Three-dimensional modeling and application of tilt photography of UAV (a sharing meeting record)

Disclaimer: The content comes from the sharing and communication of an old leader. After listening, tidy it up a little to show respect. There are inevitably some shortcomings in the sorting process. What is stated in this article does not represent any opinions.

The advantage of oblique photography technology or the most attractive to users is that it can fully automatically, efficiently, accurately and accurately establish the three-dimensional model of all elements on the surface.

The topic "3D modeling and application of UAV tilt photography" has four meanings: one is UAV, the other is tilt photography, the third is 3D modeling, and the fourth is 3D model application.

The above four aspects are the most important contents in the oblique photography technology system. In order to better promote the progress and application of oblique photography technology, it is necessary to study each aspect.

The quality of oblique photography three-dimensional model mainly depends on two factors: one is the image quality (ground resolution and image clarity), and the other is the number of photos (photo coverage in the same area).

From the actual modeling effect, in order to obtain a complete and clear three-dimensional model for high-precision measurement, the resolution of the inclined image of the building area should reach 2 ~ 3 cm, the general area should reach 5 ~ 6 cm, and the average coverage of the photos should reach more than 30 degrees. (Note: This may only be a summary of some feelings in the project, and the exact degree depends on the project requirements. )

Therefore, multi-rotor UAV is the first choice for oblique photography in building areas, and small electric vertical take-off and landing fixed-wing UAV can be selected for oblique photography in general areas.

"I have investigated and participated in the development and use of various multi-rotor drones. The quadrotor UAV can not meet the application requirements because of its poor load index and poor reliability. Six-rotor UAV often fails due to non-human factors in high-frequency flight operations, resulting in crash or damage, and its reliability is not enough; Eight-rotor UAV has certain power redundancy and flight reliability, which can improve the safety and continuity of operation and is recommended.

The take-off weight of the eight-rotor UAV should be less than 7 kg, and the working life should be 20 minutes. Using a long-range dual-lens tilt camera, about 900 photos with an effective area of 0.3 square kilometers and a resolution of 2 cm can be obtained per flight.

(Note: The choice and use of aircraft is not absolute, but more depends on choosing the right tools according to the specific geographical environment. )

The basic requirements of tilt photography flight of fixed-wing UAV are low-altitude flight, low-speed cruise, small turning radius, convenient operation and close take-off and landing.

Using oil-powered fixed-wing UAV for routine aerial photography, although the flight efficiency and performance are good, the use and maintenance requirements are high and the price remains high.

In recent two years, the electric/hybrid fixed-wing UAV launched on the market has greatly improved in terms of ease of use, stand-alone price and technical performance. , make tilt photography technology play a role in large-area three-dimensional modeling, and effectively improve the working efficiency of tilt photography flight.

Considering the comprehensive factors such as convenient operation, simple maintenance, convenient transportation and single machine price, it is recommended to use electric vertical take-off and landing fixed-wing UAV for tilt photography flight in general areas.

The electric vertical take-off and landing fixed-wing UAV has a payload of 1 ~ 2 kg, a battery life of 60 ~ 90 minutes, a relative flying height of about 300 meters, and an image ground resolution of 5 cm. (depending on the specific model)

The fixed five-lens tilt camera is one of the commonly used devices for tilt photography of unmanned aerial vehicles, which continues the traditional five-lens structure used by manned aircraft.

However, there is little research on the principle and technology of why five cameras must be used for oblique photography at the same time. We can only say, "Everyone else does this, there must be his reason, so I'll do it."

In order to explore the requirements of photo orientation and quantity in 3D modeling of oblique photography, we used 65, 438+0 cameras and 2 cameras respectively, and used the method of multiple flights and cross flights to the same area to simulate the way of five-lens cameras, respectively, to obtain down-looking, forward-looking, back-looking, left-looking and right-looking images, and carried out 3D modeling experiments with different combinations.

The main conclusions of the oblique photography experiment of simulating the five-camera structure with different numbers of cameras are as follows:

1) The modeling effect has nothing to do with the number of cameras, but is related to the number of photos and the flight interval of adjacent routes;

2) A down-looking camera is not necessary, because the real projection image is generated by the orthogonal projection of the three-dimensional model. The function of the down-looking camera is similar to that of other azimuth cameras;

3) The camera should be tilted at 20 ~ 30 degrees. The edge separation rate of camera photos with a 45-degree inclination angle is too low;

4) The tilt camera system with dual cameras and three-phase swing structure has the best comprehensive cost performance.

