KR101884920B1 - Method for underground facilities positional information by uav - Google Patents

Abstract

The present invention relates to a method for collecting position information of underground facilities using a drone, and more specifically, to a method for collecting position information of underground facilities using a drone, capable of effectively collecting location information of underground facilities by collecting location information of underground facilities at the same time as construction work of underground facilities by performing aeronautical photographing work using a drone. The method includes the steps of: a first step of preliminarily positioning a plurality of points, which are not changed or lost in a photographing section of a drone, as ground reference points and measuring the position coordinates of the points; a second step of setting up aeronautical photographing information about the photographing section in a control module of the drone; a third step of photographing the photographing section and generating a photographed image by a digital camera of the drone as the drone is flying over an installation area of the underground facility, which is the photographing section, according to the aeronautical photographing information; a fourth step of converting each of the position coordinates of the points formed in the photographed image into absolute coordinates by using image processing equipment through photographic reference point measurement; a fifth step of creating a video image by imaging and joining the photographed image using the image processing equipment, and marking the ground reference points on the video image; and a sixth step of calculating one or more first measurement information selected from a DEM (Digital Elevation Model), a DSM (Digital Surface Model) and an ortho-image for the image by calculating photo reference points based on the ground reference point marked on the image.

Description

[0001] METHOD FOR UNDERGROUND FACILITIES POSITIONAL INFORMATION BY UAV [0002]

The present invention relates to a method of collecting location information of an underground facility, more specifically, a method of collecting location information of an underground facility through construction of an underground facility through aerial photographing using a drone, And more particularly, to a method for collecting location information of an underground facility using a drone.

As the urban area of limited area continues to be developed and expanded not only in the ground but also in the underground, there is a trend of increasing the number of underground facilities such as water and sewage pipes, power and communication lines, city gas pipes, pipelines, have.

Therefore, it is necessary to accurately collect information on the installation location and the installation status of the rapidly increasing underground facilities, and to continuously maintain maintenance based on the information.

However, conventionally, information about the location and depth of underground facilities is not well-equipped, and underground facilities are embedded in the underground, so that it is difficult to visually confirm the location of the underground facilities. Therefore, maintenance of existing underground facilities, There were also many difficulties.

In order to solve this problem, there have been proposed electric detection method, electromagnetic survey method, ground penetrating radar survey method, and position detection method using magnetic markers to detect burial position, depth and direction of underground facilities already installed. In addition, when an underground facility is newly installed, two or more workers directly detect the depth and location of the underground facility based on the GPS locating equipment such as a marking table and other total stations, collect information, And how to mark the facility.

However, when an underground facility is newly installed, two or more workers wait for a relatively long period of time for burial process of underground facilities and it is necessary to measure the position of each part of the underground facility, and during the measurement, In addition, when a plurality of underground facilities are simultaneously installed in various directions, the worker has to visit a number of underground facilities and measure their position, so that there is an unreasonable necessity of a small amount of confirmation time and effort.

Prior Art Document 1. Patent Registration No. 10-1650525 (published on Aug. 26, 2014)

Accordingly, the present invention was invented to solve the above-mentioned problems, and it is an object of the present invention to provide a system and a method for constructing an underground facility by collecting location information of a new underground facility, And to provide a method for collecting location information of underground facilities using the drone.

According to an aspect of the present invention,

A first step of preliminarily positioning a plurality of points whose positions change or are not lost in the shooting interval of the drones to ground reference points and measuring the position coordinates of the points;

A second step of setting up the shooting information on the shooting interval in the control module of the drones;

A third step in which the digital camera of the drone photographs the photographing section and generates a photographed image while the drone is flying the installation area of the underground facility as the photographing section in accordance with the flying photographing information;

A fourth step of the image processing apparatus converting each of the position coordinates of the points formed in the captured image into absolute coordinates through photographic reference point measurement;

A fifth step of the image processing apparatus creating a video image by imaging and connecting a photographed image, and marking the ground reference point on the video image; And

A sixth step of calculating a photographed reference point based on the ground reference point marked on the image image to generate at least one first survey information selected from the DEM, the DSM and the orthoimage for the image;

A method for collecting location information of an underground facility using a drone.

