Topographic survey of farmland based on GNSS dualantenna combined with AHRS

Abstract: The topographic survey becomes a necessary part of the farmland leveling works in modern precision agriculture. The collected topography data can be used to design a path for the farmland leveling, and further to provide technical support for the calculation of excavation and filling earthwork. However, there are some inevitable measurement errors that caused by the mechanical structure of the devices and/or the profile of the terrain fluctuation during the field operation. This present paper therefore proposed a novel topographic measurement method that combined the GNSS (globe navigation satellite system) dual antennas and AHRS (attitude and heading reference system) to reduce the collecting errors of the farmland topography data in the process of GNSS farmland leveling, and consequently to improve the measurement accuracy of three-dimensional farmland topography. The test platform was based on the GNSS land leveling system, where the GNSS antennas were used to provide the location information, whereas the AHRS was used to obtain the attitude angle and triaxial acceleration information of the land leveler. The calculating equations were established to parse the position of the GNSS double antenna on the land leveler. The error source was also important factor in the process of data acquisition, one of which was the pin-type semi-rigid connection between the tractor and land leveler for the difficult installation of damping device. A four-step error-handling method was proposed to reduce the impact of displacement error. First, to measure the vibration acceleration by AHRS and then to calculate the vibration acceleration of the land leveler in the X, Y and Z axes using the coordinate transformation. Second, to process the acceleration trend term by LSM (least squares method). Third, to calculate the vibration displacement by FFT (fast fourier transform) in the frequency domain, and then to be subtracted by the position information from the GNSS double antenna. Fourth, to dispose the displacements on the X, Y and Z axes by the FPT (five point three times) smoothing method, then to import the final data in the terrain mapping software and to obtain the three-dimensional topographic map of the farmland. This combined GNSS dual antennas/AHRS topographic measuring method has been applied to the GNSS land leveling system, where the functional execution software was programmed by MATLAB and C++. The tests of the cement road and farmland were also carried out under the tractor speed of 1.12 m/s. The results showed that the position information of the double antennas can be obtained in the dynamic environment. After the error processing, the fluctuation data of the positioning antenna can be reduced by 9.57%, and that of directional antenna can be reduced by 8.42%, which was suitable for the GNSS land leveling work. The farmland leveling test showed that the root mean square error (RMSE) of the vertical acceleration was 0.256 m/s?, and that of the course angle, pitch angle and roll angle were 0.903°, 0.849° and 0.531°, respectively. This values proved that the AHRS can be extended to apply for the topographic survey of farmland. The accuracy of the farmland flatness measured by double antenna increased than that by single antenna topographic survey, ranking in the order of three different areas (35 m×50 m, 35 m×100 m and 35 m×200 m) by 14.286%, 14.063% and 10.084%, respectively, the accuracy of the maximum differential elevation error increased by 8.642%, 8.333% and 8.897%, respectively, and the accuracy of the elevation difference distribution increased by 1.536%, 3.357% and 2.991%, respectively. This finding demonstrated that the double antenna topographic survey system can provide much higher accuracy and more stable topographic information than that of the conventional single antenna field topographic survey method for the GNSS farmland leveling in the precision agriculture.