The dual-lens tilt camera system realizes the effect of a fixed five-lens camera with only two cameras. The system is simple in structure, low in cost, light in weight and convenient to maintain and use, and it is the first choice for multi-rotor UAV tilt photography.

Aerospace Vision's ultra-light two-lens two-dimensional tilt camera system consists of two miniature cameras and 1 two-dimensional tilt head, with a total weight of 1.2kg. ..

The size of camera photosensitive sensor should not be less than APS-C, and the number of pixels should be more than 24 million.

For example, if a multi-rotor UAV and a dual-lens tilting camera system are used for tilting photography with a resolution of 2 cm in a building area, the basic requirements for route design are:

1) The aerial photography division is rectangular as far as possible, and the route is laid along the long side of the rectangular area. The actual flight range should exceed the mission range 1 height, and the terrain elevation difference within the division should be less than 1/2 height;

2) The number of routes is even and not less than 6, and the maximum length of a single route is calculated as 40% of the effective cruising range of the multi-rotor UAV;

3) The relative altitude is designed as100m. When there are buildings over 30 meters in the aerial photography area, the minimum relative height should be calculated as 100 meters plus the building height;

4) The heading overlap is more than 75%, and the lateral overlap is more than 40%. (Note: Depending on camera parameters and specific environment)

"If you use dual cameras and fixed-wing drones for oblique photography with 5 cm resolution in ordinary areas, the basic requirements for route design are:

1) The aerial photography division is rectangular as far as possible, and the flight routes are arranged in a grid along the long and short sides of the rectangular area (cross flight). The actual flight range should exceed the mission range 1 height, and the terrain elevation difference in the sub-area should be less than 1/2 height;

2) The number of air routes is even and not less than 6. The maximum length of a single air route is designed to be 40% of the effective cruising range of the UAV, and the maximum length is not more than 5,500m m.

3) The relative height is designed to be 300m, and the minimum relative height should be more than 100m of other structures in the shooting area;

4) The heading overlap is more than 75%, and the lateral overlap is more than 40%. (Note: Depending on camera parameters and specific environment)

All aerial photographers should carry out route design in a unified way, and determine the flight parameter files of each flight by deleting redundant routes in the same route design file.

Outdoor work should be equipped with at least 10 battery, or equipped with portable generator for on-site charging to improve work efficiency.

The take-off and landing site of UAV is as close to the shooting area as possible to reduce the invalid flight distance.

"Operation Team 123 Configuration: 1 SUV, 2 multi-rotor UAVs, and 3 members (ground crew, flying hand, assistant).

The modeling accuracy of oblique photography three-dimensional model is directly related to the image resolution, which is generally about 1:3.

The plane measurement accuracy and relative elevation measurement accuracy of oblique photography three-dimensional model are basically the same.

If the resolution of the image is 2 cm, the modeling accuracy of the 3D model is generally 5 ~ 10 cm, and the corresponding measurement accuracy is also within 10 cm, which is equivalent to the accuracy of field measured points and much higher than the accuracy of 1:500 topographic map.

If the image resolution is 5cm, the modeling accuracy of 3D model is generally 15 ~ 20cm, and the corresponding measurement accuracy is less than 20cm, which is higher than that of 1: 1000 topographic map.

The emergence of unmanned aerial vehicles and various small tilt photography systems makes it very convenient to obtain tilt image data. The bottleneck of data acquisition has been solved, and the cost will be gradually reduced. .

With the development of computer cluster, GPU and 3D modeling software technology for oblique images, the efficiency of 3D modeling will be greatly improved.

As far as the current situation is concerned, the UAV, tilt photography system, computer cluster and 3D modeling software involved in 3D modeling of tilt photography can meet the requirements of batch tilt image acquisition and 3D modeling processing, and basically meet the conditions of engineering and scale.

Tilt photography technology and its UAV operation method are taking shape day by day, which makes all-factor 3D reconstruction in the real world efficient and feasible.

At present, the main use of oblique photography in the field of surveying and mapping is to quickly establish a fine three-dimensional model of the surface, which can replace the traditional manual modeling and the modeling method of "oblique image+laser scanning".

It is not appropriate to simply compare the tilt photography technology with the traditional photogrammetry technology, especially not to argue about the advantages and disadvantages of this technology, because they are studied from different directions, different ways and different purposes, but only overlap in the achievement point of reconstructing the three-dimensional model of the target.

