Method and device for locating a point of interest on a terrain from an image

Abstract

The invention relates to a method and device for locating a point of interest on terrain from an image, comprising the following steps: - obtaining an image of the terrain taken by an optical sensor; - identifying and locating the point of interest in the image; - projecting the ground position of the optical sensor onto a digital terrain model; - determining, by implementing Bresenham's algorithm, a path in the digital terrain model passing through the projection of the ground position of the optical sensor; and - searching for the projection of the point of interest within the path. Figure 2 (for the abstract)

Description

Title of the invention: Method and device for locating a point of interest on a terrain from an image

[0001] The present invention relates to the technical field of geolocation of points of interest on the Earth's surface, and in particular the field of combating vegetation fires.

[0002] Programs and algorithms have already been developed with the aim of enabling a drone equipped with a multimodal camera, i.e. operating in the visible and infrared spectra, to carry out a mission automatically or remotely in order to geolocate points of interest. The mission consists of making it follow a trajectory above a plot of land from several GPS points. During the flight over the plot, the drone is capable, thanks to an onboard computer, of detecting hot spots resulting from vegetation fires.

[0003] The document "Review of Target Geo-Location Algorithms for Aerial Remote Sensing Cameras without Control Points" Yiming Cai et al. - Applied Sciences - 2022 - pp 1-27 discloses, in particular on page 9, a method for locating a target in a terrain using a digital terrain model. The method consists of:

[0004] - estimate the geographic location of the target and extract from the digital model of terrain of target elevation information,

[0005] - initialize a possible altitude as the maximum possible altitude of the target,

[0006] - determine the longitude and latitude of the target in the Earth's ellipsoidal reference frame,

[0007] - find, from the digital terrain model, the geodetic altitude information from a point having as coordinates the possible altitude, longitude and latitude of the target,

[0008] - compare this geodetic altitude to the possible altitude; if the geodetic altitude is greater than the possible altitude, return this altitude as the result, otherwise, start again from the second step taking as the new possible altitude the previous possible altitude reduced by a small value.

[0009] No information is provided in this document regarding the method for estimating the geographic location of the target in the digital terrain model.

[0010] Document US9714833B2 discloses a principle for determining geographic coordinates of a point of interest, from a 2D image captured from a high position by a downward-facing optical sensor, with projection of the camera position onto a digital terrain model. This document discusses tracing a path in the direction of the point of interest and the use of a collision detection algorithm. It further discloses the application of this algorithm to fire detection. of forests.

[0011] A disadvantage of the principle described in the prior document is that no concrete implementation details are known. Given the hardware constraints, in particular the computing power available for a reasonable weight and cost of the equipment, the provision of a concrete solution making it possible to locate a point of interest remains non-obvious for those skilled in the art and is the subject of the present invention.

[0012] A digital terrain model or DEM, for Digital Elevation Model in English, corresponds to a representation in digital form of the relief of a geographical area. It is a discrete storage of the altitudes of a parcel defined between four points. This parcel is called a "slab". A slab is composed of a regular mesh where an altitude value is associated with each of the meshes. This altitude is generally that of a point of the digital model located at the center of the slab. The quality of a digital terrain model depends directly on the mesh interval. This interval is called resolution and defines the accuracy of the model. The smaller the resolution, the more accurate the digital terrain model.

[0013] Generally speaking, we speak more specifically of an altitude field to define a digital terrain model.

[0014] The invention aims in particular to propose a simple solution for locating a point of interest which does not require significant on-board resources or real-time processing. As soon as a point of interest is captured in the camera's field of vision, it can be located, even later.

[0015] To this end, the invention relates to a method for locating a point of interest on a terrain from an image, by selecting a mesh of a digital model of the terrain comprising meshes paving said terrain, each mesh being associated with an altitude representative of the part of the terrain that it paves, characterized in that it comprises the following steps:

[0016] - obtaining an image of the terrain taken by an optical sensor;

[0017] - identification and localization of the point of interest in the image;

[0018] - obtaining the position of the optical sensor at the time of image capture in a terrestrial reference point - for example, altitude, longitude, latitude -;

[0019] - determining the orientation of a sighting axis pointing towards the point of interest;

[0020] - projection, on the digital terrain model, of the terrestrial position of the sensor optical, according to a predetermined projection direction;

[0021] - projection, on the digital terrain model, of the line of sight, according to the same predetermined projection direction;

[0022] - determination, by an implementation of the Bresenham algorithm, of a path of meshes in the digital terrain model, including the projection of the position terrestrial of the optical sensor and following the projection of the line of sight and

[0023] - search for the path mesh whose associated altitude corresponds to the distance between said mesh and the projection of said mesh on the line of sight, according to the predetermined projection direction.

