gis

WHAT IS GIS?

1) Defining GIS

2) Maps and Geographic Information

3) Thematic Maps and Spatial Analysis

4) Map Overlay

5) Computer Cartography and the Development of GIS

6) GIS and Relational Databases

WHAT IS GIS? Visual Ways to Convey Spatial Information

GIS stands for Geographic Information System

WHAT IS A GEOGRAPHIC INFORMATION SYSTEM?

A computer-based system that is used to analyze geographic information

WHAT IS GEOGRAPHIC INFORMATION?

Geographic Information is a type of spatial information

Geographic Information is spatial information that is linked to actual earth coordinates.

So, to update the textbook’s definition:

A GIS is a computer-based system to aid in the collection, maintenance, storage, analysis, output, and

distribution of geographic data and information.

Diagram of A Pump Diagram of a Bridge

Diagram of a Car

Medical Diagram A Map

Maps and Geographic Information

Maps

Maps have long been used to communicate spatial information about the Earth

Early Maps were General-Purpose maps

A single map contained information about many themes: transportation, houses, waterways, landforms, agriculture

Thematic Maps

Special-purpose maps, which contain specific information about a particular theme

Became common in the late 19th century

General-Purpose Map: Early Map of London

Geology

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Geology of Southwest Louisiana

Thematic Map Depicting Geology of SW Louisiana

Thematic Maps and Spatial Analysis Ptolemy --astronomer and geographer from the second century B.C. --created one of the earliest known atlases --collection of world, regional and local maps --included advice on how to draw maps

Thematic Maps initiated the use of maps to solve spatial problems

EXAMPLE

In September of 1854, London experienced a cholera outbreak centered in the Soho district

Dr. John Snow theorized that the outbreak was related to untreated sewage entering the public water system

He mapped the public wells and all known cholera deaths in the district

He noted the spatial clustering of cases around a water pump on the corner of

Broad Street and Cambridge Street

The outbreak subsided soon after Snow had the pump handle on the well removed

ANALYSIS USING MAP OVERLAY TECHNIQUES

Jacqueline Tyrwhitt described map overlay analysis in the

Town and Country Planning Textbook (1950)

--brought together four themes: land elevation, surface

geology, hydrology/soil drainage, and farmland

--to create a single map called “land characteristics

1) All maps were produced to the same scale

2) Map features were duplicated so that the maps could be superimposed precisely

In Design with Nature (Ian McHargue, 1969) introduced Sieve Mapping

--several map layers are combined to identify sites meeting a number of criteria

1. Maps are transferred to transparency sheets,

2. They are then placed on a light table

--Areas of interest are highlighted

3. The layers produced are then overlain to identify areas of

overlap

Image Source: http://gisweb.massey.ac.nz/topic/SpatialAnalysis/lectures/overlaying.html

Map 1. Conservation Constraint -

the darker the tone the more suitable

the land for conservation purposes. Map 2. Recreation Constraint - the darker

the tone the better the area for recreation.

Map 3. Urbanisation Constraint - the darker

the area the more suitable for urbanisation.

Map 4. A composite map made

up from the three previous maps.

COMPUTERS AND MAPS

Before the Advent of Computer Mapping

--map overlay was a laborious process

1) Different themes had to be extracted from a map

2) This information was transferred to transparencies

3) The extracted features were color coded

--to emphasize areas of interest

--to indicate the relative importance of resulting maps

4) The maps were then overlain

--areas of overlap were identified

5) A new map was created

This process could take months when all work was done by

hand

Computer Cartography developed from the need to speed up

the process

COMPUTER CARTOGRAPHY

Problems with the development of the Atlas of Britain led cartographers at the

Oxford Experimental Cartography Unit to consider using computers to:

1. Check, edit, and classify data

2. Automatically place labels

3. Measure lines

4. Change projections

Waldo Tobler (1959) --published paper outlining simple model to use computers for cartography

His proposed MIMO (map in, map out) system contained three elements:

Map input Geocoding and data capture

Map “manipulation” Data management and analysis

Map output Printing and data display modules

All three components are now part of every GIS package in use today

FROM COMPUTER CARTOGRAPHY TO GIS

Early computer maps were printed out using primitive

printers and plotters

Still no GIS

In the 1960s newly developed modular computer

simplified the development of integrated software

Early software packages (SURFACE, IMGRID, CAM, and

SYMAP) contained sets of modules for:

1) The analysis and manipulation of data

2) The production of choropleth and isoline (contour)

maps

These modules sped up analysis using map overlay

--reduced the amount of effort used to create transparencies

DEVELOPMENT OF MAP DATABASES

By the 1960s planning agencies created databases containing complete datasets

of integrated spatial and non-spatial data

--for addressing various environmental and demographic issues

The Canadian Geographic Information System (CGIS)

Adopted a layer approach to handling map data based on:

1) Soil

2) Climate

3) Drainage

4) Physical land characteristics

The CGIS was:

1) The first general-purpose GIS

2) The first system to use raster scanning (allowed rapid map input)

3) The first GIS to employ the data structure of line segments or arcs linked together to

form polygons

4) The first GIS to offer remote interactive graphics retrieval on a national basis

