## 1 Introduction  
The longitudinal slope (angle of incline along the length) of a forest/skid road and its impact on skidder (heavy-duty vehicle used for hauling logs from the forest to the processing site) operation and fuel consumption are crucial issues that need to be considered to ensure safe and efficient logging operations. A steep slope can create several challenges for skidder operations, including decreased stability and traction, increased fuel consumption, and the potential for skidder rollovers. In addition, a steep slope can also increase the risk of soil erosion and damage to the forest floor, which can negatively impact the overall health of the forest ecosystem [1]. On the other hand, a relatively gentle slope can improve skidder operation by allowing for better traction and stability, reducing fuel consumption, and reducing the risk of forest soil erosion and damage. The diesel engine is still the main source of energy in all types of forest machinery, and hydrostatic-mechanical power transmission is mainly used [2]. The demand for forest machines that cost less, have the same efficiency and can reduce the emission of harmful gases is growing, and the interest of manufacturers in the development of electric and hybrid drives has also increased [3, 4]. By conducting this study and project, one of the consumption segments was the dependence of the fuel consumption of a skidder on the slope of the road, and precise measurements of the longitudinal slopes of forest/skid roads had to be carried out with the use of modern technologies such as GNSS (Global Navigation Satellite System) devices, drones, and total stations. The current measurement of field data of terrain configurations on forest land is difficult to carry out, primarily due to the lack of high-quality internet signal and GPS signal reception in conditions of uneven terrain configurations and canopy cover [5; 6; 7; 8]. Surveying using geodetic devices is the most accurate method for carrying out direct surveys in the field, but results in significant expenditure of time and high dependence on the human factor during the survey, which can also result in inaccurate longitudinal profile data [9]. The most common errors occur in the subjective determination of longitudinal terrain fractures during terrain surveying. Contemporary technologies provide the ability to carry out measurements from the air by defining precise reference points with precise GNSS devices to create a 3D terrain model. The use of unmanned aerial vehicles (UAVs) enables rapid and accurate data in forest research and surveys [10; 11; 12; 13], forest wind damages [14; 15], forest postharvest soil disturbances [16], and fire risk management [17]. Modern technology, such as airborne laser scanning and terrestrial mobile laser scanning, has improved our ability to assess natural resources and the accuracy of field parameters compared with some classical surveys [18], but it is still extremely expensive. Thus, in the field of forestry, one often resorts to aerial measurements and photo processing based on the photogrammetric method. This also allows for a faster data acquisition process and automated processing afterward. Therefore, these new technologies can be used in a wide array of forest-related studies [19]. Unfavorable weather conditions in the forest, such as wind, limit the quality of photographs [20] and can affect the accuracy of data obtained from the air, which are processed by the photogrammetric method. Therefore, for more precise positioning, we need to define precise ground control points (GCPs) using GNSS devices. In addition, tree density [21] and other factors, such as terrain configuration [22], acquisition time (i.e., ensuring enough position fixes per point), and satellite geometry, can also affect the accuracy of GNSS instruments [23].