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T&D > Programs Areas > Inventory & Monitoring > String Degradability Program Areas
String & Flagging Degradability

Alternate Measuring Methods

A hip chain or string box is typically used to measure distance over an uneven surface. Catching the string on vegetation at a given height allows the user to independently measure distance in rugged terrain with minimal backtracking.

In some cases, this tool could be easily substituted with other measurement techniques, such as GPS or laser rangefinders. However, in situations where these technologies are limited, like an inhibited GPS satellite signal canopy cover and/or terrain, or a heavy understory that continually blocks the intended target for a rangefinder, a hip chain may be the most suitable option. This page presents some alternate distance measurement options for consideration.

Transponder—The Haglof Vertex uses ultrasound to measure distance between the transponder and the measurement device. Using a transponder is a reasonably accurate distance measurement method; error claimed is 1% or less. However, this method requires a second person to move the transponder, or some backtracking to retrieve the transponder. There are a variety of Haglof models that use this method, the DME models provide distance and Vertex models provide angle, or slope, as well as other information. These units range in cost from $700 to $2,500.

Laser—Laser rangefinders use eye-safe laser technology, where the laser is aimed at a target object, or a reflector placed on that target, and the signal is reflected back to the unit. The total flight time, or time from sending the signal to receiving the signal, is used to calculate distance. If a reflective target is not used, then rangefinders can be operated alone, without backtracking. However, heavy understory or rangefinder configuration may require the use of a reflective target, in which case a second person or backtracking will be needed. The accuracy of these devices is dependant upon the unit selected and cost often reflects this. Many units in the $300 to $500 range claim an error of 1% or less if using a reflective target. While higher end units in excess of $2,000 claim much greater accuracy. There are a wide variety of manufacturers of laser rangefinders including, but not limited to MDL, Laser Technology, Opti-logic, Leica, and Bushnell.

GPS—GPS is a natural for distance measurement; each position calculated from the satellite signals provides a Northing, Easting, and Elevation. Position accuracy can be improved by collecting multiple positions at each point, also known as position averaging. This method can be used independently without backtracking. Unfortunately, in many forested conditions, satellite availability can be limited due to canopy cover or terrain. When sufficient satellites are available, multipathing can increase the error in the measurements. Some commercial grade units, including a few produced by Trimble and Leica provide filters to adjust for this error. With multipathing filters and post-processing, positional error under forested conditions was shown to range from 0.7 to 13 meters with an average of 3.6 (McCullough 2002). Position averaging, in this case collecting 60 positions for one point, required between 3 and 7 minutes per position.