Monitoring device and manhole cover

Abstract

A monitoring device (100) comprises: a control unit (10) that acquires an image (31) which is obtained by synthetic-aperture radar and which includes a manhole cover, estimates a degree of wear of the manhole cover on the basis of the luminance of the image (31), and generates estimated data (11) indicating the degree of wear; and an output unit (40) that outputs the estimated data (11) generated by the control unit (10).

Description

Monitoring equipment and manhole covers

[0001] The present disclosure relates to a monitoring device and a manhole cover.

[0002] Manholes are holes installed for the maintenance of underground structures. They are usually covered with a highly rigid metal cover such as cast iron. Because manhole covers are installed on roadways or sidewalks, the covers themselves must be kept in good condition to prevent damage to vehicles or pedestrians. Normally, manhole covers have uneven surfaces that prevent vehicles or pedestrians from slipping, but over time these uneven surfaces tend to wear down and become slippery. When the cover becomes worn, the company that installed it is required to take appropriate measures, such as replacing it, to ensure safe passage.

[0003] To assess the degree of wear, operators conduct periodic inspections. While inspections are typically performed visually, regularly checking the condition of manhole covers across the country is time-consuming. To address this issue, manhole covers have been developed with a two-tiered pattern that enhances visibility of wear (Non-Patent Document 1). Image processing technology is also known that enables efficient inspections by measuring the degree of wear from images captured by an onboard camera (Non-Patent Document 2).

[0004] “A new manhole cover design has been released that is resistant to wear and easy to inspect at a glance – the Tapered Diameter Manhole Cover receives the 2017 Good Design Award”, NTT, [online], [Retrieved June 7, 2024], Internet <URL: https: / / group.ntt / jp / newsrelease / 2017 / 10 / 04 / 171004a.html> Kazuhiko Murasaki et al., Manhole Cover Deterioration Estimation Technology for Efficient Inspection Using an In-Vehicle Camera, NTT Technical Journal, 2018, Vol. 30 No. 6, pp. 14-18

[0005] The techniques described in Non-Patent Documents 1 and 2 are techniques for improving the efficiency of inspections of manhole cover wear. However, these techniques require a person or a vehicle to go to the location where the manhole cover is installed, and an inspector must be dispatched to the site, so inspection costs cannot be fundamentally improved in efficiency.

[0006] The object of the present disclosure, made in consideration of such circumstances, is to efficiently inspect the wear of manhole covers without requiring inspectors to regularly visit the site.

[0007] A monitoring device according to one embodiment includes a control unit that acquires an image including a manhole cover obtained by a synthetic aperture radar, estimates the degree of wear of the manhole cover based on the brightness of the image, and generates estimated data indicating the degree of wear; and an output unit that outputs the estimated data generated by the control unit.

[0008] In one embodiment, the manhole cover has a convex portion that includes a first layer exposed upward and a second layer below the first layer that has a lower microwave absorption rate than the first layer and a higher microwave reflectance rate than the first layer.

[0009] According to the present disclosure, it becomes possible to efficiently inspect the wear of manhole covers without the need for inspectors to regularly visit the site.

[0010] Fig. 2 is a schematic diagram of a general manhole cover; Fig. 3 is a schematic diagram of a manhole cover according to one embodiment; Fig. 4 is a schematic diagram showing a state in which the manhole cover of Fig. 2 has become worn; Fig. 5 is a block diagram showing the configuration of a monitoring device according to one embodiment; Fig. 6 is a flowchart showing the operation of a monitoring device according to one embodiment.

[0011] In recent years, the number of Earth observation satellites launched has increased, and the types of observations they perform are becoming more diverse. One of the benefits of satellite-based Earth observation is that it allows us to understand the condition of the Earth's surface without having to deploy inspectors on-site. Therefore, this disclosure proposes a system that uses satellites to check the wear of manhole covers.

[0012] Some earth observation satellites are equipped with synthetic aperture radar (SAR). Synthetic aperture radar is an active radio wave sensor that emits microwaves toward the Earth and receives reflected waves to observe the physical properties, undulations, unevenness, or slope of the Earth's surface. Synthetic aperture radar is also an all-weather sensor that is virtually unaffected by weather conditions such as clouds or rain, regardless of day or night. There are various types of earth observation satellites, and they are used differently depending on the application. In this disclosure, synthetic aperture radar is used to estimate the degree of wear of a manhole cover.

