——Use 3D model to do cadastral survey for high-rise areas
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.
In this test, we choose the DJI M600PRO, equipped with the Rainpoo RIY-DG4pros oblique five-lens camera.
In response to the above problems, and to increase the difficulty, we specially selected two cells with an average building height of 100 meters for testing.
Control points are preset according to GOOGLE map, and the surrounding environment should be as open and unobstructed as possible. The distance between the points is in the range of 150-200M.
The control point is 80*80 square, divided into red and yellow according to the diagonal, so as to ensure that the point center can be clearly identified when the reflection is too strong or the illumination is insufficient, to improve the accuracy.
In order to ensure the safety of operation, we reserved a safe altitude of 60 meters, and UAV flew at 160 meters. In order to ensure the overlap of roof, we also increased the overlap rate. The longitudinal overlapping rate is 85% and the transversal overlapping rate is 80%, and UAV flew at 9.8m/s speed.
Use “Sky-Scanner” (Developed by Rainpoo) software to download and pre-process the original photos, then import them into ContextCapture 3D modeling software by one key.
From the distortion grid diagram, it can be seen that the lens distortion of RIY-DG4pros is extremely small, and the circumference is almost completely coincident with the standard square;
Thanks to the optical technology of Rainpoo, we can control the RMS value within 0.55, which is an important parameter to the accuracy of the 3D model.
It can be seen that the distance between the main point of the center vertical lens and the main point of the oblique lenses are: 1.63cm, 4.02cm, 4.68cm, 7.99cm, minus the actual position difference, the error values are: -4.37cm, -1.98cm , -1.32cm, 1.99cm, the maximum difference of position is 4.37cm, camera synchronization can be controlled within 5ms;
The RMS of predicted and actual control points ranges from 0.12 to 0.47 pixels.
We can see that because the RIY-DG4pros uses a long focal length lenses, the house at the bottom of the 3d model is very clear to see. The minimum exposure time interval of the camera can reach 0.6s, so even if the longitudinal overlapping rate is increased to 85%, there’s no photo-leakage occurs.
The footlines of high-rise buildings are very clear and basically straight, which also ensures that we can get more accurate footprints on the model later.
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.