Profile Meters for Detailed Measurement of Soil Surface Heights

Field: Agricultural and Biological Sciences (Life Sciences)

TWO surface profile meters were developed for use in soil erosion research, Both were designed for use on 4.6 m wide field plots. Surface heights were measured electronically and stored on floppy disks. Meter pin spacing of 6.4 cm and 1.3 cm allowed measurement of plot characteristics and rill cross- sections. The accuracy of the meters was determined to be better than 1 mm. Design criteria, circuitry, and performance data are presented. INTRODUCTION AND LITERATURE REVIEW Soil erosion continues to be an important research problem. In recent studies of rill erosion, detailed measurements of rill cross-sectional shape have been needed to verify models. In addition, current studies of rill pattern development further require a capability to measure the surface configuration of an erosion plot before and after rill development. For these reasons, the development of meters to electronically measure soil surface heights was undertaken. Several different rill-meters and profile meters have been developed by other researchers. A summary of commonly used meters is presented in Table 1. In many of the designs, soil surface height was measured from the displacement of pins which were lowered from a reference level onto the soil surface to be measured. Devices used by Kuipers (1957), Burwell et al. (1963), Curtis and Cole (1972), Foster and Meyer (1972), Moore and Larson (1979), McCool et al. (1981), and others utilized this concept. In many of these studies a camera was used to record the pin locations against a background grid for digitization (by hand or electronically) at a later time. Electronically read pin devices include those of Radke et al. (1981) and Van Ouwerkerk et al. (1982). These later meters used multiple pin schemes similar to those mentioned above, but the pin displacement was sensed electronically and the measurement saved on a readily accessible medium. As an alternative technique some researchers used a single height-transducer which was mounted on a movable carriage. Mitchell and Jones (1973) utilized a ten-turn rotational potentiometer to sense the height displacement of a probe as it was moved down toward the soil surface. A snap-action switch was activated upon surface contact by the sensing rod to indicate the surface level. Their device was similar to that used by Currence and Lovely (1970). Podmore and Huggins (1981) described a meter to measure sand surface profiles in which the surface height was sensed by a linear-variable-differential transformer (LVDT) mounted on a traversing carriage similar to that of the Mitchell and Jones device. The LVDT was moved 0.25 mm between measurements and provided a resolution of 0.005 mm over a 33 mm range. Non-contact meters have also been developed. Kolstad and Schuler (1980) developed a sonar device for their study of ridge-till implement tools. They tested the meter using a surface with 13 cm of relief and found that the average error was about 0.75 cm. Soil moisture content differences were found to have significant effects upon accuracy. Romkens et al. (1982) developed a non-contact meter using a light source and a sensor at a constant height above the soil surface. The necessary location adjustment of the sensor to maintain that height was monitored and recorded automatically. A 250 point-per-meter transect, 1.52 m wide, was completed in about 4 min. Another optical device was developed by Harral and Cove (1982) in which a laser diode, and position-sensing photo-detector, and appropriate lenses were utilized to determine the soil surface distance from the transducer. The device had a limited working range of about 150 mm from a point 600 mm from the source and receiver with a resolution of approximately 6 mm. For erosion research at the University of Kentucky, a profile was needed meeting the following criteria: 1. measurement accuracy of soil surface heights of 1 mm over a 450 mm range, 2. adaptable to 457 cm wide variable length plots, at slopes up to 100%, 3. measurement or rill cross-section heights on a 13 mm spacing or finer, 4. measurement of plot heights on a 64 mm spacing or finer, 5. potential for measurement of surface transects or rill cross-sections anywhere along the plot length, 6. time of measurement, reduction and recording of data from one transect or cross-section to be 1 min. or less, and 7. operable by two persons. Although some of the previously discussed meters could, if modified, meet many of the above criteria, new surface-measuring meters were developed to handle the rugged field conditions, steep slopes, and speed requirements. The design considerations led to two computer controlled multi-pin meters, with electronic sensing of the pin location. The sensing method and the overall width of the larger profile meter sets these designs apart from the previously discussed meters. The two meters were identical in general performance, the difference being a 13 mm pin spacing over aim width (72 pins) for the small meter and a 64 mm pin spacing over a 4.5 m width (70 pins) for the wide meter. The smaller meter was moved laterally on a frame across the plot as well as along the plot. The basic structures of the two meters and the smaller meter frame are shown in Figs. 1 and 2.