There are several methods that can be used to map plant locations at a botanical garden or arboretum. Regardless of the method, the goal is the same - determine the plant's coordinates. This can be done using a tape to measure the distance from the plant to known landmarks. Or, a total station electronic-optical surveying instrument can be used. It accurately measures both distances and angles from known landmarks and can be linked to a computer or data collector for automated surveying, as in the case of the BG-Map Total Station Interface. Another technology for use in plant mapping is the Global Positioning System or GPS.
GPS technology continues to improve and decrease in cost. GPS offers the advantage of efficient single person mapping - just roam around the garden with the GPS receiver. The BG-Map GPS Interface allows you to import GPS data directly into BG-Map for automatic mapping of plants and non-plant objects.
Global Positioning System, or GPS, is a method for accurately determining positions and elevations almost anywhere on earth. It uses an array of satellites placed into orbit by the U.S. government. In order to fix a position using GPS, a special receiver and data processing circuitry are required. The receiver must make contact with a minimum of 4 of the satellites. Accuracy can vary from approximately ±30 meters for inexpensive hand held receivers, to somewhere on the order of ±1 centimeter for sophisticated systems that use a base station located at a known position on earth to provide a reference signal, a technique known as "differential processing". The original user of GPS was the U.S. military, for whom the system was initially created. But GPS technology is now open and available to all, and it has found applications as diverse as tracking the locations of transit vehicles to data collection for mapping.
GPS vs. GIS
These acronyms are often confused. GPS is the Global Positioning System described above. GIS stands for Geographic Information System, a generic term for a sophisticated computer mapping system linked to a database - BG-Map, for example.
Advantages of GPS vs. Total Stations:
No Need for Control Points:
To use a total station you will need professionally surveyed ground markers, which are used as fixed reference points (control points). You will need a control point within line of sight to each plant to be mapped. And, each of these control points must be within line of sight to at least one other control point. For GPS mapping, control points are not required. This eliminates the initial cost of hiring a surveyor to install the markers.
To map plants efficiently using a total station, a two-person team is normally required. One person holds the optical target and reads plant labels while the other person operates the total station. Mapping plants with a GPS is normally a one-person operation.
Disadvantages of GPS vs. Total Stations:
Adverse Effects of Tree Canopies:
Tree Canopies, especially if wet, can affect GPS measurements, reducing their accuracy or increasing the time it takes to measure. The GPS receiver will warn you if reliable data is not available at a certain position. You can move to the side and take a measurement away from the tree, specifying an offset - a distance and an angle - between the point you are at and the actual location of the tree. You can measure this distance and angle using a tape measure and a compass. Or, you can purchase a laser rangefinder and a built-in electronic compass, which will automatically provide the offset data to the GPS.
At any given position and time, signals from a sufficient number of satellites may not be available. You can determine ahead of time when satellite coverage will not be adequate. Your GPS can vendor can help you with this. Keep in mind though that it will sometimes be necessary to schedule your work around these periods of low satellite availability.
The Question of Accuracy
Although GPS systems can provide high accuracy, it is not necessarily consistent. Changes in signal conditions can cause results to vary. Also, the time it takes to get an accurate plant position at a given point can increase greatly if signal conditions are poor. In general, you can expect GPS accuracy of less than one meter most of the time when using real-time or post-processing correction and on the order of 1 centimeter most of the time when using your own base station for correction. One-meter accuracy is adequate for mapping most woody plants while it is insufficient for mapping smaller tightly spaced plants. Despite this limitation, a GPS may serve well by mapping the larger woody plants in a bed whereas the smaller plants may be added manually by sketching in their positions in relation to the woody plants and hardscape features.
Total station accuracy does not vary as long as the equipment is properly maintained. Accuracies on the order of 1 - 10 centimeters can be consistently obtained with a total station, depending on the skill of the operator. This is perfectly adequate for mapping both woody herbaceous plants.
Some Factors to Consider Before Purchasing GPS
Some sites as not well suited to GPS, due to topography, tall buildings, large trees or sources of radio interference. Before purchasing a GPS, have the vendor demonstrate the equipment at various locations around your site to confirm that it will perform as intended.
Availability of Correction Data
For best accuracy, GPS data must be processed using a source of correction data available from an outside source or from your own base station. Correction may be done inside the GPS receiver itself (real-time correction) or later at the office (post-processing). Sources of correction data include the following:
· Coast Guard Base Stations - if you are within 100-200 miles of an ocean or the Great Lakes you will likely be able to receive correction data from a U.S. Coast Guard radio beacon. These provide real-time correction to GPS receivers equipped with a Coast Guard beacon receiver.
· Community Base stations - located throughout the U.S. and some other countries - correction data can be accessed via the Internet and used for post-processing. Contact your GPS vendor for a list of available base stations.
· Satellite Data Provider - Real-time correction data can be delivered to your GPS receiver via satellite for an annual subscription fee.
· Your Own Base Station - If other sources of correction data are not available or not practical, you could set up your own base station at a known position on or near your grounds. This means purchasing 2 GPS units, one to serve as a base station and one to be used for mapping. Some GPS vendors may offer special pricing when you buy 2 GPS units, so the cost is not doubled. Using your own base station for real-time correction can yield very high accuracy results.
Each garden will have to determine which measurement technology best meets its unique needs. For some, the solution will be a simple tape measure. Others will opt for a total station or GPS. If you have field experience using GPS to map plants, we'd like to hear from you. Please contact BG-Map..
Updated August 22, 2005
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