[0024] The mesh meeting the search criterion is the mesh containing the projection of the point of interest on the digital terrain model.

[0025] Determining the mesh path and searching for the mesh meeting the search criterion are two steps that can be intertwined. For example, it is possible that the mesh path is not fully determined but that only a first mesh of the path has been determined by implementing the Bresenham algorithm and that the search begins to be carried out on this first mesh. If the mesh does not satisfy the criterion, the path determination can resume to determine a second candidate mesh, and so on.

[0026] Another advantage of the method according to the invention is that it can be implemented using a single optical sensor.

[0027] Yet another advantage of the method according to the invention is that it can be implemented after a single shot and that the processing of the image thus obtained can be carried out later. It is therefore not necessary to carry heavy or powerful means of computing capacity on the flying machine carrying the camera.

[0028] According to a particular embodiment of the localization method, the meshes of the digital terrain model are gathered into slabs and the projections onto the digital terrain model are determined by:

[0029] - the slab containing the projection and

[0030] - the coordinates of said projection in a reference frame of said slab.

[0031] An advantage of using slabs is a simplification of the calculations required for each iteration. Determining the slab containing the projection simply results from comparing the coordinates of the projected point with the coordinates of the four vertices of the square formed by the slab. Once this slab is determined, iterating from mesh to mesh is a very simple operation, along the mesh path determined by implementing the Bresenham algorithm inside the slab. It does not require large calculations.

[0032] According to a particular embodiment of the localization method, the iteration is stopped if the mesh considered in the iteration loop is the last of the mesh path located inside the slab.

[0033] In fact, in the event of leaving the slab, it is necessary to repeat the algorithm inside the neighboring slab in the direction of the line of sight.

[0034] According to a particular embodiment of the location method, the digital terrain model is replaced by a digital surface model.

[0035] Although such an option is not preferred for a perfect implementation of the invention, it is within the scope of the invention.

[0036] According to a particular embodiment of the location method, the optical sensor is a camera operating in the visible and infrared spectra.

[0037] According to a particular embodiment of the location method, the predetermined projection direction is vertical.

[0038] Vertical projection is the most natural projection, which makes projection distance and altitude coincide. However, a non-vertical projection would not prevent the present invention from providing the expected result.

[0039] According to a particular embodiment of the location method, the expression “corresponds to” means “greater than or equal to”, so that the search criterion for the path cell whose associated altitude corresponds, according to the invention, to the distance between said cell and the projection of said cell on the line of sight, according to the predetermined projection direction, means that the altitude of the cell is greater than or equal to the distance considered.

[0040] An advantage of this simple comparison criterion is that it is sufficient to compare the altitude associated with each cell in the digital terrain model with the distance between said cell and its projection on the line of sight. If the altitude of a cell is greater than or equal to said distance, this means that the line of sight has encountered the terrain, which reveals that the cell considered contains the point of interest. Thus, the point of interest is located in the digital terrain model.

[0041] According to a particular embodiment of the location method, the search consists of an iteration from mesh to mesh, starting from the mesh containing the projection of the terrestrial position of the optical sensor, moving away from it in the direction of the point of interest.

[0042] According to a particular embodiment of the localization method, the iteration is stopped if a predetermined number of iteration loops is exceeded.

[0043] This limit avoids an infinite loop if, for some reason, the search criterion for the mesh containing the point of interest was never satisfied.

[0044] According to a particular embodiment of the location method, the representative altitude is an average altitude.

[0045] Another object of the invention is a device for locating a point of interest on a terrain from an image, characterized in that it comprises a computer-readable memory medium, with instructions encoded thereon, and at least one processor connected to the memory medium and configured to read and execute the instructions and, when it executes the instructions, implement the operations of the method. Brief description of the figures

[0046] The invention will be better understood on reading the following description given solely by way of example and with reference to the appended drawings in which:

[0047] [Fig.l] is a schematic view of the different mathematical references used to implement the invention,

[0048] [Fig.2] is a diagram illustrating the implementation of the Bresenham algorithm,

[0049] [Fig.3] represents a plane of the Bresenham frame divided into octants,

[0050] [Fig.4] provides an extract of a simple logic code of an execution loop of the Bresenham algorithm,

[0051] [Fig.5] represents a decision tree translating the execution of the code of [Fig.4],

[0052] [Fig.6] provides an extract of a refined logic code of an execution loop of Bresenham's algorithm. Detailed description

[0053] We will now describe how a point of interest can be located in an image on a terrain.

