A success case of oblique photography

——Use 3D model to do cadastral survey for high-rise areas

1. Overview

After several years of development, now in China,oblique photography has been widely used in rural cadastral survey projects. However,due to the restriction of equipment technical conditions , oblique photography is still weak for cadastral measurement of large-drop scenes, mainly because the focal length and picture format of the oblique camera lens are not up to standard. After many years of project experience, we found that the map accuracy should be within 5 cm, then the GSD must be within 2 cm, and the 3D model must be very good,the edges of the building must be straight and clear.
Generally, the camera focal length used for rural cadastral measurement projects is 25mm in vertical and 35mm oblique. In order to achieve the accuracy of 1:500, the GSD must be within 2 cm. And to ensure that，the flight altitude of drones is generally between 70m-100m. According to this flight altitude, there is no way to complete the data collection of the 100m-above-high buildings.Even if you carry out a flight anyway , it can not guarantee the overlap of the roofs, resulting in poor quality of the model.And because the fight height is too low, it is extremely dangerous for UAV.

In order to solve this problem , in May 2019, we carried out the accuracy verification test of Oblique Photography for urban high-rise buildings. The purpose of this test is to verify whether the final mapping accuracy of the 3D model built by RIY-DG4pros oblique camera can meet the requirement of 5 cm RMSE.

5. Conclusion

In this test, the difficulty is that the high and low drop of the scene, the high density of the house and the complex floor. These factors will lead to an increase in the difficulty of flight , a higher risk, and a worse 3D model , which will lead to the decrease of the accuracy in cadastral survey.

Because the RIY-DG4pros focal length is longer than common oblique cameras, it ensures that our UAV can fly at a safe enough altitude,and that the image resolution of the ground objects is within 2 cm. At the same time, the full-frame lens can help us to capture more angles of the houses when flying in high-density building areas, thus improving the quality of the 3D model. Under the premise that all hardware devices are guaranteed, we also improve the overlap of flight and the distribution density of control points to ensure the accuracy of the 3D model.

oblique photography for the high-rise areas of cadastral survey, once because of the limitations of equipment and lack of experience, can only be measured through traditional methods. But the influence of high-rise buildings on RTK signal also causes the difficulty and poor accuracy of measurement. If we can use UAV to collect data, the influence of satellite signals can be completely eliminated, and the overall accuracy of measurement can be greatly improved. So the success of this test is of great significance to us.

This test proves that RIY-DG4pros can indeed control the RMS to a small range of value, has good 3D modeling accuracy, and can be used in accurate measurement projects of high buildings.