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GPS-Mapping |
What is GPS?
The Global Positioning System (GPS) is a network of 24
satellites (owned and
operated by the United States Department of Defense) orbiting the earth
which,
through the
use of satellites and computers, is able to coordinate locations
anywhere on earth.
How Does GPS Work?
The GPS works through a system of triangulation (trilateration
as some may
say). In order to precisely pinpoint a location on earth, the receiver
must be
"in view" of at least
four satellites at the time of data collection.
The receiver's location is then pinpointed by way of trigonometrically
comparing
the distances from each satellite to the receiver.
(Thankfully, the receiver
does the calculating, not us!) The Trimble TDCI Asset Surveyor receiver
used
here at ICS is an 8-channel receiver, which has the ability of tracking
up to 8
satellites at any given moment.
How Accurate is GPS?
An accurate position is calculated by using at least four
satellites.
However, the accuracy of positions calculated by a stand-alone receiver
can be
highly variable (at times
up to variations of 100 meters). Therefore, correction
methods are required in order to produce near-exact coordinates. This
correction
occurs through use of differential
correction, which uses information from two
receivers (the hand-held rover and a permanent station) to more
accurately
pinpoint a location. The two types of correction
used here at ICS are
"real-time" and "post-processing" correction.
Real-Time Correction:
Real-time correction processes the data as it is received from
the
satellites. Correction information broadcast from a reference receiver
is used
to correct the data on-site.
The accuracy of coordinates produces in real-time
ranges within 1.5 to 3 feet.
Post-Processing Correction:
Post-processing corrections further correct the data collected
on-site by
interfacing information from a more static base station with the
real-time
corrected data. This is done
by way of computer software. The accuracy of
coordinates following post-processing correction is within 10" to 20".
Is There a Potential for Errors in the Data?
As with anything else, there is always a possibility for
error. Fortunately,
it is the sum of several errors that will ultimately diminish the
accuracy of a
GPS coordinate point.
The key to obtaining accurate data is to minimize the
number of errors occurring at any one time. Following is a list of some
of those
sources that occur often during GPS
data collection and how they affect data
accuracy along with methods of prevention:
Selective Availability:
The largest and most common source of error occurs due to
intentional
scrambling of the GPS signal by the Department of Defense. Position
accuracy can
be distorted by
up to 100 meters 95% of the time. Fortunately, this is easily
overcome by both real-time and post-processing differential correction.
Obstruction:
Environmental factors, such as trees or buildings can
sometimes simply get in
the way of signals. This is overcome by carefully planning when and
where data
collection will
occur. Utilizing data collection periods when more satellites
are directly above in the sky is a sure way to alleviate this problem.
Multipath:
Multipath errors occur when a signal is reflected off an
object prior to it
reaching the receiver. This results in a skewed distance computation
from the
satellite to the receiver.
Accuracy of a reading due to such an occurrence is
highly variable. Maintaining proper field techniques and receiver
firmware can
reduce its effects. Fortunately any major
data flaws are easily recognizable
during office processing and corrections can be made.
Satellite Coverage:
Finally, the more satellites "in view" the better. As the
strength
of satellite signals increases, so does the accuracy of data. Again
ensuring the
highest quality coordinate points
is achievable through proper data collection
planning and follow-through.
Here at ICS, GPS mapping is a matter attended to with much
pride and
seriousness. Data collection assignments are carefully planned and
carried out.
Information is
collected when the time is right, not simply when it is possible.
We have the ability to produce GPS maps for a variety of different
irrigation
needs. Whether it be for the
location of valves or an entire irrigation layout,
ICS has the ability to meet your GPS needs.