Selecting the Right GPS Receiver for Your Job
Global Positioning Systems Reveal Precise Position
GPS Collection Devices
Selecting GPS capabilities you will purchase and use will be based on computer processing capabilities of the unit. This is combined with the antenna it will use. All GPS units access data from the same satellites in orbit around the earth. The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radio navigation system. It is owned by the United States government and operated by the United States Air Force. The signals are provided to all users, free of charge. They use a radio based system to communicate from satellites to GPS units.
This is where user-collection-techniques raise to the highest levels of importance. A staff with a top mounted antenna, wired to the GPS device delivers the highest position location quality. The staff keeps the unit fixed and stable for a period of time to record a point averaging position. This technique increases the accuracy of the position recorded. The antenna mounted above the operators head-level also reduces multi-path propogation – allowing unobstructed view from antenna to satellite.
Other techniques to limit multi-path interference involves off-setting position recording from poor reception locations to clear-view points. These are the techniques each operator should learn, experiment with, and use on every GPS data collection project.
Mark Your Grade
Our first recommendation is to consider the accuracy needed based on the use of the data collected. This concept is introduced on this site, GPS Investigations, and we focus on it because it makes financial differences. It also impacts the time of data collection and processing. Time and money go hand-in-hand, and this is no exception. There are generally three grade categories of GPS Receivers:
- Recreation Grade
- Map Grade
- Survey Grade
A “real-life” example”
Take for instance the timber cruiser who needs to collect timber cruising plot center points with the GPS receiver, then collect tree data on several trees per plot. Contrast this with the professional surveyor who is collecting the positions of section corners, or property lines.
In this situation, the timber cruiser may not be able to meet timber cruising production levels while carrying an external GPS staff or backpack for the GPS antenna with a cable capable of catching on branches. But he may find himself working in rainy conditions and hiking up and down steep terrain far from a support vehicle. This user may require a unit of high durability, with replaceable batteries in case one drains in the field, but external antenna connections are not necessary since an increase of accuracy may not be justified for a timber cruise’s plot center locations. On the other hand, the mapping of proposed road center lines through the forestlands with a support vehicle within a short distance may meet the justification for an external antenna. But replaceable batteries are unnecessary if the user has an in-car charger; and the durability is a non-factor since this user is on fairly level ground. But considering a surveyor may collect over 500 seconds of data for a single point means the user will need as much data storage space as possible in the GPS receiver.
Cost Options “GPS-O-Nomics”
Some Sources we are familiar with
GPS Receiver vendors supply a variety of options for the user for all grades mentioned here. We recommend two specific brands of GPS receivers for most users involved in field collection efforts, who engage in natural resources work.
Recreational to Map Grade
For over two decades, we have been using Garmin 76 and 60 series GPS receivers with great satisfaction for the durability, versatility, and reliability of the units. Their operations are intuitive; they are weather resistant; and they run on two AA size batteries for easy replacement in the field. There is an external antenna port for times when an external antenna is desired, but the internal antenna serves the purpose well.
These units are classified as recreational grade, but the user can implement field collection techniques to move the quality of the data into the Map Grade realm. To accompany this aspect the Minnesota Department of Natural Resources developed a program to transfer GPS data from the Garmin series of GPS units to the computer and back again.
This program is offered for FREE to users and even moves the GPS data into ESRI ArcMAP programs (including version 10.x) for mapping. Sorry, no MAC support.
The 76 series GPS unit has been discontinued by Garmin, but the 60 series is still available. Other models we have used (and recommend) include the 62 series and the 78 series receivers. We recommend you look for an external antenna port and microSD card slot.
Unit photos courtesy of Garmin
We have purchased our Garmin GPS receivers and accessories from gpscity.com for over a decade and found their service and reliability to be excellent (they are not paying us for the recommendation!). They have been cost competitive and are located at: http://gpscity.com
Garmin GPS Map76 Series Field Exercises: made for the field-tech to record positional data to move to a GIS program Get the PDF
Map to Survey Grade
When higher accuracy needs are required, we have turned to the Trimble family of receivers with the Juno and GeoExplorer meeting our field collection needs. These two units are what some field staff may refer to as “data collectors” rather than simply “GPS units”. These units are, in practicality, handheld computers with features like Windows Mobile operating system, web browsing, Bluetooth connectivity, mobile software like Microsoft Word or Excel, as well as a very powerful built in GPS antenna.
Due to the sizeable difference in cost between the Trimble Juno series ($800-$1,200) and the GeoExplorer series ($2,000-$10,000) the major deciding factor between which Map Grade unit(s) to purchase is the accuracy required by the user. The Juno series is capable of 1-3 meter (3-9 feet) accuracy using good data collection techniques, while the GeoExplorer series are capable of near 10-50cm (0.3-1.3 feet). Both the Juno and GeoExplorer units come in varying levels of accuracy and/or add-ons. Of course as add-ons and accuracy increase, so will the price.
The rule of thumb when looking at a Trimble Map Grade GPS unit is: If the work requires sub-meter accuracy (within 3 feet precision) then turn to the GeoExplorer series; if not, then the Trimble Juno is a great value and wonderfully flexible unit.
The Trimble web site is located at: https://www.trimble.com
With the Trimble Store found at: https://store.trimble.com
Remember, once you get those Excellent points and lines on your GPS, you need to get them into your GIS system and know how to make them meaningful.