Tilt photography technology and photogrammetry technology are "two cars running on two roads, not taking the same road"

Tilt photography and photogrammetry can be regarded as two teams running on two roads, and now they meet at an overpass, but in fact, the destinations are different and the vehicles used are different.

Fine 3D models and measurement results can be obtained without manual observation, which is the "most fatal" advantage of tilt photography technology. This is also the goal that photogrammetry has been pursuing for decades.

The three-dimensional model constructed by oblique photography can replace manual observation in aerial photogrammetry, and realize automatic modeling and intelligent mapping with higher precision and faster speed.

With these changes, the existing technologies, products, markets, users, applications and business models will inevitably be impacted and changed.

Don't use traditional photogrammetry methods and standards to measure tilt photography technology and achievements. The only feasible method should be to compare and test the results.

The development and popularization of tilt photography technology will not only extend the spatial information from two-dimensional to three-dimensional, but more importantly, it will bring about changes in the field of surveying and mapping, and will also give birth to changes in industrial applications.

The three-dimensional model and three-dimensional spatial information service platform constructed by oblique photography will become the basic spatial information support for industrial applications.

Three-dimensional modeling of tilt photography will be a widely used three-dimensional modeling and measurement method in the future, and it is a subversive and innovative model in the field of surveying and mapping.

The rapid development of UAV tilt photography technology makes it possible to "survey the whole population, on-demand survey and dynamic survey".

It is inappropriate to overemphasize and pursue the reliability of UAV platform. It is important to consider the cost performance of UAV and tilt camera system as data acquisition tools, which should be simple to use, convenient to maintain, easy to carry and cheap.

In fact, as long as it is a drone, it will fall sooner or later. If the price of "fried chicken" is a Mercedes-Benz, neither the unit nor the flying hand is willing to bear such a loss.

We hope that the market price of "UAV+tilt camera system" which meets the requirements of large-scale operation will be within 65,438+10,000 yuan. It is hoped that UAV manufacturers and tilt camera system manufacturers can make breakthroughs in structure, technology, spare parts and cost.

Traditional photogrammetry and tilt photography are two-dimensional and three-dimensional game processes, and finally three-dimensional will win. At present, tilt photography is still in the promotion stage, which is mainly limited by the current application points and industry norms.

Tilt photography technology and achievements are not perfect, and it is still being tested and evolved. Traditional workflow and product delivery standards can not be changed in a year or two, which requires a process.

For users, using tilt photography technology, don't expect him to save money, it may require extra investment. What you do is to keep up with the development of the times and spend some learning costs, and then you will have a foundation; It doesn't matter if you don't do it There are always people who want to fall behind, and there are always enterprises who want to fall behind.

Data, platform and service are the three main contents of geographic information industry in the future, and its trend is the intensification of data production and the integration of application services.

The data content will expand from two-dimensional plane to three-dimensional space of "above ground, indoor-outdoor" integration, increasing the time dimension.

Whoever has mastered the three-dimensional total factor data will occupy the commanding heights of the three-dimensional industry.

It has refined, measurable, realistic, high-precision, objectified, rapidly updated and customized three-dimensional geographic information, which will become the basic support for industrial applications and public services.

In the era of big data and smart cities, holographic 3D will be a powerful weapon for geographic information industry to serve smart cities and big data era.

As far as the application of tilt photography technology is concerned, everyone is a novice, and the ignorant are fearless. The difference between high and low is mainly in thinking, practice and experience.

In tilt photography, experts have more background knowledge and more practical experience than we do, and making achievements is king.

Tilt photography is just a photography method. It is important that computer vision technology uses enough oblique images with enough overlap to realize three-dimensional reconstruction, thus forming oblique photography technology or oblique photogrammetry technology.

For users, the focus should be on the acquired three-dimensional model, and they don't need to care too much about the acquisition equipment and technical details behind it.

There is no engineering cluster processing software for 3D modeling of oblique images in China.

There is a lack of domestic software such as OSGB (which is not available abroad) to directly mass-produce and edit 3D data.

Lack of software tools for intelligent batch objectification of three-dimensional data.

There are many technical difficulties in the storage, management, publishing and application of 3D models of massive tilt images.

Most users' application systems cannot accept and use 3D models generated by oblique photography.

Therefore, the three-dimensional model of oblique photography has also been labeled as "beautiful but useless".