geographic information systemS
Cindy L. Piearson1, Mary O'Neill2 and Cheryl L. Reese3What is GIS and what does it do?
More
and more people around the country and around the globe would like to plug
into geographic, social, economic, political, and environmental information
digital information to answer practical questions in their lives.To
explore geo-referenced digital information, electronic tools designed for
acquiring, presenting, and interacting with information that links location
with measured values are needed. One such tool is called a geographic information
system, better known as GIS.Desktop
GIS is an immensely powerful computer mapping system. It is a tool for
managing information of any kind according to where it is located.For
example, businesses can track customer locations, optimize delivery routes,
or decide where to site future businesses using GIS; scientists use GIS
to manage sensitive wildlife habitats or track animal movements in an ecosystem;
and health care specialists track the spread of infectious disease with
GIS.GIS technology powers solutions
for telecommunications, utilities, agriculture, defense, oil, health care,
transportation, mining, environmental management, petroleum, water/wastewater,
and many other industries as well as local, state, and federal government
agencies.
How does GIS work?
Desktop
GIS represents the real world on a computer similar to the way maps represent
the world on paper.Both GIS and
paper maps convey information about places. However, desktop GIS
has power and flexibility that paper maps lack. The scale of the map influences
the size of what appears on it. With GIS, however, you can store and link
huge amounts of information about the objects represented on maps.
These objects
are called features. Each map feature has a location, a representative
shape, and a symbol that represents one or more of its characteristics.
Because features on maps are organized according to relative location or
position, maps are good for showing the relationships among feature locations. These
relationships, called spatial relationships, are important because understanding
them helps us solve problems.
Maps
use three basic shapes-- points, lines and areas to represent real-world
objects. Points represent objects that have discrete locations and are
too small to be depicted as areas. Lines represent objects that have length
but are too narrow to be depicted as areas. Areas represent objects too
large to be depicted as points or lines. Shapes alone do not give you enough
information, so maps use graphic symbols to help identify features and
provide information about them.
Most
features can be represented as more than one shape. The scale of a map
tells how the size of the map features compares with the size of the geographic
objects they represent. Map scales vary
from small-scale to large-scale.For
example, on a small-scale map a city may be represented as a point (Figure
1). That same city would be
represented as an area on a large-scale map (Figure 2).
Figure 1.Picture
of a small-scale map shown in a GIS.Black
circles represent the cities.
Figure 2.Large-scale
map representing cities, roads, lakes and stream in Brookings County, SD.
With
desktop GIS, you are not limited to the amount of information you can get
about what you see on the map. Desktop GIS stores all the information about
map features in a GIS database and links the features on the map to the
information about them. This means that you can access all the information
about a feature by simply clicking on it.For
example, in Figure 3 (shown below) a table is shown on the left (highlighted
in yellow) and a window titled a ‘View’ (highlighted in blue) is shown
on the right.‘Views’ are made of
‘themes’.‘Themes’ are used to create
the map shown in Figure 3 and highlighted in red.
Figure 3.Window
from a GIS that shows the ‘View’ area with ‘Themes’ shown on the right
side of the picture.On the left
side of the picture, a table is shown.This
table contains data about the map.
The
information that a desktop GIS stores about map features is referred to
as attribute information, or attributes. The attributes of a river, for
example, might include its name, length, average depth, rate of flow, water
quality, how many dams are on it, and how many bridges cross it. Desktop
GIS formats attributes in rows and columns, and stores them as tables.
Each column stores a different attribute and each row relates to a single
feature. The link between map features and their attributes is the basic
principle behind how a desktop GIS works, and is the source of its power.
Once the map features and attributes are linked, you can access the attributes
for any map feature or locate any feature from its attributes in a table.
GIS can also display features based on any attribute in the table. Because
the link between features and attributes is a two-way relationship, changing
an attribute in the table automatically results in a change on a map. Desktop
GIS links sets of features and their attributes and manages them together
in units called themes. A theme contains a set of related features, such
as roads, streams, parcels, or wildlife habitat areas, along with the attributes
for those features.For example,
in Figure 3, the theme shown in the view include: (1) Brookings County
Roads, (2) Brookings County Rivers and Streams and (3) Brookings County
Landcover Use.
All
the themes for a geographic area taken together make up a GIS database.The
design of a GIS database is strong because it's flexible. You can add new
themes to a GIS database or delete old ones; you can separate themes to
create more themes, or combine themes if they have common characteristics.
What you want to do with a GIS database, and what information you need,
will determine the best design for you.
The
GIS database can be ‘queried’.This
means a user can ask questions and get answers about the database. For
example, in Figure 4, the user queries the database about location of the
residential area in and around Brookings, SD.The
results of the query are highlighted in yellow in both the table and the
map.
Figure 4.Example
of a query of a GIS database.
Information
can be presented as maps, charts, and tables, along with graphics you import
from other programs or even graphics you draw yourself. The presentations
you create can be output to a printer to produce hard copy, or displayed
on your computer's screen. You decide what information to present and how
much--how much detail, which colors and symbols, and how the final pieces
will be arranged. And if you audience or your objective should change,
it's easy to make your presentation reflect those changes, without having
to start over.
What do you need to know about the
data?
Geographic
data refers to information about the earth's surface and the objects found
on it. This information comes in three basic forms: spatial data, tabular
data, and image data. Spatial data contains the locations and shapes of
map features. Tabular data is collected and compiled for specific areas
and is the descriptive data that GIS links to map features. Image data
includes such diverse elements as satellite images, aerial photographs,
and scanned data--data that's been converted from printed to digital format.
Data can be created or bought.For
example, a GPS receiver can be used to identify sites in an agricultural
field where weed data is collected.A
table can be created in the GIS showing location as well as species and
number of weeds present in the measured area.Alternatively,
data can be purchased.In most cases,
images are bought from satellite or aircraft companies that used cameras
to collect images of the Earth’s surface.
Contributions from Environmental
Systems Research Institute
Http://www.esri.com
Author Information:
1
6th grade science teacher, Mickelson Middle School, Brookings, SD,
2
Research Associate, Engineering Resource Center, South Dakota State University,
Brookings, SD
3Research
Associate, Plant Science Department, South Dakota State University, Brookings,
SD
Funding provided by:North
Central Soybean Board, South Dakota Corn Utilization Council, South Dakota
Soybean Research and Promotion Council, EPA, USDA-IPM, and South Dakota
State University Experiment Station.