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History and Background
Geographic Information Systems
(GIS)
Today, GIS is one of the
fastest growing digital technologies. It is estimated that GIS will be a $4
billion industry this year. It has emerged as a powerful and sophisticated
means of managing vast amounts of geographic data that enable organizations
to consider more effective ways of doing business.
Description
GIS has grown out of a number of technologies including cartography,
photogrammetry, information management, computer science, and remote
sensing. GIS provides a mechanism by which information on a feature's
location, spatial interaction, and geographic relationship can be assessed
and viewed in moments. It also provides an opportunity to efficiently view
and access geographic data to improve the decision-making process. The
following is a useful definition because it addresses functionality as well
as components:
"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
Others have attempted to use
the name itself to better understand the functions and components of GIS.
GIS can be viewed in this way:
Geographic: The
system is concerned with data relating to geography and geographic scales
of measurement. This is referenced by some coordinate system to locations
on the surface of the earth.
Information: The
system allows for the storage and extraction of specific and meaningful
attribute information. These data are connected to some geography and are
organized around a model of the real world. Spatial and aspatial queries
are made possible.
System: An automated
system should include an integrated set of procedures for the input,
storage, manipulation, and output of geographic information.
How to Conceptualize a GIS
One of the world's leading GIS software vendors organizes data in such a way
that they can be envisioned as digital layers or coverages of information.
Each coverage is registered to the same common map base; each has a distinct
type of feature, points, lines, or polygons. The GIS stores the spatial data
(location information - where something exists on the earth's surface) and
attribute data (characteristics of the feature; e.g., pavement condition). A
coverage represents a single theme, such as soils (polygon), streams (line),
roads (line), land-use (polygon), and wells (point). When different themes
(i.e. roads, streams, buildings, elevation, and soils) are combined into one
overlay it represents a view of the real world. This view can then be
analyzed to learn more about the spatial interactions between the various
themes in the view (i.e. what relationship is there between where roads fail
and the soil types under them).
Remote Sensing
History
In 1858 Gaspard Felix Tournachon, known as "Nadar", took the first known
aerial photograph by ascending to 80 meters above Bievre, France in a
balloon. In doing so, this Parisian photographer birthed the movement of
remote sensing. However, it was the work of an American, G. R. Lawrence, who
would bring worldwide attention to remote sensing; when following the Great
San Francisco earthquake and fire of 1906, he sailed out into San Francisco
Bay and flew a battery of 17 kites to a height of 610 meters over the city
to photograph the disaster.
Description
There are two primary methods for gathering geographic data: the first is
through direct physical contact, or in-situ, with that which is of interest.
Thermometers and wind gauges are examples of this method. The second primary
method places the measuring instrument in a location some distance from the
feature of interest. These instruments are said to be "remote", as they make
no physical contact when recording. Both of these devices contain sensors,
which permit "images" to be recorded without the device being near the
feature that the image represents. The fundamental process of remote
sensing, for geographic purposes, places an instrument at a distant
location, in order to record an image in a section of the Electromagnetic
spectrum; which when analyzed will provide valuable information.
Advantages of Remote
Sensing
The following are five basic advantages, given by Lillesand and Kiefer (1987
pp37-38), of using remote sensing over traditional "on-the-ground" data
collection. (1) The improved vantage point the imagery provides (2) The
ability to record a moment in time (3) The ability to permanently record
events (4) The broadened spectral sensitivity and (5) The improved spatial
resolution and geometric fidelity.
Four Types of Imagery
Resolution
Resolution is understood as the ability to distinguish between two different
features that are spatially near or spectrally similar. There are four types
of resolutions that are inherent in remotely sensed imagery. (1) spatial
resolution (2) spectral resolution which is the (3) temporal resolution and
(4) radiometric resolution.
Two Primary Types of
Sensors: Passive and Active
Passive sensors use only the naturally available illumination that enters
the instrument to record images. Active sensors, conversely, emit their own
energy that travels from the instrument to the target, returns to the
sensor, and is used to produce an image of the target. There are two
varieties of passive sensors: reflective and emitted. Reflective sensors are
designed to detect the natural illumination that is being reflected off a
target. Emitted sensors are designed to detect the natural illumination that
is being produced by the target itself. Active sensors transmit their own
energy in short bursts towards a target and then record the strength of the
signal that "bounces" back to the sensor. The variation in the strength of
the returned signal can be processed into an image that describes the
target's structure and/or surface.
Global Positioning
System (GPS)
History
The Department of Defense (DOD) required an accurate navigational system
that covered the entire world in order to keep track of military equipment
and men. The project was begun in 1978 and now the 24 satellites that orbit
the earth every 12 hours at 12,500 mph provide positional accuracies that
allow surveyors to locate an object's latitude, longitude and elevation to
within a few centimeters. The DOD invested $12 billion into research and
development of the Global Positioning System (GPS) and today we have a fully
operational GPS system used extensively by the military and increasingly by
civilians in cars, boats, and hand-held units.
The GPS System
GPS is an effective mapping tool because line of sight between the unknown
and a known location is NOT necessary. Only a line of site to the sky is
needed. When operated properly, GPS satellites provide accurate positioning,
user mobility, and rapid data capture. While satellite based positioning has
revolutionized the GIS/mapping data capture industry, it is important to
note that GPS is only a useful mapping tool. The GPS has three distinct
phases:
The Space Segment: The
space segment consists of satellites in six circular orbits, inclined angle
of 55 degrees and spaced so that at any time a minimum of 6 satellites are
in view to users anywhere in the world.
The Control Segment:
The control segment consists of a master control station in Colorado
Springs, with five monitor stations and three ground antennas located
throughout the world. The monitor stations track all satellites and send the
satellite's information to the master control station, which computes
extremely precise satellite orbits. That are then used to insure the
satellites are in correct orbits.
The User Segment: The
user segment consists of receivers, processors and antennas that allow land,
sea and airborne operators to receive the GPS broadcasts and compute their
precise position and velocity.
How GPS Locates Positions
To locate positions GPS utilizes a method known as Trilateration. This is
accomplished by measuring how long it takes for a radio signal to travel
from the satellite to the receiver. That time is then used to calculate the
distance. To measure travel time, GPS devices have very precise, accurate
clocks that can measure time with nanosecond timing. That's 0.000000001 of a
second. With the distance to the satellite known the receiver determines
where the satellite is located in its orbit; then using multiple satellites
signals the intersection of these "paths" pinpoint the location of the
receiver on the earth.
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