According to the present invention, it is possible to construct a space information DB and construct an underground facility map by collecting location information of a new underground facility while constructing an underground facility with only a minimum of an operator and working time.

1 is a flow chart showing an embodiment of a position information collecting method according to the present invention,
2 is a block diagram illustrating an embodiment of a measurement system according to the present invention,
FIG. 3 is a view schematically showing a flight and an image of a dron relating to flight photographing information of a position information collecting method according to the present invention,
4 is a cross-sectional view schematically showing an example of a ground reference point set on the ground in the method for collecting location information according to the present invention,
5 is a plan view schematically showing a position section of the ground reference point shown in FIG. 4,
FIG. 6 is an image showing the orthoimage image according to the position information collection method according to the present invention,
7 is a flow chart showing another embodiment of the position information collection method according to the present invention,
FIG. 8 is an image showing a pipe network diagram generated by editing an orthoimage of a post-shooting section according to a location information collecting method according to the present invention,
FIG. 9 is an image showing a pipe network diagram of an underground facility constructed according to a method for collecting location information according to the present invention,
FIG. 10 is an image showing a view of an underground facility according to a location information collecting method according to the present invention,
11 is a view schematically showing a digital camera used in the position information collecting method according to the present invention,
12 is a partial cross-sectional view schematically showing the operation of the digital camera shown in Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other features and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings, There will be. The present invention is capable of various modifications and various forms, and specific embodiments are illustrated in the drawings and described in detail in the text. It is to be understood, however, that the invention is not intended to be limited to the particular forms disclosed, but on the contrary, is intended to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

2 is a block diagram showing an embodiment of a measuring system according to the present invention, and Fig. 3 is a flowchart illustrating a method of measuring a position according to an embodiment of the present invention. Fig. FIG. 2 is a view schematically showing a flight and a shooting state of a drone in relation to flight shooting information of a dron.

The method for collecting the location information of the underground facilities according to the present embodiment includes a first step of selecting a plurality of points that are not changed or lost in the shooting interval of the drones 100 as ground reference points and measuring the position coordinates of the points S10); A second step S20 of setting up the shooting information on the shooting interval in the control module 190 of the drones 100; A third step of the digital camera 160 of the drone 100 photographing the photographing section and generating a photographed image while the drone 100 is flying the installation area of the underground facility in accordance with the flying photographing information S30); A fourth step of the image processing apparatus 300 converting each of the position coordinates of the points formed in the captured image into absolute coordinates through photographic reference point measurement; A fifth step of the image processing apparatus 300 generating an image image by imaging and joining the photographed image, and marking the ground reference point on the image image; And a sixth step (S60) of mutually calculating the land reference points marked on the image image and generating at least one first survey information selected from the DEM, the DSM and the orthoimage for the image.

The present positional information collection method will be described in more detail.

S05; Steps to plan shooting

In the present embodiment, the operator collects map information about the photographing area, selects the background map, identifies the installation place of the underground facility on the basis of the background map, and determines the range of the photographing section corresponding thereto. Generally, the map information can be downloaded and collected from the map menu of the general portal site. For reference, the range of the photographing section may be an actual distance corresponding to the underground facility construction site, and in this embodiment, it is made up of the width W and the width H of the construction site.

When the range of the shooting interval is determined, the digital camera 160 of the drones 100 determines the shooting height of the drones 100 for shooting the range of the shooting interval. Subsequently, when the range of the photographing interval and the photographing altitude are determined, the lateral redundancy, the hallway and the dron speed are determined according to the specifications of the drones 100 constituting the digital camera 160. Here, the redundancy such as the lateral redundancy and the corridor is a ratio of the overlapping interval of the adjacent shooting interval A and the shooting interval B with respect to the shooting interval A or the shooting interval B, as shown in FIG.

The control module 190 of the drone 100 in the present embodiment calculates a flight route spacing calculation formula using the lateral overlap and the photographing range as variables, the flight route number calculation formula using the photographing range and the flight route interval as variables, A solution of the photographed baseline calculation equation using the range of the photographing section and the corridor as the variables and the shutter interval grinding expression using the photographing section range, the dron speed, and the number of pictures per flight route as variables, Establish a shooting plan that determines the range of the section, the lateral redundancy, the corridor, the dron speed and the shooting height.

