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Abstract Increasing land developments, unreasonable utilization of land, earthquakes, typhoons, and torrential rain often cause hillside disasters. Soil depth is a crucial parameter influencing the scale of shallow landslides and sustainable land use in hillside areas. This paper presents a statistical model, integrated with environmental factors, for estimating soil depth in a catchment area with complex topography. The study area selected was upstream of the Houlong River in Taiwan. The model was developed using multinomial logistic regression and environmental factors such as hill slope, hill aspect, elevation, topographical curvature, and the normalized difference vegetation index. The results were then compared with those obtained from previous models applying Ordinary Kriging, Regression Kriging, and topographical wetness index methods. A classification error matrix and the Kappa index were then employed to assess the various models. The results showed that, for the data set used for model establishment, the overall accuracy and Kappa index were 76.6% and 0.65, respectively, whereas for the data set used for verification, they were 70.5% and 0.57, respectively. By contrast, the Ordinary Kriging method yielded an overall accuracy and a Kappa index of 45.7% and 0.15, respectively; the Regression Kriging method yielded results of 46.7% and 0.16, respectively; and the topographical wetness index method yielded results of 30.5% and −0.05, respectively. The proposed model was therefore determined to be superior to the others in terms of soil depth estimation accuracy. The proposed model can predict soil depth on a regional scale and serve as a reliable tool that provides reference data for future research on land use and shallow landslides. Graphical abstract Unlabelled Image Highlights • Soil depth was modelled in a regional scale with complex topography. • Predictive performance of four statistical techniques was compared. • Multinomial logistic regression proved in soil depth estimation. • Soil depth conditioned by the topographical and geomorphological characteristics. [ABSTRACT FROM AUTHOR]

A tapped wiggly-coupled technique is applied to parallel-coupled microstrip bandpass filters for multi-octave out-of-band spurious suppression. The coupled edges of tapped microstrip resonators are wiggled to equalise the odd-mode and even-mode phase velocities such that the nearest two spurious harmonics can be eliminated. The simulation and experiment show that both the second and the third spurious level are reduced below 42 dB up to 9 GHz for a 2.45 GHz and 5.7% bandwidth bandpass filter. [ABSTRACT FROM AUTHOR]