Introduction
The Basics
of a Geogrpahic Information System
Some Common GIS Software Packages
Basic Elements
of ArcView That You Should Know
Introduction
We learned how to use databases such as Filemaker
Pro in this class. These databases are relational databases and can
be used to link records and abstract information in various ways.
However, these databases do not know where the data is located on the earth.
While we can include information such as the latitude and longitude of
a measurement in our relational database, the database cannot calculate
the length between observations, the area around observations, or perform
any type of analysis where the location of the measurement is a concern.
As geologists, a key piece of information about data, the location of the
data, is lost in a relational database such as Filemaker Pro.
The Environmental Systems Research Incorporation
(ESRI) circumvented this problem in the late 1960s when they invented the
concept of a Geographic Information System (GIS). A GIS records and
understands the significance of the spatial location of data when performing
analyses. In this way, GIS is a hybrid between CAD and Filemaker
Pro- in CAD, little if any information can be associated with a spatial
position recorded by a point, line, or area; however, Filemaker Pro can
record a large amount of information but has no information about its spatial
position. A GIS contains both pieces of information and so we can
create points, lines, and areas and associate those spatial features with
data. This is a huge conceptual leap in database design that has
many applications in geology.
A good example of a set of data in which spatial
information is important is a map. Features on a map represent different
pieces of data that may include things like faults, fold axes, contacts,
assay measurements, and remotely sensed imagery. The spatial location
of each of these pieces of data is essential in understanding how these
data relate to each other. Using a CAD program to store the map data
loses information about the data, furthermore using a relational database
program to store the map data loses essential information about the data
as well. Therefore, the data only make sense when both the spatial
position of the data points and what those points are are considered.
This is a perfect application of a Geographic Information System.
The Basics
of a Geographic Information System
A GIS is "An organized collection of computer
hardware, software, geographic data, and personnel designed to efficiently
capture, store, update, manipulate, analyze, and display all forms of geographically
referenced information" (ESRI, 1993). There are two different types
of GIS that are currently available- a vector-based GIS and a raster-based
GIS. The vector-based GIS stores information as a series of nodes
and vertices. From these basic two units, vectors are defined by
creating a line between two nodes. The vertices act to better detail
the shape of the line, whereas the nodes define both ends of the line and
serve as a container for attributes related to the line. From the
vector units, areas may be defined by labeling closed areas created by
the intersections of many vectors (ESRI, 1993) In this way, all three
types of spatial data (point, line, polygon) may be represented in the
vector GIS (Figure 1.1). The ARC module of
ARC/INFO is the most popular and widely used vector GIS system. The
second variety of GIS are the raster GIS. In this type of GIS, map
data are rasterized and converted into a series of discrete square elements,
called pixels. These pixels are in turn organized as a grid of these
squares, with a square’s location being represented by a row number and
a column number. Each of these pixels holds a value representing
some sort of data at that point on the grid (Figure
1.2). These data values contained within the pixels are often
called feature types and may represent a point, an element of a line, or
part of an area. For example, if a road exists at a particular location,
the grid elements through which the road passes will be assigned a numerical
value corresponding to a "road" designation. In this way, spatial
information is represented in much the same way as graphical image data
in computers are represented (Star et al., 1990). This allows for
the easy conversion of processed image data into a raster GIS format.
IDRISI and the GRID module of ARC/INFO are two popular raster GIS systems.
Due to a recent strive to incorporate both raster and vector systems into
a single GIS, IDRISI has added fairly limited vector functionality to their
software package. Each of these types of GIS has its particular strengths
and weaknesses. The vector GIS is good at handling discontinuous
data (i.e.. property lines, geologic units, etc.) while the raster GIS
is good at handling continuous surfaces of data (i.e.. elevation data,
etc.).
Basic Elements
of ArcView That You Should Know
There are several elements of ArcView with which
you should be familiar. We will elaborate more on these elements
and on more sophisticated elements in class; however, these are the following
are the four elements that you must understand in order to get started
with ArcView. The elements are a view, a theme, a table,
and a layout.
A view is a place where you can organize and display
all of your geographic data. Many different data sets can be part
of a view and various combinations of these data sets can be displayed
within the view. A view is created by either 1) double clicking on
the "View" button in the project window, or 2) selecting the "View" button
in the project window and pressing the "New" button on the project window
toolbar.
A theme is simply a geographic data set. Examples
of themes would be geologic maps, annotation layers, hydrologic maps, contour
maps, digital elevation models, and remotely sensed imagery. To add
a theme to your project, either 1) click the add theme button on the toolbar,
or 2) select the "Add Theme" option from the "Theme" menu. Both of
these operations must be carried out in the "View" window. To turn
the theme on, simply click on the check box next to the theme's name in
the "View" window and the theme will appear.
A table is a tabular representation of your geographic
data. Each theme has an associated table which contains the attributes
of each geographic element in your data set. Although a theme must
be associated with a table, a table doesn't necessarily have to be associated
with a theme. Therefore, both data associated with spatial locations
and data unassociated with spatial locations can be stored in a GIS.
A table can be created by either 1) double clicking on the "Table" button
in the project window, or 2) clicking on the "Table" button in the project
window and then clicking the "New" button on the project toolbar.
A layout is a map that you can create out of your
geographic data. Layouts can contain scale bars, keys, north arrows,
views, and other objects you may wish to include on a map. Therefore,
the Layout area is where you can design the final mode of presentation
of your geographic analysis. A layout can be created by either 1)
double clicking on the "Layout" button in the project window, or 2) clicking
on the "Layout" button in the project window and then clicking the "New"
button on the project toolbar.
Pages last modified on Tues Nov 11 1997.