At present, drone aerial survey has become one of the most important means to obtain a suitable scale image.
It is reported that the UAV low-altitude photogrammetry technology system includes three parts: aerial photography system, ground control system and data processing system.
    Among them, the aerial photography system mainly includes flight platform, digital camera and autopilot for completing aerial survey photography; the ground control system is mainly composed of ground transportation, drone ground control and data receiving and exchange, which is used to complete Man-machine control and data signal receiving work; the data processing system mainly includes route design, impact quality inspection and data post-processing software, which is used to complete the preliminary route formulation and post-processing data processing of photogrammetry work.
    The workflow of UAV low-altitude photogrammetry technology is: measuring regional route planning - controlling drones to take photos according to routes - storing shooting data - data exchange processing with ground control system - mission end drone Landing - Select whether to make a reshoot based on the data - complete the low-altitude photogrammetry of the drone.
   1 preparation stage
First, determine the specific project requirements in the mapping task to select a reasonable model of the UAV low-altitude camera. According to the Yuchen network reporter, at present, in addition to tilt photography and small area surveying and mapping, multi-rotor may be used. General engineering surveying and mapping is based on fixed-wing drones.
Secondly, collect and adjust the overall data of the surveying and mapping project.
    Finally, carry out the route design, site survey and aircraft inspection work of the hydropower engineering surveying and mapping project, so as to prepare for the measurement.
2 field implementation stage
    First, conduct on-site measurement of environmental and equipment inspections. If photographic holes are found, it is necessary to further redesign the route according to the terrain of the site, and restart the aerial aerial photography work; if there are no loopholes, technical processing can be performed according to the original aerial route.
    Secondly, according to the distribution of image control image points, the measurement and recording of the image control points are completed. In this process, the image control points are corrected and corrected according to the image data corrected by the distortion difference, and the complete and accurate field is completed. Measurement records, laying the foundation for internal data processing.
    According to Mr. Gao, an industry professional who specializes in aerial surveying, the effect of the field operations is most affected by topographical factors. The most feared is the mountainous terrain with complex terrain. “Mountain work is inefficient, because the terrain is complex, the transition is very troublesome, and the road is often detoured; sometimes the mountain is very high, there is no signal to fly the plane, increasing the risk of operation, and the plane may be lost if it is out of control.â€
    In addition, places with complex terrain like a good control point are also a problem. If some ground planes do not fly, it is difficult to produce a picture with higher precision.
Aerial survey drone/graph source network serving rural land contracting rights project
    "The drone takes off mainly by ejecting parachute, or ejecting and sprinting. It is relatively easy on the plains and the transition is convenient. In the mountains, it is difficult to climb because of the mountain drone climbing. It is like being in a well. If the mountain is particularly high, even if it flies up, it is difficult to climb. Many times, it has to climb to the top of the mountain. But in many cases, the top of the mountain is not going up. This place is difficult to fly. This is for the outside industry and the inside industry. It is a big challenge. When the internal processing is done, because the height difference is large, the accuracy of each place is different. It is difficult to come up with a high-precision picture,†said the industry insider.
There are now planes that take off and land vertically, but they can only solve the problem of take-off and landing. In the mountains, the drones are directly faced with climbing space and signal transmission problems. However, the advantage of the vertical take-off and landing drone does not need to find a landing site. In the mountainous area, you can try to get to a higher road, and the climbing space and signal will be better.
It is reported that in terms of precision control, there is now a differential RTK and PPK post-difference processing. RTK has a wide range of applications in the topographic mapping industry. Compared with traditional measuring instruments, RTK is not only easy to operate, but also has high measurement accuracy.
    For example, when mapping urban built-up areas and planning areas, it is necessary to set control points for these areas. In order to ensure these control points, RTK technology must be applied. Because if the control points can be provided quickly and accurately, the construction efficiency will be greatly improved. After applying RTK technology, regular measurements can be made very well.
    However, due to the lack of RTK in the mountain area, the post-differential PPK is mainly used to reduce the image control points, which can reduce the workload and improve the accuracy.
    According to Yuchen.com, the field work of aerial survey is very hard and the risk is relatively high. It is difficult to deal with the situation that the aerial survey data is incomplete. Generally, it will be tested in the field on the day of flight. If the data is not complete, it can only be re-flyed.
    3 internal page data processing stage
    This stage is mainly divided into two parts: data acquisition and data editing. The main tasks of this part are: on the one hand, generating and processing DEM (Digital Elevation Model) and DOM (Digital Orthographic Data Set), using technical means and data processing means to realize the generation of basic data model; on the other hand, DEM and The results of DOM data generation are tested for quality of results, ensuring the authenticity of data output, outputting DEM and DOM results, and then completing DLG (Digital Planning Map) production and outputting DLG results.
    Industry insiders told reporters that compared with the "soil-filled" field work, the work in the industry can be described as "fine and cumbersome", but they are equally important.
    The data collection work is mainly to look at the stereoscopic image of the stereoscopic display device and draw the map of the first hand. In the data acquisition process, the accuracy is very high, that is, all visible features should be drawn, contours of all areas should be drawn (including residential areas), and all elevation points of the mapped areas should be collected.
Point cloud generated DSM results / map source network
    The work of data editing is to trim the first-hand map drawn by the collector. That is, splicing the map, trimming the contour lines, deleting the elevation annotation points, adding the frame, adding text annotations, and the like. By trimming the map, the map is met with the requirements of the party or the specification regarding the mapping.
    It is often the case that the map cannot be produced at one time. Because the collection of visible stereo pairs may have missing, fuzzy, and uncertain areas, the editors will mark these areas, and then hand over the semi-finished products to the fielding personnel to make up the test. Then add the map of the test.
    According to industry insiders, the aerial survey industry has a strong work intensity. At present, it is still labor-intensive, with low salaries and the problem of lack of high-quality comprehensive talents.
    However, for now, the existing software can realize the function of automatically generating the contour line, and also provides the human-computer interaction of the curve editing, and can generate contour lines for the batch processing of the point cloud block according to the frame. With the continuous advancement of technology, it is expected to improve the accuracy and efficiency of processing aerial survey data.
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