Knowledge of site-specific crop parameters such as plant height, coverage and biomass density is important for optimising crop management and harvesting processes. Sensors for measuring crop parameters are essential pre-requisites to gather this information. In recent years, laser rangefinder sensors have been adopted in many industrial applications. In agricultural engineering, the potential of laser rangefinders for measuring crop parameters has been little exploited. This paper reports the design and the performance of a measuring system based on a triangulation and a time-of-flight laser rangefinder for estimating crop biomass density in representative crops under field conditions. It was shown that the mean height of reflection point is a suitable parameter for non-contact indirect measurement of crop biomass by laser rangefinder sensors. The main parameters for potential assessment were the coefficient of determination (R 2 ) and the standard error (RMSE) for the relation between crop biomass density and the mean height of the reflection point in crop stands from oilseed rape, winter rye, winter wheat and grassland during the vegetation period in 2006. For the triangulation sensor, R 2 was in the range from 0.87 to 0.98 and for the time-of-flight sensor in the range from 0.75 to 0.99 for both fresh matter and dry matter density. The triangulation sensor had a reduced suitability caused by masking effects of the reflected beam and because of limited measuring range. Based on the results of experiments and technical data, it was concluded that the time-of-flight principle has good potential for site-specific crop management.

Laser rangefinders and laser scanners are widely used for industrial purposes and for remote sensing. In agriculture information about crop parameters like volume, height, and density can support the optimisation of production processes. In scientific papers the measurement of these parameters by low cost laser rangefinders with one echo has been presented for short ranges. Because the cross section area of the beam increases with the measuring range, it can be expected that laser rangefinders will have a reduced measuring accuracy in small sized crops and when measuring far distances. These problems are caused by target areas smaller than the beam and by the beam striking the edges of crop objects. Lab tests under defined conditions and a real field test were performed to assess the measuring properties under such difficult conditions of a chosen low cost sensor. Based on lab tests it was shown that the accuracy was reduced, but the successful use of the sensor under field conditions demonstrated the potential to meet the demands for agricultural applications, Insights resulting from investigations made in the paper contribute to facilitating the choice or the development of laser rangefinder sensors for vehicle based measurement of crop parameters for optimisation of production processes.

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