All concepts and techniques that influenced the development of modern GIS

systems

GBF-DIME

Stands for: Geographic Base File, Dual Independent Map Encoding

Developed by the US Bureau of the Census

1. As experiment in digital mapping and encoding

2. Recognized that census data and maps could be used to search for

geographic patterns and distributions

3. Also recognized the need to use map topology to overcome problems

with data redundancy and storage space needs

4. Map topology refers to using codes to provide information about

adjacency, containment and connectivity

DIME was a practical innovation

1. Assisted with data input, and with error removal

2. Supported choropleth mapping of census results

3. It is at the heart of many vector data models, including that used by

ArcInfo and ArcView

4. It is also an important part of the National Spatial Data Infrastructure

SPATIAL DATA STRUCTURES

RASTER DATA MODELS

Use individual cells (pixels) to create images of point, line, area, network and surface entities

VECTOR DATA MODELS

The Vector Approach to constructing a spatial data model uses 2-D Cartesian coordinates to store the shape of a spatial entity

The point is the basic building block from which all spatial entities are constructed --represented by a single x,y pair

Line and area entities are constructed by connecting a series of points into chains and polygons

INFORMATION SYSTEMS

Information systems are Computer Systems that Are Designed to Collect Data (bits of information)

They also allow us to sift and sort this data, then to rebuild it to answer a question

Three Basic Types of Geodatabase Management System

1) Flat File Database

Stores rows of information in a text or binary file

--finding information requires parsing the table and selecting the records of interest

--simple, but inefficient

2) Hierarchical Database

Has multiple files

--each contains fdifferent recordsand fields

--parent tables can be linkedto child tables trhough a specified field called a key

--relationships between tables are fixed

--speeds retrieval of information

--relationships are inflexible, since they are designed to permit a predefined set of operations 0

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Development of GIS

In the 1960s, the Laboratory for Computer Graphics and Spatial Analysis

(Harvard) developed a data structure that captured polygon information

using a series of nodes:

1) A beginning node, and an ending node

2) An arc that connected the two nodes

Arcs could be assembled to form a polygon, because the structure contained

information about adjacency and connectivity between features

Many GIS packages, including Arc/Info and ArcView are based on this

simple model of geographic databases

This spatial model is called:

The Vector Data Model

RELATIONAL DATABASES

In 1969, Jack Dangermond founded the Environmental Systems

Research Institute (ESRI)

--introduced ARC/INFO GIS software package in early 1980s

--successfully married standard relational database management

system (INFO) to handle attribute tables

--with specialized software to handle objects stored as vectors,

which they called arcs (ARC)

THE RELATIONAL DATABASE MODEL

How it works: Data are organized in a series of two-

dimensional tables

1) Each table contains records for one thematic entity

2) Tables are linked by common data known as keys

3) Queries are possible on individual tables or on groups of tables

General Database Structure:

1) Data are organized into rows and columns

2) Columns contain attributes and each has a distinct name

Very flexible, and allows a variety of queries to be performed on the same data

1. On one table at a time, or

2. On several tables at once by linking through key fields

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What Does a GIS Do?

One GIS Analysis Technique: Map Overlay Using GIS

Thematic map layers can be overlain using a GIS --to select out areas of interest based on combined characteristics of

each layer --allows user to create a new map containing the desired characteristics

Map Overlay Tools Union Combines spatial and attribute information for both input layers

Output layer contains all polygons and attributes from both input map layers --areas inside area of overlap contain attributes for both layer

files --areas outside area of overlap contain attributes of one layer

file

Clipping (Extractive Function—Extracts Subset of Data) Uses features from one layer to extract a subset of features from another layer --often referred to as “cookie cutting”

Output layer only contains polygons and attributes from the “clipped” file for the area of overlap

Map Overlay Tools

Intersecting Works like “clipping” except: --keeps features from both layer files --combines attributes for both sets of

features in the area of overlap

Erasing (Extractive Function— Extracts Subset of Data) The opposite of clipping --all features within area of overlap are removed --all features and attributes outside area of overlap are

retained

Roads were overlain with parcels

--landuse codes attribute field

for each parcel appended to

the section of road that

intersected that parcel

--forced one-to-one relationship

Other Analysis Tools

Querying (Extractive Function—extracts subset of dataset) Select Features by Attribute

An attribute query specifies a certain condition based on fields in the attribute table to select the records that meet that criteria

--spatial relationships are NOT examined

--do not need a GIS to perform an attribute query

--finding counties with populations over 1,000,000

Select Features by Location (Spatial Query—also an extractive function—extracts subset of data)

Spatial Queries are unique to GIS analysis

--used to select features based on the spatial

relationships between them

Used for finding features

--finding features in one spatial dataset (layer) that are located within features in another spatial dataset (layer)

--finding features in one spatial dataset (layer) that are within a certain distance of a particular feature or set of features within a second spatial dataset (layer)

Query: Locate all cities with population greater than 1,000,000

Query: Locate all US counties that contain major rivers

What Else Does a GIS Do?

This is Just One Type of Analysis Using GIS

Other GIS Functions Used for Spatial Analysis

Measurement

Find the length of a river

Find driving distance between cities

(Example: MapQuest)

Find a country, state, or county’s area in square miles

Buffering and Neighborhood Functions

Find which areas of a city lie within 5, And 10, miles of a cell phone tower (a)

Find which hotels are within 200, and 400, meters of a highway (b)

Create a 20-mile, and a 40 mile buffer around a nature preserve in the Amazon Basin (c)

Geoprocessing Geoprocessing applies one or more GIS functions in a sequence

to solve a problem or investigate the properties of data sets

Research Question: Where is the best habitat for an endangered species of snail located?

Best Snail Habitat: 1) Dense conifer

2) Limestone outcrops

3) Above X foot elevation and below Y foot elevation

Data Extraction: Attribute Queries