A non-contact measurement method for the diameter at breast height of standing trees based on topographic conditions

Abstract

The present invention provides a non-contact method for measuring the diameter at breast height (DBH) of standing trees. Based on low-cost passive optical devices and active optical devices, it can be effectively applied under various forest stands. By integrating these two low-cost optical devices, namely the passive optical device and the active optical device, the passive optical device is used to obtain two-dimensional image information containing the target standing tree, and the active optical device is used to obtain the distance information from the integrated device to the target standing tree. Calibrate the integrated device to obtain the basic parameter information of the device. Estimate the breast height position in the two-dimensional image considering the terrain conditions. Rotate the integrated device to the horizontal and aim at the target standing tree to obtain the two-dimensional image containing the target standing tree and the distance information from the integrated device to the target standing tree. Use the algorithm of the present invention to calculate the DBH of the target standing tree and complete the non-contact measurement of the DBH of the target standing tree. The structure of the present invention is simple and easy to integrate, improving the measurement universality of the device and enabling effective measurement under complex terrain conditions.

Description

Technical Field

[0001] The invention relates to a non-contact method for measuring the diameter at breast height of a standing tree, and more specifically, to a method for realizing non-contact, low-cost and rapid measurement of the diameter at breast height of a standing tree in a complex forest stand, belonging to the technical field of forestry structural parameter measurement. Background Art

[0002] Diameter at Breast Height (DBH) is an important parameter in forest resource surveys and is generally defined as the diameter perpendicular to the trunk axis at 1.3m above the ground. Accurate measurement of DBH plays an irreplaceable role in inverting forest volume, biomass, and revealing forest carbon flow.

[0003] DBH measurement methods can be divided into contact method and non-contact method. Traditional contact measurement tools are generally based on calipers or tree measuring. Although their measurement accuracy is high and they are often regarded as true values, they are time-consuming and laborious in actual forest resource surveys and are sometimes limited by the difficulty of approaching the target area. The new contact measurement method replaces traditional contact measurement tools with integrated electronic sensors, and realizes automated measurement of DBH based on the changes in the electrical signals of the sensors. Although it realizes automated measurement of parameters, the inaccessibility of some areas during actual measurement still limits its effective application.

[0004] With the continuous development of current smart terminals, the quality of their optical lenses and their own computing power have been effectively improved. Using smart terminals and integrating active and passive optical measurement systems composed of laser rangefinders can realize non-contact measurement of DBH, which will improve the efficiency of forest resource surveys and reduce the time and material costs of data measurement. At present, some ideas use smart phones and other special sensors to measure DBH, such as using specific smart terminals equipped with depth cameras or designing specific calibration objects to estimate depth information to measure DBH, which to a certain extent meets the needs of some research. However, due to the differences in smart terminal models and ease of implementation, a DBH measurement mode with high universality and sufficient accuracy based on smart terminals and integrated multiple sensors remains to be studied.

[0005] First, the non-contact measurement of DBH using a smartphone requires establishing a conversion relationship between the two-dimensional image space and the three-dimensional real space. Existing space conversion methods based on intelligent terminals generally rely on specific built-in sensors and are limited to specific models of smartphones, such as smartphones equipped with TOF cameras and Google Tango frameworks, or iPhone 13 Pro configured with lidar scanning modules. When additional external integrated sensors are installed, specific markers are generally designed to perform spatial relationship transformation. Although it is not limited to the specific models of intelligent terminals, the complex design of the markers restricts the portability of the model and also brings a large number of repeated calibration problems in field work. Secondly, existing measurement modes generally focus on simple forest stands such as plantations with flat terrain, and how to design an effective measurement mode to adapt to DBH measurement in complex forest stands such as mountain forests is still a problem to be solved. Specifically, existing research generally assumes that the terrain of the measurement scene is flat and directly converts the height of the device from the ground into the breast height of the target tree, which limits its effective application in natural forests with rugged terrain. Thirdly, for existing non-contact measurement modes of DBH based on intelligent terminals and integrating multiple sensors, in order to continuously and stably measure DBH in field experiments, relatively complex device calibration experiments need to be carried out before the experiment to accurately obtain the basic parameter information of the device. Therefore, a simplified integrated device calibration mode needs to be designed to improve the practicality and convenience of measuring DBH with integrated devices based on intelligent terminals. Summary of the Invention

[0006] A brief overview of the present disclosure will be given below to provide a basic understanding of certain aspects of the present disclosure. It should be understood that this overview is not an exhaustive overview of the present disclosure. It is not intended to identify the key or important parts of the present disclosure, nor is it intended to limit the scope of the present disclosure. Its purpose is merely to present certain concepts in a simplified form as a prelude to the more detailed description that follows.

