Accuracy and Precision of Producing Topographic Maps in Linear Projects by UAV Photogrammetric Method

The production of topographic maps by drone photogrammetry has replaced traditional mapping in many civil projects. UAV photogrammetry due to the rapid development of hardware and software can be used in many geomatic applications including agriculture, forestry, archeology and architecture, emergency management, and traffic monitoring. Nowadays, mapping the boundaries of linear projects such as roads, water transmission canals, power transmission lines (electricity and gas), railway lines, and the like, is done by UAV photogrammetry. UAV photogrammetry has emerged as a viable alternative due to its lower cost, greater spatial and temporal resolution, and flexibility in capturing images compared to other conventional aerial and space methods. Despite various researches in this field, the effect of the imaging method on the accuracy of topographic maps has not been directly studied and tested. In this paper, the experimental conditions of UAV photogrammetric imaging methods in two modes of height-fixed and scale-fixed were considered. The effect of using accurate positioning equipment on image centers on reducing the number of required control points has also been investigated.

Assessing and ensuring accuracy is very important for topographic maps. Knowledge of the number and distribution of control points throughout the project area and the optimal imaging method are effective in producing these maps. In this paper, to achieve the most optimal imaging mode and the highest accuracy in the production of topographic maps for this type of project, the effects of flight design parameters, number, and distribution of control points were studied. Also, the effect of using accurate positioning equipment of image centers on reducing the number of required control points has been investigated. Image processing without accurate information of image centers with control points, image processing with accurate information of image centers and without using control points, and image processing with accurate information of image centers and control points, were considered in two flight modes with fixed altitude and fixed scale. Of the 25 points whose exact coordinates were measured differentially by the Global Positioning System, 18 were ground control points and 7 checked points. The control of the elevation coordinates of the points was performed using the direct geometric leveling method. To evaluate the accuracy of the results, the amount of error and the accuracy of the work performed, the leveling operation was performed in a round trip between all control and checkpoints.

Evaluating the accuracy of the UAV photogrammetric method in producing topographic maps related to this project, according to the criterion, the root means square of errors has been done for the control and checkpoints. In addition to calculating this criterion for control and checkpoints, the difference between the digital models prepared and the reference model was considered another criterion for comparison and evaluation. According to the results, the mean height error of all points in the constant-scale mode has the lowest value in Scenario 3. The numerical value of the mean error, in this case, was equal to 0.010 meters for control points and 0.020 meters for checkpoints. The accuracy of the models obtained from point clouds with dimensions of 0.5 meters is higher than the point clouds of 2 and 4 meters. The largest difference from the reference model is related to model 1 in the category of models obtained from 0.5-meter point clouds, which vary numerically in the range (-1.68 to 10.56) meters.

The results of the height error evaluation of control and checkpoints show if aerial triangulation and justification of images use image center observations alone, in challenging projects the mapping of linear areas where the mapping is done in a strip area will not have the desired accuracy. The use of ground reference images alone is not sufficient and the simultaneous use of ground control points and ground reference images improves the accuracy of the results. As a result, the use of ground control points, fixed scale images, and image center information in the image processing process to produce corridor maps provides the best elevation accuracy compared to other modes. Also, the use of the initial digital elevation model of the project area in performing flight operations and capturing images with fixed scales has a significant effect on increasing the accuracy of the elevation component. Based on the comparison of the final digital elevation models compared to the reference model, the accuracy of the models obtained from the resolution of 0.5 meters is higher than 2 and 4 meters. Also, the effectiveness of filters to correct and reduce errors in digital elevation models has better results.