Why GIS ?

Many organization sinvest significant funds in geographic information systems (GIS) and geographic databases. It is anticipated that billions of dollars will be invested in these products over the next decade. Why is it like this now, when only a few years ago geographic information systems were rare? There are two answers: First of all, the prices of computer hardware required for this task are rapidly dropping, and a larger and larger target audience can afford to purchase it. Furthermore, geography (and the data describing it) is part of our daily world; almost every decision we make is limited, affected or dictated by some kind of geographic fact. Fire engines are dispatched to blazes via the quickest possible routes, central authority grants are given to local authorities based on population dispersal, and diseases are studied by the areas they afflict and their rate of spread. This potential explains the need for geographical information systems and their increasing popularity. Such a general explanation does not explain, however, why and how a GIS can help you. For this purpose, you must use a GIS. You must initially understand what a geographic information system is, and how it can help you. This lesson presents a number of issues that will help you gain better understanding of the following concepts:

What is a geographic information system (GIS)?

What are the questions a geographic information system can answer?

GIS applications.

GIS components.

How is a GIS used?

The definition of the GIS (Geographic Information Ssytem)

A GIS is an organized collection of hardware, software, geographic data and personnel designed to efficiently receive, process, analyze and present all types of data of geographic orientation.

"A computer system capable of storing and using information that describes places on earth."

To all practical purposes, GIS can be considered as an equivalent to an atlas connected to a calculator. GIS provides the ability to extract information from different types of data with a spatial dimension. For example, this data can be a road, settlement or flora map. The main use of a GIS is storing this data in the form of thematic layers. GIS stores data in digital format (i.e. in files on a hard disk). The difference between GIS and other information systems (such as MIS) is the former's ability to understand and manipulate spatial relations.

GIS, through using computers, allows the user to analyze and build models from spatial information. The product of this analysis is usually expressed in maps, tables and graphs.

The GIS is used for various applications - from environmental models through to maintaining municipal services, from finding old routes (Columbus, for example) through to paving new roads.

Spatial processes

Many common programs, such as spreadsheets (Excel, for example) or drawing software (such as AutoCAD) can handle a limited amount of geographical or spatial information. So why are they not considered to be GIS applications? The conventional answer is that a GIS is a GIS only if it provides the ability to perform spatial queries on data. For a simple example, check the following table:

Name	Latitude	Longitude	GIS Population
London	51N	0	80
Zurich	47N	8E	25
Utrecht	52N	5E	40
Santa Barbara	34N	119W	50
Orono	45N	69W	30
Buffalo	42N	78W	30

This table shows the approximate number of people working with certain aspects of a GIS, in selected centers of activity, in 1989.

Non-spatial queries: The question of "what is the average number of people working with a GIS at each site?" is a non-spatial question. The answer does not require information on latitude or longitude; neither does it describe places relative to one another.

Spatial queries

"How many people work with GIS in large centers in Western Europe?"

"What centers are situated 1,000 miles from one another?"

"What is the shortest way passing through each of them?"

These are spatial queries that can be answered only using information on latitude and longitude, and additional information such as earth's radius. GIS will answer these questions. GIS can perform all these queries because it uses geography, or space, as a common key between files. Data items are connected only if they refer to a common geographic element.

Why is the linking of information so important? Imagine you have two files describing a given geographic area, one describing, for example, the average income per district, and the other the average price of houses for that area. One can analyze or map each of these files separately, but they may also be combined. In the case of two files, only one combination is possible, but if you had twenty files for this district, there would be more than a million possible combinations. Not all of these have meaning (such as land types and unemployment), but one can answer far more queries than on separate files. By linking these files, you provide your database with added value.

Questions that GIS can answer

So far, we have described geographic information systems in two ways:

1. By formal definition.

2. By its ability to perform spatial analysis involving linking files and using location as a key.

However, it is possible to identify a geographic information system by specifying the types of questions it is able (or should be able) to answer. If you provide a general view, disassociated from any specific application, you can see that there are five types of questions that GIS can answer:

Location

What can I find at...?

The first of these questions attempts to find out what can be found at a certain location. The location can be described in many ways, for instance by stating the name of a place, a zip code, or longitude and latitude.

State

Where is...?

The second question is opposite to the first, and a spatial analysis is required to answer it. Instead of identifying what can be found at a given location, you wish to know what the location where certain conditions hold is (for example, an unforested tract of land of at least 2,000m2 area, which has a road at least 100 meters away, whose land is suitable for building).

Processes

What has changed since...?

The third question incorporates the first two, with the intent of finding differences in a given area over time.

Patterns

What spatial patterns are there?

This is a more sophisticated question. It can be asked in order to check whether cancer is a major cause of death for residents around a nuclear power station. The same degree of importance is given for locating exceptions that do not conform with the pattern, their number and location.

Prediction

What will happen if...?

Questions of the type "what will happen if...?" are presented in order to find out the outcome if, for example, a new road is added to the road network, or whether a poisonous substance will seep into an underground water system. To answer such questions, geographic and other information (possibly even scientific laws) is required.

An example of GIS applications

The first applications of various geographic information systems in the world arose based on the needs of each place. In Europe, the great impetus was achieved in the direction of land registration systems and environmental databases. In Britain, the greatest investment of the 1980s was for infrastructure company purposes and for creating a comprehensive nationwide topographic database, cut mainly out of 1:1250-1:2500 scale maps.