When the lateral width W and the lateral redundancy in the photographing section range are inserted as variables, the control module 190 calculates the air line spacing SP as a variable, So that the driving motor 110 and the stator 170 can be controlled. Here, the driving motor 110 is a device for applying a rotational force to the propeller for flying the dron 100, and the steering device 170 adjusts the rotational gear of the driving motor 110 under the control of the control module 190, (100).

If the horizontal width W and the flight route spacing SP of the photographing section range are inserted as variables, the control module 190 calculates the number of flight routes ( NFL) so as to control the driving motor 110 and the steering unit 170.

If the length H of the range of the photographing interval and the corridor of the range are inserted as variables, the control module 190 calculates the distance between the photographing point of the continuous photographing image The photographing base line (B) can be confirmed.

If the length H of the range of the photographing interval, the dron speed, and the number of pictures per flight route (NIM) are inserted as variables, the control module 190 controls the digital The digital camera 160 can be controlled by setting the shutter interval SI of the camera 160. [ If the vertical length H of the photographing section range and the photographing line B are inserted as variables, the control module 190 calculates the number of photographs per flight route NIM, (NIM) per flight route.

Meanwhile, the ground sample distance (GSD) can be confirmed according to the specifications of the drone 100 and the digital camera 160, and the control module 190 inserts the shooting height as a variable in Equation (6) You can see the ground sample distance (GSD).

Since the shooting height of the drone 100 is 30 m, the ground sample distance GSD is 12.96 mm according to Equation (6).

Meanwhile, in the present embodiment, the digital camera 160 of the drone 100 has a focal length f of 3.61 mm, a lens having a width of 4000 pixels, a height of 3000 pixels, and a pixel size of 0.00156 mm / pixel to be.

Also, in this embodiment, the overlapping is 80% and the overlapping is 50% or more, resulting in the flight path spacing SP and the photographic base line B, respectively. Also, the speed of the drone 100 is 5 m / s to derive the shutter interval SI.

The relationship between the speed of the drone and the shutter speed in the shooting plan described above is an experience value through various types of simulations. Generally, when the dron 100 is flying at high speed, , And if the image is taken at a very low speed, an image of excessive redundancy is photographed by the digital camera 160. [ In this case, all of them affect the planned quality and running cost, so proper flight speed is required.

For reference, images with low redundancy can not produce high-quality stereoscopic images, and images with excessive redundancies exceed image processing time excessively. Therefore, optimal overlapping roads It is preferable to take a photograph.

FIG. 4 is a sectional view schematically showing an example of a ground reference point set on the ground in the position measuring method according to the present invention, FIG. 5 is a plan view schematically showing a position interval of the ground reference point shown in FIG. Is an image showing the appearance of the orthoimage image according to the position information collection method according to the present invention.

S10; Ground reference point pre-position and position coordinate measurement stage

Underground facilities are used for real-time first public survey for renewing or establishing pipelines in urbanized areas, real-time second public survey based on the installation of new pipelines for local waterworks and sewerage, and new pipelines buried due to the development of large- And real - time third public survey.

Here, in the case of the first and second public measurements, the installation area where the underground facilities are installed has no concern with the existing roads or additional facilities. Therefore, before the photographing using the drone 100, A plurality of points whose positions change or are not lost in the photographing interval of the camera 100 are selected as the ground reference points P1 and the GPS position coordinates of the points are measured. Accordingly, the position coordinates of the ground reference point P1 are determined before the shooting using the drones 100.

This practice is to apply for a copy of public survey results and survey records to the surveyor (ordering party) by preliminary examination of the status of the public reference points in the installation area, and to receive the survey in advance according to the issued public reference point scorecard. To check whether it is lost.

Also, existing 2 and 4 class ground reference points should be utilized to prevent duplicate installations through preliminary investigation.

In order to improve the efficiency of management and maintenance of underground facilities and data utilization, a 4th grade reference point connected with 2nd level ground reference point installed at the client is installed, and the location is set in the area where there is no risk of loss or damage.

