Considerations on the Purchase of a GPS

Currently, The American Chestnut Foundation uses the Garmin 60CSx and Garmin 62s for its data collection and fieldwork.  This statement is not an endorsement of those products specifically or Garmin in general. - sff

by Hugh Irwin

The specs for GPS units that would be suitable for [The Appalachian Trail Mega-Transect] shouldn’t be too difficult. Many of the current units would be adequate for the job.

First to clarify two concepts related to GPS units that are sometimes confused: accuracy and sensitivity. Some GPS uses need very accurate readings. GPS units are available in meter and sub-meter and even centimeter accuracy and for some uses (survey work; locating set markers accurately; determining building locations) the push for greater accuracy is worth paying a premium price for. However, some of these units cost tens of thousands of dollars - or more.

Sensitivity is somewhat different. For much biological field work the highest priority is to have a GPS unit that will get and keep a signal under a forest canopy, in canyons, and around mountain sides – sensitivity. You could easily think the more accurate the unit the greater the sensitivity and vice-versa. However, this isn’t the case. I found out in discussions with a Trimble representative (manufacturer of high end very accurate GPS units) that they filter out low quality signals to achieve greater accuracy. So these accurate (and expensive) units actually can have trouble maintaining readings in difficult situations because they don’t take advantage of all the available signals, essentially sacrificing sensitivity for greater accuracy.

On the other hand, some of GPS units generally thought of as recreational units are very sensitive. Especially units that have “high sensitivity receivers” that have come out in the last couple of years. You won’t get meter accuracy with these, but for most field purposes you don’t need this. We’ve done a lot of field work using GPS units (e.g. old growth forest surveys; surveys of forest communities; identifying stream ranges for brook trout reintroduction). For all of these types of surveys accuracy within a few dozen feet is fine. Even when identifying individual old growth trees, if you have a GPS reading that’s accurate to 30 or even 50 feet is more than sufficient to relocate a tree. I typically get readings accurate to 20-30 feet with the unit I use now. I suspect this would be adequate for recording American chestnut locations. It would be accurate enough to re-locate large trees and also accurate enough to delineate areas (e.g. of chestnut occurrence vs non-occurrence) for regression analysis and other spatial analysis relating the data you collect to other available data.

Another consideration is a unit that has memory for storing waypoints. Most units I’m familiar with store 500 or 1,000 points. The 1,000 points capacity would allow someone to record a lot of tree points in a day of field work.

An easy way to download data and convert it to GIS shapefiles would be essential for good project workflow. I’m most familiar with Garmin units, but it could likely be done with most units that have capability to download data to a computer. It’s particularly easy with Garmin units as the Minnesota Dept of Natural Resources has developed a free ArcGIS (and ArcView) extension (called DNR Garmin) that allows you to export points directly into GIS shapefiles.