BASIC CONCEPTS OF GIS 

Michael F. Goodchild
UC Santa Barbara

OUTLINE

1. GEOGRAPHIC REPRESENTATION

2. THE FUNDAMENTAL PROBLEM

3. OBJECTS AND FIELDS

4. GIS DATA MODELS

5. THE NATURE OF GEOGRAPHIC INFORMATION

Geographic information

information about some place on the surface of the Earth

or near the surface

at some point in time

one of the earliest forms of shared information

hunters and gatherers reporting back to the band

"there's good hunting near the old tree"

early stick maps for navigation in the Pacific

drawings on cave walls

storing on paper

the printing press in the 15th Century

information accessible to all

shared knowledge as a human community asset

Prince Henry the Navigator, 1394-1460

massive new capability for sharing, communicating geographic information

in digital form

paradigm change

from GIS as the personal engine performing calculations on data

to GIS as the medium for communicating knowledge of the planet

humans work in a vague world

GIS as a precise medium acts as a filter

communicating a vague fact

The atom of geographic information

<location, time, attribute>

it's cold today in Ottawa

at 45 North, 75 East at 12 noon EST the temperature was -10 Celsius

general methods for describing location

everyone around the world understands latitude and longitude

similarly for time

attributes must also be generally understood

"cold" is subjective and relative

-10 Celsius is generally understood

complete representation of the planet

past, present, and future

a "mirror world"

“Imagine, for example, a young child going to a Digital Earth exhibit at a local museum. After donning
a head-mounted display, she sees Earth as it appears from space. Using a data glove, she zooms in,
using higher and higher levels of resolution, to see continents, then regions, countries, cities, and
finally individual houses, trees, and other natural and man-made objects. Having found an area of the
planet she is interested in exploring, she takes the equivalent of a ‘magic carpet ride’ through a 3-D
visualization of the terrain. Of course, terrain is only one of the numerous kinds of data with which she
can interact. Using the system’s voice recognition capabilities, she is able to request information on
land cover, distribution of plant and animal species, real-time weather, roads, political boundaries, and
population. She can also visualize the environmental information that she and other students all over
the world have collected as part of the GLOBE project. This information can be seamlessly fused with
the digital map or terrain data. She can get more information on many of the objects she sees by using
her data glove to click on a hyperlink. To prepare for her family’s vacation to Yellowstone National
Park, for example, she plans the perfect hike to the geysers, bison, and bighorn sheep that she has just
read about. In fact, she can follow the trail visually from start to finish before she ever leaves the
museum in her hometown.
She is not limited to moving through space, but can also travel through time. After taking a virtual
field-trip to Paris to visit the Louvre, she moves backward in time to learn about French history,
perusing digitized maps overlaid on the surface of the Digital Earth, newsreel footage, oral history,
newspapers and other primary sources. She sends some of this information to her personal e-mail
address to study later. The time-line, which stretches off in the distance, can be set for days, years,
centuries, or even geological epochs, for those occasions when she wants to learn more about
dinosaurs.” (U.S. Vice President Al Gore, in a speech written for presentation at the California Science
Museum, Los Angeles, January 1998)

www.digitalearth.gov

How many atoms are there?

an infinite number

to make a two-word description of every sq km on the planet would require 10 Gigabytes

to store one number for every sq m on the planet would require 1 Petabyte

that's too many for any system

how to limit?

ignore the water

that's 2/3 of the planet

one temperature for all of Ottawa

one number for an entire area

definition of the area is shared and implicit

definition of the area is finite and digital

sample the space

only measure at weather stations

because temperature varies slowly

all geographic data miss detail

all are uncertain to some degree

all geographic phenomena vary slowly

there are many ways of doing this

a GIS user must make choices

GIS designers must allow for many options

geographic description is complex

description of the differences between a representation and the truth can be as important as the representation

need to know what is missing

e.g. a space-time representation

the uncertainty about reality associated with a representation

The most important of the options

how we think about the world

how we interpret the contents of a database

not inherent in the database

points, lines, areas (or volumes) having known properties

littering an otherwise empty space

may overlap

can be counted

how many lakes are there in Minnesota?

how many mountains in Scotland over 3000 ft?

how many clouds in the sky?

how many cities over 1 million population?

how many atmospheric lows in the northern hemisphere today?

represent as shapefiles

points

polylines

polygons

things it's worth measuring at every location on the planet

temperature

soil pH

soil type

land cover type

elevation

source of language, metaphor

did Newton, Leibnitz think that way?

rainfall

ownership

population density

explicit scale (property of convolution)

each of these variables has one value everywhere

variable is a function of location

field = a way of conceiving of geography as a set of variables each having one value at every location on the planet

z = f(x,y,z,t)

six alternative representations:

a raster of points

sample points (weather stations)

a triangulated irregular network

a raster of cells

a coverage

contours

how many are there?

fronts, highs, lows, or pressure surfaces?

fields are more associated with science

two point-data sets

Discrete object implementations

shapefiles

ArcView

collections of points, lines, and areas

point

multipoint

polyline

a line made by connecting points with straight lines

multipart polyline

polygon

an area made by connecting points with straight lines

multipart polygon

slide

slide

coverages

the relational model applied to GIS

the georelational model

representing maps in the relational model

ARC/INFO circa 1980

polygons

arcs

nodes

Coverage slide 1

Coverage slide 2

Coverage slide 3

avoids double representation of internal boundaries

easier to build the database

easier to edit and maintain

keeps track of 'topology'

which nodes are connected by which arcs

which polygons are separated by which arcs

Data that fit the coverage model

all points within one polygon have the same attributes

all points must lie in exactly one polygon

resource management

forest stands

soil type

soil map

vegetation cover class

land use class

the cadaster

land ownership parcels

demographics

census data

data by state

data by county

marketing data by market area

population by ZIP

the choropleth map

the area class map

coverages capture the field view of the world

a continuous world

one value of a variable at every point

sharp changes in value as boundaries are crossed

points

lines

arcs

TINs

grids

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