[0007] The object of the present invention is to provide a non-contact measurement method for the diameter at breast height (DBH) of standing trees. Based on low-cost active and passive optical devices, it can be effectively applied in various forest stands. By integrating and calibrating two low-cost optical devices, namely a passive optical device such as an optical camera or image sensor of a smart terminal and an active optical device such as a laser rangefinder, and relying on the algorithm of the present invention, non-contact measurement of DBH can be achieved. The present invention makes full use of the characteristic advantages of active and passive optical devices. Firstly, the structure is simple, easy to integrate and only requires a small amount of calibration to perform non-contact measurement of DBH. Secondly, it can achieve DBH measurement in various forest stands without adding additional sensors, improving the measurement universality of the device. Therefore, the non-contact measurement mode of DBH achieved by the present invention is more simple and effective and can be effectively measured in complex terrains.

[0008] To solve the above technical problems, the present invention provides the following technical solutions:

[0009] The present invention provides a non-contact measurement method for the diameter at breast height of standing trees, comprising the following steps:

[0010] A non-contact measurement method for the diameter at breast height of standing trees, characterized by comprising the following steps:

[0011] Step 1: Assemble an active optical device and a passive optical device to obtain an integrated device, and fix it on a bracket. Among them, the passive optical device is used to obtain two-dimensional image information containing the target standing tree, and the active optical device is used to obtain the distance information from the integrated device to the target standing tree;

[0012] Step 2: Calibrate the integrated device to obtain basic parameter information of the device, including separately calibrating the passive optical device to obtain the internal parameter matrix of the passive optical device, and globally calibrating the integrated device to simultaneously obtain the relative distance between the ranging centers of the passive optical device and the active optical device inside the integrated device and the optical field angle of the passive optical device;

[0013] Step 3: Estimate the breast height position in the two-dimensional image considering the terrain conditions. Rotate the integrated device to the horizontal and aim at the target standing tree to obtain the two-dimensional image containing the target standing tree and the distance information from the integrated device to the target standing tree. The breast height position in the two-dimensional image is expressed by the following formula:

[0014]

[0015] where P dbh represents the breast height position in the two-dimensional image in pixels; H imageis the height of the two-dimensional image in pixels, which is a constant value when the shooting mode of the passive optical device is fixed; d is the distance value of the distance information obtained by the active optical device; α and β are respectively half of the optical field of view and the angle change of the integrated device from horizontal downward rotation to the bottom of the target standing tree; D0 is the relative distance between the passive optical device and the active optical device in the integrated device; and

[0016] Step 4: Calculate the breast diameter of the target tree. According to the breast height position in the two-dimensional image obtained by formula 3, crop the image into a rectangular area centered on the breast height position, establish a region of interest containing the breast height position of the trunk, and use the distance value d to identify and calculate the trunk width N in the region of interest in pixels, and establish a conversion relationship from the image two-dimensional space to the real three-dimensional space as expressed in formula 4.

[0017]

[0018] Where f is the focal length of the passive optical device, d is the distance value, N is the width of the tree trunk in pixels in the two-dimensional image, and d x is the calibrated single pixel spacing in image space, obtained by the separate calibration of the passive optical device in step 2, L pixel is the physical size of the target tree's DBH in the two-dimensional image space, D0 is the relative distance between the passive optical device and the active optical device in the integrated device, and DBH is the actual size of the target tree's DBH.

[0019] Furthermore, the passive optical device is selected from an optical camera or image sensor built into a mobile phone, tablet computer or laptop computer.

[0020] Furthermore, the active optical device includes a laser rangefinder.

[0021] Furthermore, the integrated device also includes an angle gyroscope, and the angle change β is obtained by rotating the integrated device downward from the horizontal direction to aim at the bottom of the target standing tree.

