Geography Reference
In-Depth Information
surveyed) point, a user can get accuracies of 1 to 3 meters.)
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At least two factors promote such
accuracy:
First, with GPS, we work with primary data sources. Consider two alternatives to using GPS to
generate spatial data: the physical digitizer and heads-up digitizing on a computer screen.
A digitizer is essentially an electronic drawing table, wherewith an operator traces lines or
enters points by “pointing”—with “crosshairs” embedded in a clear plastic “puck”—at features on
a map. Or, in heads-up digitizing, the operator manipulates the crosshairs on a computer screen
with a mouse.
One could consider that the ground-based portion of a GPS system and a digitizer are analogous:
the Earth's surface is the digitizing tablet, and the GPS receiver antenna plays the part of the
crosshairs, tracing along, for example, a road. But data generation with GPS takes place by record-
ing the position on the most fundamental entity available: the Earth itself, rather than a map or
photograph of a part of the Earth that was derived through a process involving perhaps several
transformations.
Second, GPS itself has high inherent accuracy. The precision of a digitizer may be 0.1 millimeters
(mm). On a map scale of 1:24,000, this translates into 2.4 meters (m) on the ground. A distance of
2.4 m is comparable to the accuracy one might expect of the properly corrected data from a medium-
quality GPS receiver. It would be hard to get this out of the digitizing process. A secondary road on
our map might be represented by a line five times as wide as the precision of the digitizer (0.5 mm
wide), giving a distance on the ground of 12 m, or about 40 feet.
One larger-scale maps, of course, the precision one might obtain from a digitizer can exceed
that obtained from the sort of GPS receiver commonly used to put data into a GIS. On a
“200-scale map” (where 1 inch is equivalent to 200 feet on the ground), 0.1 mm would imply a
distance of approximately a quarter of a meter, or less than a foot. While this distance is well
within the range of GPS capability, the equipment to obtain such accuracy is expensive and is
usually used for surveying, rather than for general GIS spatial analysis and mapmaking activities.
In summary, if you are willing to pay for it, at the extremes of accuracy, GPS wins over all other
methods. Surveyors know that GPS can provide horizontal, real-world accuracies of less than one
centimeter.
Ease of use
—Anyone who can read coordinates and find the corresponding position on a map can
use a GPS receiver. A single position so derived is usually accurate within 4 meters or so. Those who
want to collect data accurate enough for a GIS must involve themselves in more complex procedures,
but the task is no more difficult than many GIS operations.
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GPS data points are inherently three-dimensional
—in addition to providing latitude-longitude (or
other “horizontal” information), a GPS receiver may also provide altitude information. In fact,
unless it does provide altitude information itself, it must be told its altitude in order to know
where it is in the horizontal plane. The accuracy of the third dimension of GPS data is not as
great, usually, as the horizontal accuracies. As a rule of thumb, variances in the horizontal accu-
racy should be multiplied by 1.5 (and perhaps as much as 3.0) to get an estimate of the vertical
accuracy.
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These accuracies pertain to “mapping grade” data collection. By spending more money and much more time one can
shift to “survey grade” GPS, with accuracies down to a centimeter.
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