In addition, the ground control points should be identified in the installation area and pre-survey should be conducted so that at least one other ground control point and the traffic network are available considering the burial density and network conditions. In addition, additional ground reference points can be placed in the outskirts of the installation area in order to utilize the aerial triangulation (AT) performance in the shooting range within the working position at the level 4 reference point, so as to obtain good performance. Also, as shown in FIG. 5, the ground reference point is selected so as to be clearly and clearly seen on the photograph.

4 and 5, the ground reference point P1 is the corner P1 of the traffic light control box EP located in the underground facilities installation area, In the image, the traffic light control box (EP) is specified.

On the other hand, the position coordinates of the ground reference point P1 are finally measured by a method of analytical processing, that is, a method derived from a baseline analysis calculation unit, a baseline vector check calculation unit, and an adjustment calculation unit.

S20; Flying shot information setup section for shooting section

When all of the shooting plan establishment and the ground reference point preoccupying are completed, the flight shooting information for the aerial photographing of the drones 100 is set up in the control module 190.

As described above, the flight photographing information includes the range of the photographing section, the lateral redundancy, the corridor, the dron speed, and the photographing altitude, and the control module 190 processes data of the drones 100), the number of flight routes, the photographic base line and the shutter interval.

The control module 190 controls the flight of the drones 100 and the operation of the digital camera 160 according to the data calculated according to the designated process. For this purpose, the drones 100 include an altimeter 120 The INS 140, the GPS recognizer 130, and the like.

The drones 100 according to the present embodiment may be configured to have an automatic navigation function in the control module 190 so that the drones 100 can automatically operate according to flight shooting information set by an operator, May be used to directly adjust the flight of the drones 100. To this end, the drones 100 may further include a wireless communication device 180 for communication with the wireless controller 200.

The wireless controller 200 includes a wireless communication device 210 for communicating with the drones 100 and an input and output device 230 for manually operating a lever or a handle and outputting the flying state of the drones 100, Output unit 230 to the drones 100 through the wireless communication unit 210 and receives the output signals received from the drones 100 from the wireless communication unit 210 and outputs the processed signals to the input / And a control module 220 for outputting the control signal. The configuration and process of the radio controller 200 for controlling the drone 100 are well known in the art, and a description thereof will be omitted here.

S30; The digital camera in the flying drones takes the shooting section

The information and the current status of the shooting area of the installation area is used as the basic road of this shooting plan, and the location and interval of each shooting area, the overlap of the side, An examination chart for each course is prepared.

In the case of a continuous development of a city below 50 m and a location of a real-time underground facility such as an industrial complex is photographed by a drone (100), even if a drone (100) Falls. The drone 100 of this embodiment is a PHANTOM 4 manufactured by DJI of China and has a tap fly function as an automatic navigation function. However, there is a deviation between a predicted path and an actual flight path, and a range It is restricted on the screen.

In this embodiment, the operator marks the image of the shooting region with the white chalk for the marker in accordance with the shutter interval at the spot on the field, etc., and shoots the corresponding shooting region continuously with the digital camera 160, The number of copies and the degree of redundancy. Through such processing, it is possible to perform positional coordinates and image image joining for each point of the captured image.

For reference, since the installation area designated as the photographing area is a vast area, it is necessary to carefully take the same altitude photographing in consideration of the irregular wind speed of the wind, and it is possible to cope with the error due to the altitude change and the route change by repeatedly photographing several times.

The control module 190 controls the driving of the digital camera 160 according to the set shooting information. The digital camera 160 photographs a designated photographing section under the control of the control module 190 to generate a photographing image and stores the photographing image data in the storage device 150 of the drone 100.

For reference, the storage device 150 may be a removable storage medium such as a USB or a RAM module, which can be easily detached from the main body of the drones 100 by an operator.

S40; Converting the point of the photographed image into absolute coordinates through photographic reference point surveying

(X, Y, Z, X, Y; X, Y, Px, Py) of the countless points formed in the photographed image are measured and then the measurement results of a small number of the ground reference points The coordinates of the innumerable points are converted into absolute or absolute coordinates by means of a computer, a block regulator and an illustrative method.

The above-described photographic reference point measurement process is a known aerial triangulation (AT) technique, and a detailed description thereof will be omitted here.