In Canada, an important application in the field of forestry helped the planning of timber volume to be cut, identified means of access to wood and reported the results to the government. In Japan and China, strong emphasis was placed on controlling and regulating possible environmental quality changes.

In the USA, all these fields have been important. Furthermore, the use of GIS technology in the US Bureau of Census and US Geological Survey TIGER project (Geographic Topologically Integrated Enclosing & Referencing) is worth noting.

The largest collection of geographic data ever collected is from satellite photographs. These differ from most other geographic data by being represented as rasters (pixels), as opposed to vectors. These pixels on the ground are represented by one ore more numbers that describe their properties.

GIS components

The user becomes part of the system each time it is necessary to perform complex activity such as spatial analysis and prediction. These require skills of choosing and using GIS tools and a familiarity with the data. Today, every off-the-shelf GIS program depends on users knowing what they are doing - pressing a button is not enough in itself.

GIS - what does it not do?

GIS is not a computer system that produces maps, although being able to produce maps at different scales with different projections and colors. GIS is an analytic tool. Its great benefit is identifying spatial relations between features on a map.

GIS does not "store" a map in the regular sense of the word. Neither does it store a picture of a geographic area. GIS saves data from which the desired map can be created for any purpose.

GIS associates spatial data with geographical information on a feature on a map. The information is stored as the properties of the graphically represented feature. For example, a network of roads can be represented by the road's midline. Using this method, the visual representation of the road does not provide a lot of information, such as width or type, for which purpose you must refer to the database. Now you can create a picture with representation of roads according to the information type you wish to show.

GIS can also calculate new information on map features based on existing properties. For example, calculating the length of a given road or establishing the total area of a given type of land.

Geographic database

In brief, GIS does not store maps or images but a database. The concept of the database is central to GIS, and it is the important difference between a simple drawing system and a computerized mapping system capable of issuing good graphic output. All the geographical information systems contain a database management system.

If you want to get more than just a picture, you need to know three things about each feature stored in the computer: what it is, where it is located and how it relates to other features (for example, which roads intersect).

GIS will allow you to associate data with entities on the map and identify their relations.

It will allow you to estimate ecological effects, calculate the volume of agricultural harvests, identify the most suitable site for a new factory, and so on.

Showing all intersections (feature records) on which road accidents have occurred on Saturdays.

Data affinity

Exact match:

When one has two files with different information on the same geographic entity (e.g. district). Their joining action is simple, and merely requires the use of a common key - in this case, the district name. Pairs of lists with an identical district name need be extracted from the two files and joined.

Hierarchical (nested) match

There are types of information, however, that are gathered at higher resolution or lower frequency than others. For example, financial information and employment information over a large area are gathered at high frequency. In contrast with these, population data is gathered in smaller areas, but at low frequency. If the small areas nest into a large one (i.e. fit into its exact borders), a state can be achieved in which the two information types relate to the same area by using a "hierarchical match"- a summation of data describing the small areas according to their groups belonging to large ones, followed by an "exact match".An example of this would be statistic areas (census data) within settlements (current data).

Vague / complex match

In many cases, the boundaries of the small areas do not precisely fit those of the large areas. This is correct mainly in the case of environmental information, for example: boundaries of agricultural crops, which are usually defined by the boundaries of fields, do not always match the borders between soil types. If you wish to determine the most fertile land type for a certain crop, you must use the two files and calculated the amount of crops for each soil type. Essentially, this is a process that is similar to placing maps on one another and marking combinations of crop quantities and soil types.

Using GIS

The first stage of the GIS project is setting up a computerized database. After you understand the model, you can start setting up the database.

The issues to be covered in the lesson are as follows:

Two data types: spatial and descriptive.

Important entity types: points, lines and polygons.

How points, lines and polygons are represented in the computer using X and Y coordinates (and sometimes Z, vertical coordinates).

A. Basic map components:

Spatial information

Describes the location and shape of geographic entities and their spatial relations with other geographic entities. Descriptive information Descriptive information of the geographic entities.

B. Important entity types:

Entity types	Format	Examples
Points, nodes	A pair of X, Y coordinates	Electrical pylons,traffic lights...
Lines, arcs	A collection of coordinates, including nodes at beginning and end of line	Roads, rivers...
Polygons	A collection of coordinates, the starting and ending point being identical	Lakes, borders of countries...

Notes:

1. Points along an arc (vertices) define its shape.

2. Each arc has two end points (nodes).

3. finding all the arcs meeting at a certain end point, the program establishes the relations between the arcs. This is an example of one kind of topology process.

Basic concepts

Arc	A series of X, Y coordinates representing a linear geographic feature
Attributes	Relational data on geographic features
Coverage / layer	Contains geographic features of a certain type (roads, plots).
Feature	A geographical feature (point, line or polygon)
Item	A data field (descriptive / spatial).
Label point	X,Y coordinates used as an polygon identification point.
Node	The end of a line
Point	X, Y (Z) coordinate representing a focal geographic feature)
Polygon	A series of X, Y (Z) coordinates representing a geographic area.
Record	Relational data on a single geographic item.
Relate	An associative action between two compatible records in two different relational files.
Rational join	The joining of two compatible records from two different files to create a record in a third file
Spatial overlay	The overlaying of two layers to create a third layer. The details of the new layer have properties of both the first and the second layers.
Topology	Relations between geographic features.