[0022] Furthermore, the angle gyroscope is an angle gyroscope built into a mobile phone, a tablet computer or a laptop computer.

[0023] Furthermore, the calibration of the optical field angle of the passive optical device in step 2 is based on the actual object height represented by the image at different distances. The actual height h represented by the image is determined by recording the specific positions corresponding to the vertical upper and lower boundaries of the current image on the calibration plate, and the optical field angle θ is determined in combination with the distance measurement value d obtained by calibration, which is specifically expressed by the following formula:

[0024]

[0025] Among them, d is the ranging value obtained during calibration; h is the actual distance represented by the pixel height of the captured image; θ is the optical field of view angle to be determined, which is expressed as the average value of multiple calibration experiments in actual calculation, and D0 is the relative distance between the passive optical device and the active optical device in the integrated device.

[0026] Furthermore, among them, the calculation of the relative distance D0 between the passive optical device and the active optical device is based on the camera imaging model, and is realized by obtaining the proportional relationship between the pixel length and the actual length of the calibration plate in the image, which is specifically expressed by the following formula:

[0027]

[0028] Among them, l is the pixel length of the calibration plate in the captured image; L is the fixed length of the calibration plate; f is the focal length of the passive optical device; d is the ranging value obtained during calibration; D0 is the relative distance between the passive optical device and the active optical device in the integrated device.

[0029] Compared with the existing DBH measurement methods, the beneficial effects of the present invention are: a new non-contact measurement method for DBH integrating passive optical devices such as smart terminals and active optical devices such as laser rangefinders is proposed. Compared with the existing DBH measurement methods derived from passive optical devices such as smart terminals, the measurement accuracy of the present invention is higher, which can meet the DBH measurement requirements under various stand conditions at the same time, has high measurement stability and is easy to use, greatly improving the efficiency and convenience of DBH measurement. Brief Description of the Drawings

[0030] The specific content of the present disclosure will be described below with reference to the drawings, which will help to more easily understand the above and other objects, features and advantages of the present disclosure. The drawings are only for showing the principle of the present disclosure. The dimensions and relative positions of the units do not have to be drawn to scale in the drawings.

[0031] Figure 1 It is a schematic diagram of the DBH calculation process of the present invention.

[0032] Figure 2 It is a schematic diagram of breast height position estimation under different terrains, Figure 2 a and Figure 2 b show schematic diagrams of breast height position estimation when the bottom of the tree trunk is visible in the two-dimensional image; Figures 2c and Figure 2 d show schematic diagrams of breast height position estimation when the bottom of the tree trunk is not visible in the two-dimensional image.

[0033] Figure 3 It is a top view schematic diagram of the DBH calculation model.

[0034] Figure 4It is a scatter plot comparing the measurement results of the present invention and the chest diameter circumference ruler and an equivalent test result graph based on regression. DETAILED DESCRIPTION

[0035] Exemplary disclosures of the present disclosure are described below in conjunction with the accompanying drawings. For the sake of clarity and conciseness, not all features of implementing the present disclosure are described in the specification. However, it should be understood that many decisions specific to the present disclosure may be made in the process of developing any such implementation of the present disclosure in order to achieve the developer's specific goals, and these decisions may vary from one disclosure to another.

[0036] It should also be noted here that in order to avoid obscuring the contents of the present disclosure due to unnecessary details, only the pipe network structure closely related to the scheme according to the contents of the present disclosure is shown in the accompanying drawings, while other details that are not closely related to the contents of the present disclosure are omitted.

[0037] It should be understood that the present disclosure is not limited to the described implementation forms due to the following description with reference to the accompanying drawings. Herein, where feasible, features between different embodiments may be replaced or borrowed, and one or more features may be omitted in one embodiment.

[0038] The present invention is a method for non-contact measurement of DBH based on active and passive optical devices, which uses an intelligent terminal with a passive optical device and an active optical device to obtain images and distance information to a target tree, i.e., depth information, respectively in field measurement, and determines the chest height position in combination with an angle gyroscope, so that DBH can be calculated. Preferably, the intelligent terminal in the present invention can be any device including a passive optical device, such as a mobile phone, a tablet computer or a laptop computer, etc., and the passive optical device is preferably an optical camera or an image sensor, etc., the active optical device is preferably a laser rangefinder or other device with similar functions, and the angle gyroscope is preferably provided by the intelligent terminal, or it can be a separate angle gyroscope.

