What are the technologies:
Global Positioning Systems (GPS)
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GPS Units for Rental
GPS Field Protocol
Handheld GPS Buyer's Guide
Accuray and Differential Correction
Until the last few years, data from the satellites was deliberately downgraded by the U.S. military to impede accurate real-time location measurements. Because of the removal of this scrambling, called Selective Availability (or SA), it is now much easier for the public to use GPS technology. For high levels of accuracy, measurements still need to be corrected for atmospheric conditions and irregularities in satellite orbits. This is called differential correction, and is accomplished by referencing a nearby "base station."
More Information on Types of GPS Units
Recreation-grade GPS units, such as Garmin units, have the advantage of being inexpensive, commonly available, and user-friendly. The level of accuracy (between 3 and 30 meters depending on conditions) is generally adequate for natural resource science. These units can store the point and line locations for you to download to a computer back in the office. Otherwise you need to write down coordinates (this is good practice in case batteries die).
Mapping-grade GPS units, such as the Trimble GeoXH, not only have the capacity for higher accuracy but are designed to facilitate data collection for a GIS program. Features can be captured as points, lines, or polygons (areas).
The major differences between mapping-grade units and recreation-grade units include:
- The mapping-grade units allow the collection of point, line,
or polygon data , while recreation only (no polygon features);
- They
are not designed to store data in a GIS-ready format such as a
shapefile for ArcGIS, allowing data generated in the
field to be automatically transferred into a GIS system on the
computer; and
- You will not be able to use the unit alone to digitally collect descriptive (attribute) data together with your geographic data. There are certainly options for solving these challenges, however, such as software packages that convert simple waypoint coordinates into data that can be used in a GIS program.
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