The number of GPS units needed in circulation for any department is largely dependent on scheduling and demand. How many staff members are interested in using the unit? What times of day or year will the units be in use? How many projects do you have in mind, and do certain projects have a higher priority than others?
Keep in mind that the majority of any GPS unit's time can be spent in the field. The only times the GPS receivers need to be in the office are when data are being uploaded or downloaded to the unit, when data collection software needs to be tested on the unit, or when its battery needs to be charged.
One approach to introduce the use of GPS in a department could be to purchase just one or two GPS units and monitor their use over a period of several months. This "trial" period will provide the staff in the department a chance to learn about the units' capabilities and understand the potential for their use in future projects. After this trial period, consult with each of the potential users about the needs of projects you have in mind for the next two years. At this point, you should be able to make a better informed decision about how many GPS units the department can use.
Keep in mind that you may find it more cost effective to purchase two units that you know will be heavily used for at least the next two years instead of purchasing six units that are used heavily at first, then spend their remaining useful lifetime stashed away in a storage closet.
A final suggestion: If you do not have access to someone with GPS expertise to help you get started, think about purchasing one or two recreational grade GPS receivers to learn the basic concepts before making a large investment in higher accuracy, more expensive GPS units.
