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UAS Photogrammetry and GNSS Used in Hydroelectric Survey Project

Tersus GNSS

The Zhuxi River Reservoir was built in 1968. And from 2002 to 2004, the government invested 14.2 million CNY state bonds in reinforcing the dam to prevent the flood and spillway. The reservoir has a rain-bearing area of 120 square kilometres and an average annual runoff of 65 million cubic meters. Since July 2020, various parts of the south have been hit by torrential rains, with flooding rare for many years. The Yangtze River, Huaihe River, and other river basins have suffered severe flooding, which has caused widespread damage and large economic losses, making it one of the most severe years in history.

The project mainly involves the landscape and riverbed of the Xiahou River section, Maoguping section, and Taihegou section of Zhuxi County, with a total length of about 37 kilometres and a coverage area of about 80 square kilometres. The 3D Modeling and topographic surveys are used to assess the area of damage and budget for reinforcement of damaged structures on both sides of the river and post-reinforcement work.

UAS Photogrammetry and GNSS Used in Hydroelectric Survey Project River Points

Summary of the Surveying Area

The terrain in this survey area is long and narrow, with undulating terrain and a maximum elevation difference of about 300 m. There are also high mountain cliffs on both sides of the river. The main features are riverbeds, villages, dense forests, mountains, and so on. The flight time is mainly from 1:00 to 3:00 p.m. During this time, the weather in the survey area is sunny, with moderate light and grade 2 to 3 breeze, suitable for flying. In the survey area, RTK was used to measure 75 image control points along the river's two banks, and the points were clear and easy to identify. Above is the distribution map.

Equipment List 

1. Tersus Oscar Receiver,
2. Drone,
3. Metashape and Context Capture 3D modelling software, 
4. High-performance Desktop computer,
5. South Cass3D software.

Details

1. Tersus Oscar GNSS Receiver

The project adopts the Oscar GNSS receiver provided by Tersus GNSS Inc. According to the project requirement, riverbank elevation, depth of creek, and control points need to be collected. It is also very important to check the accuracy of 3D models via GNSS receivers. As a new generation of GNSS receiver, its stable satellite searching capability, fast fix mode, and IMU can significantly improve surveyor fieldwork speed.

UAS Photogrammetry and GNSS Used in Hydroelectric Survey Project Tersus Tilt Survey

2. Unmanned Aerial Vehicle

As the terrain is complicated and obstacle wires are crossed, the vertical take-off fixed-wing UAV can carry high-resolution cameras, although the efficiency is high. As the terrain is complicated and obstacle wires are crossed, the vertical take-off fixed-wing UAV can carry high-resolution cameras and the efficiency is high. For this terrain, the risk factor is high. A five-way obstacle avoidance function and supports terrain follow flight mode are perfectly qualified for this project.

3.Metashape & Context Capture

For 3D modelling software, Metashape and Context Capture ("CC" for short) are used in this project. The project has a long and narrow terrain, and there is a bending phenomenon at both ends of the survey area during aerial triangulation. Therefore, Metashape calculates the aerial triangulation and interpolates more points by generating dense point clouds. The accurate dense points are critical for 3D Modeling and 1:1000 topography.

Flight Plan

In this project, we designed an altitude of 200 meters. The ground resolution (GSD) is about 2cm. It operates according to the heading/sideways overlap of 80%/70%, using the terrain follow mode to overcome large differences in mountain altitudes. The measurement area's effective area is about 20km², and the actual flying area is about 28km², according to the distance of doubling the outer height. With 80 sorties in total, the flight time of a single flight is about 25min. More than 8000 images are obtained, with a clear outline and uniform colour, which meets the project's requirements.

Data processing and modelling result

1. Aerial triangulation result

UAS Photogrammetry and GNSS Used in Hydroelectric Survey Project Aerial triangulation result

2. 3D Modeling

The model established in this project has a good effect, no pulling or hole phenomenon, and more distinct edges and corners, which is suitable for the post DLG production.

UAS Photogrammetry and GNSS Used in Hydroelectric Survey Project 3D Modeling

3. Topographic drawing

Based on the real three-dimensional model completed in the project, we use CASS3D software to draw the DLG based on the model and accuracy verification. The verification results have been made into the accuracy table; the results meet our project's requirements.

UAS Photogrammetry and GNSS Used in Hydroelectric Survey Project Topographic drawing

4. Accuracy Examination 

We use the 3D model to collect checkpoints at the same location points on CASS3D and comparing with the checkpoints collected by Oscar receivers. The 3D model meets the 1:1000 topographic map standard (GBT17160-2008).

Summary of the Project

This project uses UAV aerial photography measurement as a technical means to produce effective 3D model data for river construction and disaster assessment through processes such as UAV aerial field flight, image-controlled measurement, 3D model production, and DLG productions.

Overall, the combination of Tersus Oscar IMU RTK and the UAV was a good fit for the project. Compared to traditional manual mapping, the UAV photography technology and IMU RTK are a quantum leap in efficiency.

For areas like this project, where the survey area is not ideal, and the terrain and aerial environment are complicated, UAV photography is an excellent technical means. The progress of surveying and mapping technology has reduced surveyors' work intensity, labour and make a great significance in ensuring project safety and improving project efficiency.

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Daniƫl Haanskorf Peter Tapken Sjors Kloosterman

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