another that have a common value, e.g.

cover group, but differ in other characteristics, e.g. tree height, species, will be treated

and appear as one class. In raster based GIS software, numerical values are often

used

to

indicate

classes.

Reclassification

is

an

attribute

generalization

technique.

Typically

this

function

makes

use

of

polygon

patterning

techniques

such

as

crosshatching and/or color shading for graphic representation.

In

a

vector

based

GIS,

boundaries

between

polygons

of

common

reclassed

values

should

be

dissolved

to

create

a

cleaner

map

of

homogeneous

continuity.

Raster

reclassification

intrinsically

involves

boundary

dissolving.

The

dissolving

of

map boundaries based on a specific attribute value often results in a new data layer

being

created.

This

is

often

done

for

visual

clarity

in

the

creation

of

derived

maps.

Almost all GIS software provides the capability to easily dissolve boundaries based on

the results of a reclassification. Some systems allow the user to create a new data

layer for the reclassification while others simply dissolve the boundaries during data

output.

One

can

see

how

the

querying

capability

of

the

DBMS

is

a

necessity

in

the

reclassification

process.

The

ability

and

process

for

displaying

the

results

of

reclassification, a map or report, will vary depending on the GIS. In some systems the

querying process is independent from data display functions, while in others they are

integrated

and

querying

is

done

in

a

graphics

mode.

The

exact

process

for

undertaking a reclassification varies greatly from GIS to GIS. Some will store results

of the query in

query sets

independent from the DBMS, while others store the results

in

a

newly

created

attribute

column

in

the

DBMS.

The

approach

varies

drastically

depending on the architecture of the GIS software.

Topological Overlay

The

capability

to

overlay

multiple

data

layers

in

a

vertical

fashion

is

the

most

required and common technique in geographic data processing. In fact, the use of a

topological data structure can be traced back to the need for overlaying vector data

layers. With the advent of the concepts of mathematical topology

polygon overlay

has

become

the

most

popular

geoprocessing

tool,

and

the

basis

of

any

functional

GIS

software package.

Topological

overlay

is

predominantly

concerned

with

overlaying

polygon

data

with

polygon

data,

e.g.

soils

and

forest

cover.

However,

there

are

requirements

for

overlaying

point,

linear,

and

polygon

data

in

selected

combinations,

e.g.

point

in

polygon, line in polygon, and polygon on polygon are the most common. Vector and

raster based software differ considerably in their approach to topological overlay.

Raster based software is oriented towards arithmetic overlay operations, e.g.

the addition, subtraction, division, multiplication of data layers. The nature of the

one

attribute

map

approach,

typical

of

the

raster

data

model,

usually

provides

a

more

flexible

and

efficient

overlay

capability.

The

raster

data

model

affords

a

strong

numerically modelling (quantitative analysis) modelling capability. Most sophisticated

spatial modelling is undertaken within the raster domain.

In vector based systems topological overlay is achieved by the creation of a

new

topological

network

from

two

or

more

existing

networks.

This

requires

the

rebuilding

of

topological

tables,

e.g.

arc,

node,

polygon,

and

therefore

can

be

time

consuming

and

CPU

intensive.

The

result

of

a

topological

overlay

in

the

vector

domain is a new topological network that will contain attributes of the original input

data layers. In this way selected queries can then be undertaken of the original layer,

e.g.

soils

and

forest

cover,

to

determine

where

specific

situations

occur,

e.g.

deciduous forest cover where drainage is poor.

Most GIS software makes use of a consistent logic for the overlay of multiple

data

layers.

The

rules

of

Boolean

logic

are

used

to

operate

on

the

attributes

and

spatial properties of geographic features. Boolean algebra uses the operators AND,

OR, XOR, NOT to see whether a particular condition is true or false. Boolean logic

represents all possible combinations of spatial interaction between different features.

The implementation of Boolean operators is often transparent to the user.

Generally, GIS software implements the overlay of different vector data layers

by

combining

the

spatial

and

attribute

data

files

of

the

layers

to

create

a

new

data

layer.

Again,

different

GIS

software

utilize

varying

approaches

for

the

display

and