[0054] Localization is carried out in two stages: detection and localization of the point of interest in the image and geographical localization of the point of interest on the ground.

[0055] A camera 6 on board a drone 7 is an optical sensor which provides an image 8 of the monitored area. In this image 8, the point of interest constitutes a pixel 9 having its own coordinates in this image 8. Different detection algorithms can be implemented to detect the point of interest. For example, if it is a hot spot in an image for fire detection, the camera can be an infrared camera and the temperature of each pixel of the image is measured so as to determine the hot spot which then constitutes the point of interest.

[0056] Once this point of interest has been determined, the geographical location of this point is obtained by the ray-tracing method, according to which a line of sight 10 going from the camera to the point of interest is considered. This line of sight 10 has an orientation provided by a vector. The camera 6 has coordinates. The combination of the information consisting of the coordinates of the camera 6 and the orientation of the vector makes it possible to find the geographical coordinates of the hot spot according to the method which will be described.

[0057] First of all, it is a matter of carrying out mathematical transformations to move from the frame of reference of camera 6 to the terrestrial frame of reference.

[0058] This succession of transformations of reference points requires the following reference points.

[0059] - Terrestrial reference “East North Rising” or “ENU” 101, Rmenu, in which the X axis of the abscissa points east, the Y axis of the ordinate points north and the vertical Z axis points upwards. The origin point is located at a reference point of a digital terrain model 12. This reference point is considered the reference mesh of the digital terrain model.

[0060] - Terrestrial reference point “North East Descending” or “NED” 102, RMNED, in which the axis X is facing north, Y is facing east, and Z is facing down. The origin of the coordinate system is placed at the center of gravity of drone 7. This coordinate system is mainly used to determine the orientations of drone 7.

[0061] - Bresenham marker, RB 103, oriented as the East North Rising marker RMENU, but whose origin is placed at the starting point of the algorithm which will be described.

[0062] - Drone reference point, RD 104. This reference point is placed at the center of gravity of the drone 7 with the X axis looking forward, the Y axis looking to the right and the Z axis looking down. The angles between the drone 7 and the North East Descending reference point are called the drone pitch (Dp), the drone roll (Dr) and the drone yaw (Dy).

[0063] - Gimbal reference, RG 105. This reference is oriented and placed like the drone reference. The angles between drone 7 and camera 6 are called camera pitch (Cp), camera roll (Cr), and camera yaw (Cy).

[0064] - Camera reference, Rc 106, which is placed at the optical center of camera 6, with the Z axis looking forward along the optical axis, the Y axis looking down, and the X axis looking to the right.

[0065] - Image reference, R! 107. This reference is placed in the center of image 8 with the X axis looking to the right, the Y axis looking down and the Z axis pointing into the image.

[0066] - Pixel marker, RP 108. This marker is oriented like the marker in image 7, but its origin is placed at the upper left corner of image 8.

[0067] The distance between the origins of the drone reference frame 104 and the gimbal reference frame 105, as well as the distance between the origins of the gimbal reference frames 105 and camera 106, are negligible. Thus, it is considered that the origins of the terrestrial reference frames NED RMned 102, drone RD 104, gimbal RG 105 and camera Rc 106, are merged.

[0068] [Fig.l] represents each of the previously explained reference points, as well as the successive coordinate transformation operations to move from the image reference point R! 107, in image 8, to the Bresenham reference point RB 103, in the digital terrain model 12, passing successively through the pixel reference point RP 108, the camera reference point Rc 106 on camera 6, the gimbal reference point RG 105, the drone reference point RD 104 on drone 7, the terrestrial reference point RMned 102, the terrestrial reference point RMenu 101.

[0069] To geolocate the point of interest in the image, we proceed in two steps. First, the direction vector 1 li of the line of sight 10 is determined. Its coordinates are transformed to pass from the pixel reference RP 108 to the terrestrial reference Rmenu 101; Then, the Bresenham algorithm will be used to browse the digital terrain model and find the location of the point of interest, in accordance with the invention.