In this embodiment, the photographic reference point measurement process is performed in the image processing apparatus 300, and the image processing apparatus 300 of the present embodiment utilizes the 'Photoscan PRO' image processing program produced by 'Agis soft'. However, image processing equipment for map production can be variously modified within the scope of the following rights.

For reference, the image processing apparatus 300 receives corresponding shot image data from the storage device 150 of the drones 100, and performs imaging processing on the shot images. Communication between the storage device 150 of the drones 100 and the image processing equipment 300 may be accomplished by connecting the removable storage device 150 to the image processing equipment 300 as described above, 100 may be in wireless communication with the image processing equipment 300.

S50; Marking the ground reference point on the video image

The image processing apparatus 300 completes an image image by combining the image image or a plurality of captured images. To this end, the image processing apparatus 300 may be configured to search and output a captured image, a plurality of captured image alignments and joins, Dense Cloud editing of the target section, Mesh editing of the target section, texture editing of the target section, ground reference marking, and image alignment of the ground reference mark are sequentially performed.

If an error is detected for the ground reference point after aligning the marked image with the ground reference point, the ground reference point for which the error has been identified can be corrected or deleted.

The above process is repeated several times to complete the imaging process of a plurality of shot images, and position coordinates of each point in the image image are grasped.

S60; The first survey information generation step

When a video image is obtained through the joining of the captured images, an orthoimage image and a digital elevation model (DEM) as shown in FIG. 6 based on the position coordinates of the ground reference points marked on the video image, And a digital surface model (DSM), and constructs a spatial information DB for 'Geo Referenced' images.

Since the process of generating the digital elevation model, the numerical surface model, the orthoimage, and the like is a well-known technique, detailed description of the model and image generation will be omitted here.

For reference, 'Geo Referencing' is a calculation process of linking an image and a map projection system and assigning map coordinates to individual pixels of the image. The map coordinates are formed by polynomials of the relationship between the image and map coordinates using the ground reference point .

FIG. 7 is a flow chart showing another embodiment of the position measuring method according to the present invention. FIG. 8 is a view showing an image of a pipe network generated by editing an orthographic image of a post- FIG. 9 is an image showing a pipe network diagram of an underground facility constructed according to the method of constructing an underground facility map according to the present invention, and FIG. 10 is an image of an underground facility map according to a method of collecting location information according to the present invention Respectively.

S70; A step of generating second survey information based on the collected information by photographing a post installation area

When all of the underground facilities are installed and the pavement of the road pavement is completed, steps S30 to S60 are repeated for the re-photographing section described above, and an orthoimage based on the captured image and a Digital Elevation Model (DEM) Elevation Model) and a digital surface model (DSM). That is, after the burial work such as the road pavement, the installation area in which the underground facility is not seen as the exterior is re-taken as the drone 100, and the image shown in FIG.

For reference, as shown in Fig. 8, the editing of the underground facilities map is performed according to the regulations for creating underground facilities for each point by using a CAD program. In addition, the image processing apparatus 300 performs a 'Draw point' function and a 'Draw Poly Line' function through a photo scan processing process to display 'Point' and 'Line' in a video image as shown in FIG. Each point is linked to a Shapes file. That is, the data file is generated in the form operated in 'Arc GIS'.

Here, the 'Shapes' file is a Esri vector data storage format that stores the position, shape, and attributes of spatial features. Shape files that are stored as related filesets and that contain a single feature class have been used previously in GIS desktop applications such as ArcMap, and many of them contain large, feature-rich features. If there is a small amount of data in the 'Shapes' file (typically less than 1,000 features), the map created by the Map viewer will have * prj, which contains files with * .zip, * .shp, * .shx, * .dbf extensions. It can also be added as a file with an extension so that others can view it through a web browser.

In this embodiment, the piping network shown in FIG. 8 and FIG. 9 is constructed such that the points of the chain spacing of 20 m are acquired as a 'Shapes' file by the 'Point', and the piping lines of the underground facilities are 'Poly Line Shapes' Since the files are formed, the attribute charts are created by dividing them into the respective layers. Therefore, it is possible to generate and manage other information such as pipeline, pipe diameter, pipe type, and depth desired by the operator for underground facilities within the installation area.

