What are the technologies:

Global Positioning Systems (GPS)

Quick link to...
GPS Units for Rental
GPS Field Protocol
Handheld GPS Buyer's Guide
Buyer's Guide for Field PDA w/ GPS
Geospatial Resources for Agricultural Field Research
Other GPS Resources

Good articles...
Where in the world are my field plots? Johnson & Barton, 2004

GPS units have become a common way to collect location data. By receiving signals from a network of satellites, GPS units are now providing accurate location information in a variety of applications.

There are several choices of GPS units that differ in features and capability for accuracy (more info on accuracy). Overall, GPS units are separated into three categories: recreation-grade, mapping-grade, and survey-grade. These categories are based upon the usual intended use.

• Recreation-grade GPS units are sold mostly for outdoor sports or recreational activities. These GPS units are the least accurate of all the grades, usually having an error/offset of 3-30 meters, depending on conditions. More info...

• Mapping-grade GPS units are used in general land inventories and often in research projects where higher accuracy is important. Mapping-grade units can be further thought of as sub-divided into those that have the capability of about 1-meter accuracy and those capable of sub-meter accuracy due to differences in receiver and antennae instruments. Mapping-grade GPS units use differential correction for high accuracy. More info...

• Survey-grade GPS units are used for surveying tasks that require very high accuracy (within 1 cm), such as bridge construction. Survey-grade GPS units use differential correction for highest accuracy, and are required for some types of natural resource science such as acquatic sedimentation studies.

As the chart below shows, increasing accuracy comes with increasing price (these costs reflect other features as well).

GPS Unit Comparison Chart

Grade	Accuracy	Cost
Recreation	3-30 m *	$200-600
Mapping	1 m	$5,000
	Sub-meter	$10,000
Survey	0.1 m	$20,000

* Accuracy depends on conditions, e.g. canopy cover, terrain, satellite position, etc.