[0070] The geolocation algorithm starts with pixel 9 of the point of interest identified in image 8. The coordinates of this pixel 9 are (u, v, 1)T and the vector 1 Ip composed of its coordinates is named Tp. Five transformations are required to transform the coordinates of this point of interest, expressed in the reference frame 107 of image 8, into coordinates expressed in the terrestrial reference frame “East North Upward”, RMenu 101.

[0071] First transformation 1: from the pixel reference frame RP 107 to the camera reference frame Rc 106 of camera 6. This transformation uses the intrinsic matrix of camera 6. For a theoretically perfect camera, the intrinsic matrix takes the following form.

[0072] [Math.l] / A 0 CA K - 0 Cy \0 ü 1 J

[0073] in which fx and fy are respectively the focal length on the u and v axes of the pixel frame and Cx and Cy the coordinates of the point of interest in the pixel image. The transformation of the vector Tp in the pixel frame to Tc, designated 1 le, in the frame of the camera 6 is

[0074] [Math.2] A :::: A 7 A

[0075] Second transformation 2: from the camera frame Rc 106 to the gimbal frame Rg 105. This transformation is necessary to align the camera frame and the gimbal frame. It requires two rotations, one around the ZG axis of the gimbal frame, the other around the YG axis of the gimbal frame, each by +90°. These two rotations are summarized but in the following matrix.

[0076] [Math.3] / 0 1 0\ «G-Æ :::: 0 0 1 \ioo /

[0077] to transform the vector Tc of the camera reference frame into the vector Tg of the gimbal reference frame, we apply the formula

[0078] [Math.4] ? - THAT

[0079] Third transformation 3: from the gimbal frame RG 105 to the drone frame RD 104 of the drone 7. This transformation takes into account the camera orientation angles 6 Cy, Cp and Cr. As illustrated in [Fig.3], the rotational matrices between the gimbal frame RG 105 and the drone frame RD 104 can be calculated as follows: 0 1. —sin(Cp) 0 £W(Cf / ) 0 ' nin(Cf) cos{Ctl),

[0080] [Math.5] Ky-jG — RsXCw (Cp).RXj (Cr)

[0081] [Math.6] (œs(Cv) sin(Cv) 0' -SÏHfC,,) C 0

[0082] [Math.7] Æos(Cp) 0 Rv,C.CP) 0 j \sni(Cp,i 0

[0083] [Math. 8] / 1 0 Rx^ (Cr ) ~ I 0 OJS'Cr) \0 -shhC;.)

[0084] Thus, to transform the vector Tg expressed in the gimbal frame RG 105 into a vector Td expressed in the drone frame RD 104, the formula to be applied is the following.

[0085] [Math.9]

[0086] Fourth transformation 4: from the drone reference frame RD 104 to the terrestrial reference frame “North East Descending” Rmned 102. This transformation takes into account the drone orientation angles Dy, Dp and Dr. From [Fig.2], the rotational matrices between the drone reference frame RD 104 and the terrestrial reference frame RMned 102 can be calculated. The formats of each of these matrices are the same as for the third transformation, namely:

[0087] [Math. 10] ( Df )

[0088] Thus, to transform the vector Td expressed in the drone reference frame RD into a vector Tmned expressed in the terrestrial reference frame Rmned, the formula is as follows.

[0089] [Math. 11] T^ - Ta

[0090] Fifth transformation 5: from the terrestrial reference frame Rmned 102 to the terrestrial reference frame Rmenu 101 in the digital terrain model 12. This transformation is necessary due to the orientation angles of the drone defined according to the terrestrial reference frame Rmned 102. Given that the digital terrain model 12 is defined in the terrestrial reference frame Rmenu 101, the transformation from the reference frame Rmned 102 to the reference frame Rmenu 101 requires a rotation of +90° around the zmenu axis, then another rotation of +180° around the ymenu axis. These rotations can be summarized by the following matrix.

[0091] [Math. 12] / 0 1 0 \ :::: PO 0 \oo -:i /

[0092] Finally, to transform the vector Tmned of the terrestrial reference frame Rmned 102 into a vector Tmenu = T, designated 11, in the terrestrial reference frame ENU 101, the formula is as follows

[0093] [Math. 13] T - RL v .7?

[0094] We will now determine the altitude of the line of sight 10. Once the direction vector 11 of the line of sight is expressed in the terrestrial reference frame RMenu 101, the Bresenham algorithm can be used to traverse the digital terrain model 12 and find the location of the point of interest. [Fig.2] shows this route.