[0039] See also Figure 1 , is a schematic diagram of the calculation flow of DBH non-contact measurement of the present invention, and the following steps should be followed:

[0040] Step 1: Select appropriate active optical devices and passive optical devices for assembly to obtain an integrated device to ensure the stability of the measurement process. Specifically, the smart terminal with the passive optical device and the active optical device are assembled and fixed on a bracket such as a tripod. Preferably, by fixing the smart terminal on the bracket and ensuring that the smart terminal is vertical, and also fixing the laser rangefinder on the bracket and keeping the rangefinder horizontal, a certain relative distance is maintained between the two devices and obtained through calibration in the later step 2 to ensure the vertical shooting and horizontal ranging of the optical camera of the integrated device during the later measurement process, thereby realizing the integration of the device. It is further preferred that the active optical device and the smart terminal are fixed on the connection platform according to the preset card position, and further fixed on a common tripod gimbal equipped with a rotating platform to achieve the preliminary integration of the DBH non-contact measurement device. Among them, the optical camera or image sensor of the smart terminal is used as a passive optical device to obtain two-dimensional image information containing the target tree, and the laser rangefinder is used as an active optical device to obtain the distance information from the integrated device to the surface of the target tree, that is, the depth information, and then the above two types of data information are transmitted as input data to the solution program to achieve non-contact measurement of DBH. Among them, the passive optical device is preferably an optical camera or image sensor that comes with the smart terminal, and the active optical device is preferably a laser rangefinder or other devices with similar functions. Furthermore, the integrated device also includes an angle gyroscope, which is preferably provided by the smart terminal, or it can be an assembled separate angle gyroscope.

[0041] Step 2: Calibrate the integrated device to obtain basic parameter information of the device.

[0042] like Figure 1 As shown in FIG. 1 , the calibration of the integrated device is mainly used to obtain three calibration results: the intrinsic parameter matrix of the passive optical device, the relative distance D0 between components, and the optical field angle θ of the passive optical device. According to the implementation process, it is specifically divided into the following two steps: 1) The passive optical device of the smart terminal is calibrated separately to obtain the intrinsic parameter matrix of the passive optical device, specifically, to obtain the single pixel spacing d in the image space x ; 2) calibrate the integrated device as a whole, and finally obtain the relative distance D0 between the ranging centers of the passive optical device and the active optical device of the smart terminal inside the integrated device and the optical field of view angle θ of the passive optical device of the smart terminal.

[0043] In the separate calibration step for passive optical devices, a checkerboard calibration plate with known size is mainly used in combination with the traditional Zhang Zhengyou camera calibration method to obtain an intrinsic parameter matrix including the physical size of a single pixel in the image, thereby obtaining the single pixel spacing d in the image space. x。In the method for calibrating the integrated device as a whole, calibration experiments for the optical field of view and the relative distance between the two components are carried out by simultaneously placing two calibration plates with known sizes on a vertical wall. During the actual experiment, the integrated device is placed on a tripod rotating platform to keep it horizontal, aligned with the two calibration plates for taking pictures and laser ranging, and the distance between the integrated device and the calibration plates is continuously changed for multiple measurements. The calculation of the relative distance between the passive optical device and the active optical device is based on the camera imaging model, and is realized by obtaining the proportional relationship between the pixel length and the actual length of the calibration plate in the image, specifically expressed by the following formula:

[0044]

[0045] where, l is the pixel length of the calibration plate in the captured image; L is the fixed length of the calibration plate; f is the focal length of the passive optical device of the intelligent terminal; d is the ranging value obtained by the active optical device during calibration; D0 is the relative distance between the passive optical device and the active optical device in the integrated device. In the actual experiment, L and f are fixed known values, d is measured and calculated by the active optical device, l is obtained by manually selecting the left and right edges of the calibration plate in the image and calculating the pixel distance between the two points, and the average value is taken after multiple experiments to obtain the relative distance D0 between the ranging centers of the passive optical device and the active optical device of the intelligent terminal inside the integrated device. The calibration of the optical field of view of the passive optical device is based on the actual object height represented by the image at different distances. The actual height h represented by the image is determined by recording the specific positions corresponding to the vertical upper and lower boundaries of the currently captured image on the calibration plate, and the optical field of view angle θ is determined in combination with the ranging value d, specifically expressed by the following formula:

[0046]

[0047] where, d is the ranging value obtained during calibration; h is the actual distance represented by the pixel height of the captured image; θ is the optical field of view angle of the passive optical device of the intelligent terminal to be obtained, and this value is usually the average value of multiple calibration experiments in actual calculation.

[0048] Step 3: Estimation of breast height position in the two-dimensional image considering terrain conditions.

[0049] Such as Figure 1As shown in the figure, breast height estimation in two-dimensional images is a necessary prerequisite for DBH measurement, and it is necessary to accurately estimate the breast height position of the target tree in any natural scene. By rotating the integrated device to a horizontal position and aiming at the target tree, a two-dimensional image containing the target tree and the distance information from the device to the trunk surface are obtained, and the breast height position is determined according to the angle change during the rotation of the device by effectively utilizing the built-in angle gyroscope of the smart terminal. Specifically, in natural forests with large terrain undulations, the positional relationship between the integrated device and the target tree can be divided into four cases according to the difference in elevation and slope shape between the two. As Figure 2 As shown in the figure, four cases are listed respectively considering two different slope shapes and the difference between the integrated device and the target tree elevation. Finally, a unified expression for determining the breast height position is established according to the angle change of the integrated device, realizing the effective estimation of the breast height position in DBH measurement. When the slope shape is divided into ascending or descending, the actual breast height position is the tree diameter 130mm above the bottom of the trunk. When the bottom of the tree is visible in the obtained two-dimensional image ( Figure 2 (a) and (b)), with the bottom of the tree as the reference, the chest height position in the two-dimensional image is 130 mm above the bottom of the tree in the image; when the passive optical device does not capture the complete tree, that is, the bottom of the tree is not visible in the obtained two-dimensional image ( Figure 2 (c) and (d)), with the bottom of the tree as the reference, the chest height position in the two-dimensional image is 130 mm above the bottom of the tree that is not visible in the two-dimensional image. In actual measurement, the integrated device will rotate downward from the horizontal to aim at the bottom of the tree each time to obtain the longitudinal angle change β of the angle gyroscope. The angle gyroscope is preferably provided by the smart terminal, or it can be a separately assembled integrated angle gyroscope. Combined with the distance measurement value and the optical field angle calibrated in step 2, the chest height position P in pixels in the two-dimensional image is estimated. dbh (i.e. the number of pixels included). Based on Figure 2 The geometric transformation relationship shown in the figure, the chest height position in pixels in the two-dimensional image, that is, the number of pixels included in the bottom of the image and the chest height position, can be uniformly expressed as the following formula:

[0050]

[0051] Among them, P dbh Indicates the chest height position in the two-dimensional image in pixels; H image is the height of the two-dimensional image in pixels (i.e., the number of pixels included), which is a fixed value when the passive optical device shooting mode is fixed; d is the ranging value of the active optical device; α and β are half of the optical field angle and the angle change of the angle gyroscope, respectively. In actual measurement, only the angle change β of the angle gyroscope and the ranging value d are variables, and the other parameters are fixed values ​​after the equipment calibration is completed.

[0052] Step 4: Calculate the diameter at breast height (DBH) of the target standing tree.

[0053] The calculation of the DBH of the target standing tree is based on the imaging principle of passive optical devices, and realizes the non-contact measurement of the DBH of the target standing tree by establishing the conversion relationship from the two-dimensional image space to the real three-dimensional space. As Figure 1 shown, according to the breast height position accurately estimated by formula 3 considering the terrain influence, the image is cropped into a rectangular area centered on the breast height position, an interested area containing the breast height position of the tree trunk is established, and the distance information d obtained by the active optical device is integrated and the tree trunk width N in pixels in the interested area is identified and calculated, and the conversion relationship from the two-dimensional image space of the image to the real three-dimensional space as shown in formula 4 is established. For a single measurement as Figure 3 shown, the ratio of the ranging value d to the focal length f of the passive optical device is regarded as the fixed proportional relationship in the current space conversion process. After calculating the specific value L of the DBH in the two-dimensional image space through the image processing algorithm pixel the true value of the DBH of the target standing tree in the three-dimensional space can be determined according to the above fixed proportional relationship, and the specific calculation formula is expressed as:

[0054]

[0055] where f is the focal length of the passive optical device; d is the ranging value, representing the distance information from the device to the target tree trunk; N is the tree trunk width in pixels in the two-dimensional image, obtained by segmenting the tree trunk area through the image processing algorithm; d x is the calibrated single-pixel pitch in the image space, obtained when obtaining the internal parameter matrix through the individual calibration of the passive optical device in step 2; L pixel is the physical size of the DBH in the two-dimensional image space, and DBH is the actual size of the diameter at breast height of the target standing tree.

[0056] Specific measurement example:

[0057] In the actual field measurement process, as Figure 1 shown, after the assembly and integration and calibration of the device are completed, when measuring a single standing tree in the target quadrat in the field, first rotate the integrated device to the horizontal and aim at the target standing tree to obtain the two-dimensional image containing the target standing tree and the distance information from the integrated device to the standing tree; then, based on the breast height position estimation method in the two-dimensional image described in step 3, rotate the integrated device downward from the horizontal to aim at the bottom of the standing tree again, and obtain the device angle change information through the angle gyroscope, that is, the gyroscope angle change β, and automatically estimate the position of the breast height in the two-dimensional image; after the above operations are completed, calculate the DBH based on the DBH calculation method of the target standing tree in step 4 to obtain the actual DBH of the target standing tree, and finally complete the non-contact measurement of the DBH of the target standing tree.

[0058] Specifically, measurement experiments were carried out in natural mountain forests with large topographic undulations, artificial forests with relatively flat terrains, and an urban park forest according to the above steps. The main tree species in the mountain forest is Chinese pine, accompanied by a certain number of Oriental arborvitae and Populus canadensis, etc. A total of 206 standing trees were measured; the terrain in the artificial forest is flat and the tree species composition is single, and the main tree species is Populus euramericana. Finally, 128 standing trees were measured; finally, 37 standing trees in the mixed forest composed of Koelreuteria paniculata, Cinnamomum camphora, and Oriental arborvitae were measured in the urban park forest. When measuring based on this device, the breast height diameter tape was also synchronously applied for measurement as a reference value to evaluate and compare the measurement accuracy. The accuracy evaluation method uses six dimensions: mean absolute error (MAE), relative mean error (reBias), root mean square error (RMSE), relative root mean square error (reRMSE), standard deviation (SD), and minimum / maximum error. At the same time, a regression-based equivalence test was also used to verify the accuracy of this method.

[0059] As can be seen from Table 1, the accuracy evaluation results of DBH measurement using the non-contact measurement method of the present invention are shown. Among them, MAE is 1.12 cm (reBias = -0.23%), RMSE is 1.55 cm (reRMSE = 6.64%), and SD is 1.07 cm, which all indicate the good measurement accuracy of the measurement method of the present invention. In addition, in the Figure 4 results of the regression-based equivalence test as shown, the 95% confidence intervals of the intercepts are all included within the ±10% intercept equivalence intervals, indicating that the null hypothesis of a deviation in the measurement results can be rejected and the measurement mean is equivalent to the reference value; and the 95% confidence intervals of the slopes are also all included within the ±10% slope equivalence intervals, which indicates that the proportional null hypothesis is accepted and the measured values and the reference values have good consistency.

[0060] Table 1 Comparison of measurement results between this method and breast height diameter tape

[0061]

[0062] The above specific embodiments are only for explaining the technical concept and structural features of the present invention, aiming to enable those skilled in the relevant art to implement it accordingly. However, the above content does not limit the protection scope of the present invention. Any equivalent changes or modifications made according to the technical characteristics of the present invention shall fall within the protection scope of the present invention.

[0063] The above describes the present disclosure in combination with specific implementation schemes, but those skilled in the art should understand that these descriptions are exemplary and not a limitation on the protection scope of the present disclosure. Those skilled in the art can make various variations and modifications to the present disclosure according to the spirit and principle of the present disclosure, and these variations and modifications are also within the scope of the present disclosure.

