Filtering out vegetation from a point cloud based on color is only rarely used, largely due to the lack of knowledge of the suitability of input information (color, vegetation indices) and the thresholding methods. We have evaluated multiple vegetation indices (ExG, ExR, ExB, ExGr, GRVI, MGRVI, RGBVI, IKAW, VARI, CIVE, GLI, and VEG) and combined them with 10 methods of threshold determination based on training set selection (including machine learning methods) and the renowned Otsu’s method. All these combinations were applied to four clouds representing vegetated rocky terrain, and the results were compared. The ExG and GLI indices were generally the most suitable for this purpose, with the best F-scores of 97.7 and 95.4, respectively, and the best-balanced accuracies for the same combination of the method/vegetation index of 98.9 and 98.3%, respectively. Surprisingly, these best results were achieved using the simplest method of threshold determination, considering only a single class (vegetation) with a normal distribution. This algorithm outperformed all other methods, including those based on a support vector machine and a deep neural network. Thanks to its simplicity and ease of use (only several patches representing vegetation must be manually selected as a training set), this method can be recommended for vegetation removal from rocky and anthropogenic surfaces.

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This paper presents the results of over a year-long experiment dealing with a temperature measurement to calculate the theoretical effect of the atmosphere on the measured zenith angle in engineering surveying. The measurements were performed to determine the accurate and specific temperatures (temperature gradients), which can be recorded in different seasons in the low level of the atmosphere (up to 2 m above the ground, where most Engineering Surveying measurements take place) for the geographical area of Central Europe - specifically the Czech Republic. A numerical model was then applied to the resulting determined temperature gradients to calculate the path of the beam passing through an inhomogeneous atmosphere. From these values, the apparent vertical shifts caused by refraction in a given environment and time were finally determined.