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© 2006 Plant Management Network.
Accepted for publication 26 May 2006. Published 25 August 2006.

GPS-Enabled Rising Plate Meter with Data Logging Capability

E. Scott Flynn, Research Analyst, Charles T. Dougherty, Professor, Department of Plant and Soil Sciences, and Benjamin K. Koostra, Engineer Associate, Department of Biosystems and Agricultural Engineering, University of Kentucky, Lexington 40546

Corresponding author: E. Scott Flynn. esflyn2@msn.com

Flynn, E. S., Dougherty, C. T., and Koostra, B. K. 2006. GPS-enabled rising plate meter with data logging capability. Online. Forage and Grazinglands doi:10.1094/FG-2006-0825-01-BR.

Measuring and georeferencing of grassland biomass for regression against vegetative indices is laborious, time-consuming, and subject to recording errors. Clipping and weighing of biomass and georeferencing sampling sites limits the number and the intensity of samples that can be collected in a reasonable time-frame by a single observer. Larger and more densely-spaced datasets may be assembled by multiple observers and by employing indirect methods of biomass estimation, but labor and equipment costs may be limiting. To address these issues, we fitted a laser distance meter to a rising plate meter (RPM) to record compressed sward surface height (CSSH) (1) and used a Bluetooth-enabled personal digital assistant (PDA) equipped with GPS data-logging software to record CSSH and spatial coordinates.

Harmoney et al. (2) reported that CSSH determined by a RPM was linearly correlated with herbage over normal ranges for cool-season grasses, such as tall fescue (Festuca arundinacea) (R² = 0.85). Some RPM device configurations use center poles with 0.5-cm grooves that are counted with gear counters or read directly from a scale. We fabricated a custom RPM using aluminum and fitted a laser distance meter (Figs. 1 and 2) to measure compressed sward surface height and transmit data wirelessly to a PDA.

Fig. 1. Design and specifications of the GPS-enabled rising plate meter (RPM). The GPS antenna is normally mounted to the top of the RPM, but it may be carried by the operator as long as the offset is corrected.

Fig. 2. The GPS-enabled rising plate meter allows for easier biomass data collection and georeferencing.

The Leica Disto plus laser distance meter (Leica Geosystems AG, Heerbrugg Switzerland) was chosen for this design because of its accuracy (± 1.5 mm) and its ability to transfer data via a Bluetooth wireless connection. The Leica Disto plus has a continuous measurement function but only sends CSSH to the data logger when the user initiates a command. The weight (335 g) and dimensions (172 × 73 × 45 mm) make it easy to mount on the RPM and its operating distance range (0.2-200 m) make it well suited for our application. However, due to the reflective properties of aluminum, the compression plate was coated with red oxide primer to allow the distance meter to measure properly. To record the CSSH from the laser distance meter, we used a HP iPAQ PDA (Hewlett Packard, Palo Alto, CA) and ESRI ArcPad 6 software (ESRI, Redlands, CA) with a custom script created using ESRI ArcPad Application Builder software (ESRI, Redlands, CA). Spatial coordinates were obtained using a Trimble AgGPS 132 GPS receiver (Trimble Navigation Limited, Sunnyvale, CA) with submeter accuracy (± 99 cm) connected via serial port to the PDA. ESRI ArcPad 6 software has standard built-in functionality for connecting to GPS receivers and recording GPS coordinates. The script and a custom configuration file for ArcPad were bundled into an ArcPad Applet and deployed to the PDA. When a CSSH measurement is initiated by the user, the measurement is transmitted from the distance meter wirelessly to the PDA and a point data collection event is triggered in ArcPad. The latitude, longitude, elevation, and accuracy information obtained from the GPS, along with the distance obtained by the distance meter, are stored on the PDA in an ESRI shapefile format data file.

Our integrated system allows for CSSH and geographical positions to be logged simultaneously within 1 to 3 seconds of the RPM being placed on a sward canopy. The single-user design eliminates much of the human error in collecting and georeferencing data, and the accuracy of the distance meter (± 1.5 mm) makes it well suited for collecting biomass information. When calibrated in a tall fescue sward, the new design exhibited a strong correlation with DM (R² = 0.90) (Fig. 3).

Fig. 3. Relationship between DM and the compressed sward surface height recorded by the GPS-enabled RPM. Data was collected in a tall fescue sward.

The components used with our particular RPM may be used with other canopy height-measuring methods. Also, certain components of this design, such as the GPS, distancemeter, and PDA, may be substituted with less expensive but suitable devices recognizing the accuracy specifications of the GPS receivers. The approximate cost of materials and equipment needed to assemble the design described here or a similar design are as follows:

Literature Cited

1. Earle, D. F., and McGowan, A. A. 1979. Evaluation and calibration of an automated rising plate meter for estimating dry matter yield of pasture. Aust. J. Exp. Agric. Anim. Husb. 19:337-343.

2. Harmoney, K. R., Moore, K. J., George, J. R., Brummer, E. C., and Russell, J. R. 1997. Determination of pasture biomass using four indirect methods. Agron. J. 89:665-672.