Using the information gathered above, the drawing system of CAD and the like is diagrammed to complete an underground facility diagram shown in FIG.

FIG. 11 is a view schematically showing a digital camera used in the position information collecting method according to the present invention, and FIG. 12 is a partial cross-sectional view schematically showing the operation of the digital camera shown in FIG.

The digital camera 160 of the present embodiment includes a camera body 161 on which a lens 161a is mounted; A magnetic panel 162 installed on one side of the camera body 161; An enclosure 163 for horizontally movably holding the camera body 161 and connected to the camera body 161 through a tension spring 163b to maintain the camera body 161 in one side position; An electromagnet 164 mounted on the enclosure 163 to face the magnetizable panel 162; And a switch 165 for receiving a drive signal of the control module 190 and applying a current to the electromagnet 164.

On the other hand, the control module 190 sends a drive signal to the switch 165 in response to the shutter drive of the digital camera 160.

The electromagnet 164 is provided at a position opposite to the direction of the movement of the drones 100 for photographing based on the camera body 161 and the tension spring 163b is mounted on the camera body 161, The camera body 161 is positioned on the housing 163 at a position opposite to the electromagnet 164 on the basis of the reference position. Therefore, the movement of the camera body 161 is restricted in the direction in which the electromagnet 164 is positioned.

On the other hand, the electromagnet 164 includes a conductor 164a disposed to face the camera body 161, a plunger 164b that draws out from the conductor 164a, and a plunger 164b that electrically surrounds the plunger 164b, And a coil 164c connected thereto. Therefore, when the switch 165 is closed, a current is applied to the coil 164c to generate a magnetic field, and the plunger 164b and the conductor 164a are magnetized.

The magnetic body of the electromagnet 164 causes the camera body 161 to rotate in the direction of the arrow b (b) against the tension of the tension spring 163b, because the magnetic panel 162 is provided on one side of the camera body 161 As shown in the drawing, for the purpose of joining with the electromagnet 164.

As a result, when the digital camera 160 photographs the ground in the flying dragon 100, the digital camera 160 maintains the position of the shooting point at the backward speed corresponding to the flying speed of the dragon 100, The digital camera 160 photographs the ground in a fixed photographing posture irrespective of the movement of the drones 100. For this, the control module 190 controls the switch 165 simultaneously with the shutter control of the digital camera 160.

Then, when the switch 165 is opened, the camera main body 161 returns to the home position by the tension of the tension spring 163b, and prepares for the subsequent photographing.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (3)

A first step of preliminarily positioning a plurality of points whose positions change or are not lost in the shooting interval of the drones to ground reference points and measuring the position coordinates of the points;
A second step of setting up the shooting information on the shooting interval in the control module of the drones;
A third step in which the digital camera of the drone photographs the photographing section and generates a photographed image while the drone is flying the installation area of the underground facility as the photographing section in accordance with the flying photographing information;
A fourth step of the image processing apparatus converting each of the position coordinates of the points formed in the captured image into absolute coordinates through photographic reference point measurement;
A fifth step of the image processing apparatus creating a video image by imaging and connecting a photographed image, and marking the ground reference point on the video image; And
A sixth step of calculating a photographed reference point based on the ground reference point marked on the image image to generate at least one first survey information selected from the DEM, the DSM and the orthoimage for the image;
Wherein the location information of the underground facility is collected by using a drone. The method according to claim 1,
The flight photographing information includes a range of a photographing section, lateral redundancy, corridors, drone speeds, and photographing altitudes;
Wherein the control module includes: a flight route spacing calculation equation having the lateral redundancy and the photographing range as variables; a flight route number calculation formula having the photographing interval range and the flight route interval as variables; Processing the drones by processing them according to data calculated by a shutter interval grinding formula using the shooting range, the dron speed, and the number of pictures per flight line as variables;
A method for collecting location information of underground facilities using a dron. The method according to claim 1,
The digital camera captures an after-photographing section while the drone is flying in an after-installation area in which the underground facility is buried in accordance with the flying photographing information, A seventh step of re-performing the sixth step to generate second survey information for constructing an underground facility;
Wherein the location information of the underground facilities is collected using a drone.

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