[0095] The path starts at mesh 13 (xdep, ydep) of the digital terrain model which corresponds to the location of the drone, by projection according to a predetermined projection direction, here the vertical. The drone is located on the Z axis at a certain altitude (DroneAiü).

[0096] A maximum number of iterations k is set to execute the algorithm.

[0097] The algorithm consists of a succession of iterations, with a maximum number of k, during which the altitude deduced from the line of sight is compared to the altitude provided by the digital terrain model. As illustrated by [Fig.2], the digital terrain model is run over a succession of cells 13 to 18. Each cell is assimilated to the point which is located in its middle and, according to an approximation considered acceptable, although geometrically false, it is considered that these middles are aligned with the orthogonal projection of the line of sight. At each iteration, the altitude is calculated by taking into account simple trigonometric considerations. If we consider in a simplified manner the triangle ABC in [Fig.2], to find the altitude of the line of sight, we execute the product of the vectors Tc and N where N points downwards and constitutes the normal vector of the plane xbyb.The purpose of this scalar product is to find the delta angle, which we call the "propagation angle": .

[0098] [Math. 14] (has, ô ~ arccûs

[0099] with ::: (Nf, and so, for the inner product, as a term-by-term product:

[0100] [Math. 15] (N, Tf} ■■■ (¾.¾} +

[0101] where and are normalized: a HHB 1.

[0102] Once the propagation angle is calculated, we have:

[0103] [Math. 16] . . b '■.Vile: “ -a OR 6 = yZ Ù3 + 0 / 3 - ÿde?}Z-

[0104] Thus, the altitude of the sighting axis (Altiray) is:

[0105] [Math. 17] Attira, ■■■■ DroneAiti ~ a

[0106] For each mesh, the altitude of the line of sight is the distance between said mesh and the projection of said mesh on the line of sight, according to the projection direction.

[0107] Knowing the two altitudes at each iteration, the comparison can be made.

[0108] If the altitude of the line of sight 10 is less than or equal to the altitude provided by the digital terrain model 12, then this means that the line of sight 10 intercepts the Earth's surface and that the point of interest is located in the cell of the digital terrain model 12 considered at this iteration. Otherwise, the Bresenham algorithm provides a next cell to explore and the comparison is carried out again on this next cell, and so on until the line of sight 10 intercepts the digital terrain model 12. When this interception occurs, the altitude of the line of sight 10, that is to say the distance between said cell and its projection on the line of sight, according to the projection direction, is greater than the altitude associated with the cell in the digital terrain model.

[0109] We will now provide a more detailed description of Bresenham's algorithm.

[0110] The Bresenham algorithm is an algorithm for tracing a straight line in a mesh of dimension n, by determining which mesh cells must be retained to form said straight line. In the case of the invention, the dimension n is equal to 2.

[0111] As shown in [Fig.3], the plane xbyb is separated into eight parts 19 to 26 designated "octants", based on the inclination of the vector. In this way, the method can be carried out for the first octant 19 only and extended to the others octants 20-26 using the following symmetries:

[0112] • invert the x and y components, which amounts to going from octant 19 to octant 20,

[0113] • change the sign of the x and y components, which amounts to going from octant 22 to octant 26.

[0114] [Fig.4] provides an extract of a logic code of an execution loop of Bresenham's algorithm. From a starting point with coordinates (xO, yO), a vector (dx, dy) is plotted on the first octant 19. The coordinates dx and dy refer to the inclination of the vector on the x-axis and on the y-axis respectively.

[0115] Considering a pixel with coordinates (x, y), we calculate as follows a decision variable named “d”:

[0116] [Math. 18] d — 2.dy — dx

[0117] in order to determine which next pixel should be chosen from the following possibilities: (x +1, y) and (x+1, y+1), depending on the sign of the value d. If d is negative, then the next pixel will be (x +1, y). Otherwise, the next pixel will be (x+1, y+1).

[0118] The errors accumulated during the exploration of the mesh are taken into account. When crossing a pixel with coordinates (x+1, y), the error is ex and if the pixel with coordinates (x+1, y+1) is crossed, the error is exy.

[0119] [Math. 19]

[0120] [Math.20] ™ 2.{dy - dx ■

[0121] By executing the algorithm in [Fig.4], we follow a decision tree represented in [Fig.5]. Four situations can arise:

[0122] - if dx = 0, only y is incremented,

[0123] - if dy = 0, only x is incremented,

[0124] - if dx = dy, the exploration must be done on the diagonal, which means that both x and y must be incremented.