Claims

1. A non-contact method for measuring the diameter at breast height of standing trees, characterized in that, The following steps are involved: Step 1: Assemble the active optical device and the passive optical device to obtain an integrated device, and fix it on a bracket, wherein the passive optical device is used to obtain two-dimensional image information containing the target standing tree, and the active optical device is used to obtain the distance information from the integrated device to the target standing tree; Step 2: calibrate the integrated device to obtain basic parameter information of the device, including calibrating the passive optical device separately to obtain the internal parameter matrix of the passive optical device, calibrating the integrated device as a whole, and obtaining the relative distance between the ranging center of the passive optical device and the active optical device inside the integrated device and the optical field of view of the passive optical device; Step 3: Consider the estimation of the breast height position in the two-dimensional image of the terrain conditions, rotate the integrated device to the horizontal and aim at the target tree, obtain the two-dimensional image containing the target tree and the distance information from the integrated device to the target tree, and the breast height position in the two-dimensional image is expressed as the following formula: where P dbh represents the breast height position in the two-dimensional image in pixels; H image is the height of the two-dimensional image in pixels, which is a fixed value when the shooting mode of the passive optical device is fixed; d is the ranging value of the distance information obtained by the active optical device; α and β are respectively half of the optical field of view angle and the angle change amount of the integrated device rotating from horizontal downward to aim at the bottom of the target standing tree; D0 is the relative distance between the passive optical device and the active optical device in the integrated device; and Step 4: Calculate the breast diameter of the target tree. According to the breast height position in the two-dimensional image obtained by formula 3, crop the image into a rectangular area centered on the breast height position, establish a region of interest containing the breast height position of the trunk, and use the distance value d to identify and calculate the trunk width N in the region of interest in pixels, and establish a conversion relationship from the image two-dimensional space to the real three-dimensional space as expressed in formula 4. Among them, f is the focal length of the passive optical device, d is the ranging value, N is the trunk width in pixels in the two-dimensional image, d x is the calibrated single-pixel pitch in the image space, obtained by the individual calibration of the passive optical device in step two, L pixel is the physical size of the breast diameter of the target standing tree in the two-dimensional image space, D0 is the relative distance between the passive optical device and the active optical device in the integrated device, and DBH is the actual size of the breast diameter of the target standing tree.

2. The measuring method according to claim 1, wherein, The passive optical device is selected from an optical camera or image sensor provided in a mobile phone, tablet computer or notebook computer.

3. The measuring method according to claim 1, wherein, The active optical device includes a laser rangefinder.

4. The measuring method according to claim 1, wherein The integrated device also includes an angle gyroscope, and the angle change β is obtained by rotating the integrated device downward from the horizontal to the bottom of the target standing tree.

5. The measuring method according to claim 4, wherein The angle gyroscope is an angle gyroscope built into a mobile phone, a tablet computer or a notebook computer.

6. The measurement method according to claim 1, wherein, The calibration of the optical field of view angle of the passive optical device in step 2 is based on the actual object height represented by the image at different distances. The actual height h represented by the image is determined by recording the specific positions corresponding to the vertical upper and lower boundaries of the current image on the calibration plate, and the optical field of view angle θ is determined in combination with the distance measurement value d obtained by calibration. It is specifically expressed by the following formula: Wherein, d is the distance value obtained during calibration; h is the actual distance represented by the pixel height of the captured image; θ is the desired optical field of view, which is expressed as the average value of multiple calibration experiments in actual calculations; and D0 is the relative distance between the passive optical device and the active optical device in the integrated device.

7. The measuring method according to claim 1, wherein The calculation of the relative distance D0 between the passive optical device and the active optical device is based on the camera imaging model and is achieved by obtaining the proportional relationship between the pixel length of the calibration plate in the image and the actual length, which is specifically expressed by the following formula: Wherein, l is the pixel length of the calibration plate in the captured image; L is the fixed length of the calibration plate; f is the focal length of the passive optical device; d is the distance value obtained during calibration; D0 is the relative distance between the passive optical device and the active optical device in the integrated device.

Images