Geographic Information Systems (GIS)

The use of Geographic Information System
(GIS) has grown dramatically to become
commonplace in business, universities
and governments where they are now used
for many diverse applications.
Consequently, many different definitions
of GIS have developed. 

A definition defines GIS as an organized
collection of computer hardware software
geographic data, and personal designed
to efficiently capture, store, update,
manipulate, analyze & display all forms
of geographically referenced info. 

Another definition defines GIS as a
computer system capable of holding 
and using data describing places on
the earth's surface.

It is generally accepted that a GIS
permits spatial operations on data. 
An aspatial query is a query with an
answer that does not require the 
stored value of latitude and longitude;
not does it describe where the places
are in relation to each other.

A spatial query, on the other hand, 
is a query which can only be answered
using latitude an longitude data 
and other information. 

Asking "What's the average number
of people working with GIS in each 
location?" is an aspatial query.
The answer doesn't require the stored 
value of latitude and longitude;
nor does it describe where the places 
are in relation to each other.

"How many people work in GIS in the
major centers of Western Europe?" 
"Which centers lie within 1,000 miles
of each other?" "What's the shortest
route passing thru all these centers?"
These are examples of spatial queries
that can only be answered using
latitude and longitude data and other
information, such as the radius of
the earth. A geographic information
sys can readily answer such questions. 

Data linkage in a GIS typically links
data from different sets. As an 
example, suppose you need to know what
percentage of each country's total 
food production is grown for export.
You've located the data you need, but 
your total food production for each
country is stored in one computer 
file, and the food export data is
contained in a separate file.  You
must combine these files to solve the
problem. Once the files are combined, 
it's a simple process to have the
computer perform the arithmetic to 
produce your answer.

Data linkage in a GIS can be either
exact matching or non-exact matching. 
Exact matching occurs when you have
information in one computer file 
about many geographic features
(e.g., counties) and additional 
information in another file about
the same set of features.
The operation to bring them together
is easy, archived by using a key
common to both files, in this case,
the county name. So, the record in
each file with the same county name
is extracted and the two are joined
and stored in another file.

For non-exact matching, it can be
divided as hierarchical matching
and fuzzy matching. 

Some types of information, however,
are collected in more detailed o more 
frequently than other types of
information. For example, finance and 
unemployment data covering large
areas is collected frequently. On the 
other hand, population data is
collected for small areas, but at
less frequent intervals. If the smaller
areas nest (i.e., fit exactly) within 
the larger ones, then the solution for
matching these data is to use 
hierarchical matching. Group the small
areas together until they cover 
the same area as the larger area,
total their data, and then perform an 
exact match.

On many occasions, the boundaries of the
smaller areas do not match those 
of the larger ones. This is especially
true when dealing with environment 
data. For example, crop boundaries,
usually defined by field edge, rarely 
match the boundary between types of soil.
If you want to determine the most
productive soil for a particular crop,
you need to overlay the two data sets &
compute crop productivity for each and
every soil type. In principle, this is
like laying one map over another and
noting the combinations of soil and
crop productivity.

A GIS can perform all these operations
because it uses geography, or space, as
the common key between the data sets.
Information is linked only if it relates
to the same geographic area.

Why is data linkage so important?
Consider a situation where you have
two data sets for the same area, such
as yearly income by county and average 
cost of housing. Each data set might
be analyzed and mapped individually. 
Alternatively, they can be combined
to produce one valid combination. If, 
however, you have 20 data sets for the
county, you have over one million 
possible combinations. Although not
all combinations are meaningful 
(e.g., unemployment and soil type),
How can answer many more questions 
than if  the data sets are kept
separate. Combining them adds value
to the database. To do his,
you need a GIS.

GIS links a spatial data with geographic
information about a particular 
feature on a map. The information is
stored as attributes of the 
graphically represented feature.
For example, the centerline that 
represents a road on a map doesn't
tell you much about the road except 
its location. To find out the road's
width or pavement type, you must 
query the database. Using the info
stored in the database, you could
create a display symbolizing the
roads according to the type of 
information that needs to be shown.

A GIS also uses the stored feature
attributes to compute new information 
about map features; for example, to
calculate the length of a particular 
road segment or to determine the total
area of a particular soil type.

Essentially, a GIS gives you the
ability to associate information with
a feature on a map and to create new
relationships that can determine the 
suitability of various sites or
development, evaluate environmental 
impacts, calculate harvest volumes,
identify the best location for a new 
facility, and so on.


ARC/INFO

If you want to go beyond just making
pictures, you need to know three 
things of what about every feature
stored in the computer: what it is, 
where it is, and how it relates to
other features. Database systems 
provide the means of storing a wide
range of such information and 
updating it without the need to
rewrite programs.

In ARC/INFO, ARC handles where the
features are, while the INFO component 
handles the feature descriptions and
how each feature is related to others.

ARC/INFO stores two types of data for
coverage features: locational data, 
maintained by ARC, and attribute
data, which you can maintain. Attr 
data is stored in INFO data files.
These files contain rows (records)
& columns (items) in tabular format.
This type of table holds the 
thematic attribute data related to
the spatial information recorded from 
a map. It can hold any number of
records and items, but all records
must contain the same items.

The INFO data file created
automatically when topology is
established for a coverage is referred
to as the feature attribute table
(such as a PAT or an AAT).

Feature attribute tables are a
special kind of INFO data file 
and always contain certain attribute
information about coverage features. 
These standard items are
automatically created in a specific
order.

ArcView

ArcView is a display and query tool
that can perform many of the tasks 
involved in the spatial analysis of
geographic data bases. ArcView can be 
applied to more than just base data.
Because display and query are 
fundamental to the interpretation of
the results of spatial analysis, 
ArcView complements spatial analysis
by enabling investigation of the 
results and new spatial relationships
derived from analytical procedures 
and models.

ArcView can be used to achieve the following tasks:
1) display current status of your db
2) draw spatial data with the colors and patterns you want
3) examine images and display remotely sensed data
4) integrate vector and raster data
5) perform spatial and logical queries
6) browse the tables of your database
7) save graphics and reports to hand off to other application
8) match addresses to street networks