[0125] A differentiation must be made between the eight octants of the plane.

[0126] - if dx > dy, the bytes considered are octants 19, 22, 23 and 26. In this case, the propagation can directly follow the sign of the vector components. The signs of the components are stored in two specific variables sx and sy.

[0127] - if dx < dy, the octants considered are 20, 21, 24 and 25. Thanks to the properties sy metrics, we come back to the case in which dx is greater than dy, by playing on the symmetries. For this, we must consider two sub-cases:

[0128] - if (dx > 0 and dy > 0) or (dx < 0 and dy < 0), that is to say dx and dy have the same sign, the octants processed are 20 and 24 and the x and y coordinates must be reversed

[0129] - if (dx > 0 and dy < 0) or (dx < 0 and dy > 0), that is to say dx and dy have signs opposite, the octants considered are 21 and 25. The x and y coordinates must be reversed, then their signs changed.

[0130] An offset variable is introduced to take into account the fact that the center of the xbyb plane is not always zero. In this case, the algorithm can be refined according to the logic code presented in [Fig.6].

[0131] Other, less efficient algorithms can also be used instead of the Bresenham algorithm, such as the midpoint algorithm or the DDA (for "Digital Differential Analyzer") algorithm. An advantage of the Bresenham algorithm in the context of the invention is that the digital terrain model can be considered as a large screen whose pixels are the meshes of the digital model.

[0132] In summary, the steps that are carried out for the execution of the Bresenham algorithm are as follows:

[0133] (a) Initialization of the variable k = 0.

[0134] (b) At the coordinates (x, y) of point O, projection onto the digital terrain model 12 from the position of drone 6, the corresponding height h is searched for in the digital terrain model.

[0135] (c) The value h is then compared with the altitude of the drone 6, taken as the value initial of the variable H.

[0136] (d) As H > h, k is incremented by 1 and the coordinates of the next point are calculated.

[0137] (e) This process is repeated as many times as necessary, giving the variable H the altitude of the next point and making the comparison between H and h.

[0138] (f) When the condition H < h is satisfied, the point is identified.

[0139] This point corresponds to the intersection between the mesh path and the Earth's surface.

[0140] In a variant, when the mesh path propagates on the digital terrain model, it may be that for one reason or another, the stopping condition H < h is never satisfied. Two additional stopping conditions can be introduced to overcome this problem. The first condition concerns a maximum number of iterations to be carried out for propagation. The second condition makes the program stop when the mesh path leaves the slab of the digital terrain model. This last condition will also make it possible to know, if the starting point is in one slab and the sought point in another, at which point one leaves the first slab and by extrapolation to know at which point one enters the second. Thus, one can travel through the digital terrain model in all possible directions.

[0141] The device used for the present implementation of the invention comprises four entities (not shown):

[0142] - the first, in the form of an Android application, corresponding to an interface between the drone and the remote pilot. The latter can choose to perform a flight in manual piloting or to create a trajectory for an automatic mission. This automatic mission is generated on the basis of any polygon made up of at least three points entered on a satellite map. There are two modes for generating this trajectory from the flight height, the overlap rate as well as the camera parameters: scanning the surface of the polygon; and tracing the contour of the polygon, with an additional parameter: the width to be scanned entered by the user. If the requested width is greater than that possible at this height, the trajectory generation will then automatically create one or more polygons, uniformly dilated, in order to fly over an area around the polygon in relation to the requested width.These trajectory generation strategies are adapted to the needs of firefighting operators for monitoring burnt areas and fire edges. Once the trajectory has been validated by the remote pilot, the mission is then generated based on the drone's performance and the speed entered by the remote pilot. This mission is transferred to the drone. During the flight, the application will allow these essential parameters to be monitored and its route to be followed;

[0143] - the second, which can be developed in C++, retrieves the video stream from the camera thermal imager and processes it to automatically detect hotspots based on a set temperature threshold. Once detected, each hotspot is assigned its pixel coordinates, the drone's GPS position, and the camera orientation at the time the image was taken.

[0144] - the third entity, which can be developed in Python, retrieves the attributes of each hotspot to determine its geographic coordinates on the Earth's surface. This operation is performed by tracing a ray propagating on a digital terrain model via a Bresenham algorithm;

[0145] - finally, in a fourth and final entity, the data collected during the flight are archived on the on-board computer and transmitted to the remote pilot without a wired connection.

