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A Noncontact Laser System for Measuring Soil Surface Topography

National Soil Erosion Lab., USDA-ARS, Purdue Univ., West Lafayette, IN 47907

I. White^*

Div. of Environmental Mechanics-CSIRO, GPO Box 821, Canberra City, ACT 2601, Australia

E. G. Thwaite and A. Bendeli

Div. of Applied Physics-CSIRO, PO Box 218, Lindfield, NSW 2070, Australia

* Corresponding author.

ABSTRACT

Soil surface topography profoundly influences runoff hydrodynamics, soil erosion, and surface retention of water. Here we describe an optical noncontact system for measuring soil surface topography. Soil elevation is measured by projecting a laser beam onto the surface and detecting the position of the interception point. The optical axis of the detection system is oriented at a small angle to the incident beam. A low-power HeNe (Helium-Neon) laser is used as the laser source, a photodiode array is used as the laser image detector and an ordinary 35-mm single lens reflex camera provides the optical system to focus the laser image onto the diode array. A wide spectrum of measurement ranges (R) and resolutions are selectable, from 1 mm to 1 m. These are determined by the laser-camera distance and angle, the focal length of the lens, and the sensing length of the diode array and the number of elements (N) contained in the array. The resolution of the system is approximately R/2N. We show for the system used here that this resolution is approximately 0.2%. In the configuration selected, elevation changes of 0.16 mm could be detected over a surface elevation range of 87 mm. The sampling rate of the system is 1000 Hz, which permits soil surfaces to be measured at speeds of up to 1 m s^–1 with measurements taken at 1-mm spacing. Measurements of individual raindrop impacts on the soil and of soil surfaces before and after rain show the versatility of the laser surface profiler, which has applications in studies of erosion processes, surface storage and soil trafficability.

Contribution from CSIRO, Australia.

Received for publication April 13, 1987.

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