[0146] The steps of the method can thus be summarized as follows:

[0147] - projection of the image capture position onto the digital terrain model,

[0148] - determination of the direction in which the point of interest is located on the model digital field,

[0149] - from the projected position, tracing a mesh path in the direction of completed, each mesh of the path being determined using the Bresenham algorithm adapted to vectorization, with, at each mesh of the traced path, the comparison between the altitude of the corresponding geographical point and that of the mesh,

[0150] - the process is stopped if the altitudes cross.

[0151] The result of this process is the reference of the mesh 18 corresponding to the point of interest. List of references

[0152] 1 to 5: transformations of change of reference frame

[0153] 6: camera

[0154] 7: drone

[0155] 8: image

[0156] 9: pixel

[0157] 10: sighting axis

[0158] 11, 11c, 1 li, 1 lp: direction vector

[0159] 12: digital terrain model

[0160] 13-18: meshes of the digital terrain model

[0161] 19-26: octants of the xbyb plane

[0162] 101 = Rmenu: Terrestrial reference “East North Upward” or “ENU”

[0163] 102 = Rmned: Terrestrial reference point “North East Descending” or “NED”

[0164] 103 = Rb: Bresenham landmark

[0165] 104 = Rd: Drone reference point

[0166] 105 = RG: Cardan reference

[0167] 106 = Rc: Camera reference

[0168] 107 = R! : Image reference

[0169] 108 = RP: Pixel reference

Claims

Claims

1. Method for locating a point of interest (9) on a terrain from an image (8), by selecting a mesh of a digital terrain model (12) comprising meshes (13-18) paving said terrain, each mesh being associated with an altitude representative of the part of the terrain that it paves, characterized in that it comprises the following steps: - obtaining an image (8) of the terrain taken by an optical sensor (6); - identifying and locating the point of interest (9) in the image (8); - obtaining the position of the optical sensor (6) at the time of image taking in a terrestrial reference frame (101); - determining the orientation of a sighting axis (10) pointing towards the point of interest (9); - projecting, onto the digital terrain model (12), the terrestrial position of the optical sensor (6), according to a predetermined projection direction;- projection, on the digital terrain model (12), of the line of sight (10), according to the same predetermined projection direction; - determination, by an implementation of the Bresenham algorithm, of a path of meshes (13-18) in the digital terrain model (12), including the projection of the terrestrial position of the optical sensor (6) and following the projection of the line of sight (10) and - search for the path mesh whose associated altitude corresponds to the distance between said mesh and the projection of said mesh on the line of sight (10), according to the predetermined projection direction.;

2. A localization method according to claim 1, wherein the search consists of an iteration from mesh to mesh and wherein the meshes of the digital terrain model are gathered into slabs and the projections onto the digital terrain model (12) are determined by - the slab containing the projection and - the coordinates of said projection in a reference frame of said slab.

3. A localization method according to claim 2, wherein the iteration is stopped if the mesh considered in the iteration loop is the last of the mesh path located inside the slab.

4. A location method according to any one of claims 1, 2 and 3, wherein the digital terrain model is replaced by a digital surface model.

5. A location method according to any one of claims 1, 2, 3 and 4, wherein the optical sensor is a camera (6) operating in the visible and infrared spectra.

6. A locating method according to any one of claims 1, 2, 3, 4 and 5, wherein the predetermined projection direction is vertical.

7. A locating method according to any one of claims 1, 2, 3, 4, 5 and 6, wherein the expression "corresponds to" means "greater than or equal to".

8. A location method according to any one of claims 1, 2, 3, 4, 5, 6 and 7, wherein the search consists of an iteration from mesh to mesh, starting from the mesh containing the projection of the terrestrial position of the optical sensor, moving away from it in the direction of the point of interest.

9. A localization method according to claim 8, wherein the iteration is stopped if a predetermined number of iteration loops is exceeded.

10. A location method according to any one of claims 1, 2, 3, 4, 5, 6, 7, 8 and 9, wherein the representative altitude is an average altitude.

11. Device for locating a point of interest on a terrain from an image, characterized in that it comprises a computer-readable memory medium, with instructions encoded thereon, and at least one processor connected to the memory medium and configured to read and execute the instructions and, when executing the instructions, implement the operations of the method according to any one of claims 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10.

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