[0013] The most important factor in selecting a satellite equipped with a synthetic aperture radar is the observation frequency. The higher the observation frequency, the lower the image resolution, making it possible to capture the state of the Earth's surface in detail. Among the observation frequencies, the X-band is a frequency band with particularly low resolution that is suitable for understanding the state of man-made objects. Satellites equipped with sensors with a resolution of 1 m have been launched, and in recent years there have been successful examples of imaging with a resolution of 46 cm, and technological development is progressing. Therefore, this disclosure is premised on the use of a synthetic aperture radar satellite equipped with a sensor using the X-band observation frequency.

[0014] When observing the Earth's surface using a synthetic aperture radar satellite, a monochrome image is obtained. The white areas are areas where the electromagnetic waves emitted from the synthetic aperture radar are strongly reflected by the Earth's surface. The black areas are areas where the electromagnetic waves are weakly reflected. The condition of the Earth's surface can be estimated from the strength of this reflection. It is known that the conditions for reflection are related to the electromagnetic waves and the conductivity of the surface material, as well as the roughness of the surface material.

[0015] Here, the wavelength of the X-band is approximately 25-37 mm. On the other hand, the unevenness provided on manhole covers to prevent slipping is on the order of centimeters, which is significantly larger than the wavelength of the X-band. Therefore, when electromagnetic waves with an X-band wavelength are irradiated onto a manhole cover made of highly conductive cast iron, scattered reflection occurs due to the surface roughness of the manhole cover.

[0016] This scattered reflection is observed by a synthetic aperture radar satellite, so in synthetic aperture radar images, manhole covers with normal irregularities appear white. On the other hand, if a manhole cover is worn, the irregularities disappear and there is no surface scattering on the manhole cover, resulting in a state close to specular reflection. In this case, electromagnetic waves are not reflected back to the synthetic aperture radar sensor, so the worn manhole cover appears black in synthetic aperture radar images.

[0017] The scattering state of a manhole cover changes between normal and worn conditions, and by utilizing this, the degree of wear of the manhole cover can be determined from the change in brightness (luminance) of the synthetic aperture radar image. The following explains the structural and image processing innovations of the manhole cover.

[0018] FIG. 1 shows a schematic diagram of a manhole cover 1000 made of a metal such as a typical cast iron. The manhole cover 1000 may be a conventional manhole cover. When observing the manhole cover 1000 with a synthetic aperture radar, the incident wave is indicated by a dotted line, and the reflected wave is indicated by a solid line. If the manhole cover 1000 has no irregularities, the incident wave is scattered and reflected by the surface of the manhole cover 1000. Therefore, as described above, a new manhole cover 1000 appears white (high brightness) in the synthetic aperture radar observation image. If the manhole cover 1000 has worn down and no longer has any irregularities, the surface of the manhole cover 1000 no longer scatters (is almost specularly reflected), and the worn manhole cover appears black (low brightness) in the observation image. Below, a method for checking the wear of a manhole cover using a synthetic aperture radar image will be described, taking advantage of the fact that the scattering state changes depending on whether or not the manhole cover 1000 has irregularities.

[0019] Formally, surface scattering occurs due to the unevenness of the surface of the manhole cover 1000 when the following condition is satisfied: σ≧λ / (8cosθ) (Equation 1). In Equation 1, σ is the surface roughness, and may be the standard deviation of the unevenness from a reference plane (datum plane) where the average height of the top surface of the manhole cover 1000 is set to 0. λ is the wavelength of the incident wave, and θ is the angle of incidence of the microwave from the synthetic aperture radar (called the off-nadir angle in synthetic aperture radar).

[0020] If the uneven shape of the manhole cover 1000 when it is healthy is known, it is possible to calculate the height of the upper surface of the convex parts at which scattering no longer occurs due to wear. Based on the brightness of the manhole cover 1000 in the image acquired by the synthetic aperture radar, the user can know to what extent the manhole cover 1000 has worn down (degree of wear). More specifically, if it is determined from the brightness of the manhole cover 1000 that scattering is occurring on the surface of the manhole cover 1000, it can be inferred that the degree of wear is small. If it is determined from the brightness of the manhole cover 1000 that scattering is not occurring on the surface of the manhole cover 1000 (almost specular reflection is occurring), it can be inferred that the degree of wear is large.

[0021] Furthermore, if the resolution of the synthetic aperture radar image is low, the manhole cover 1000 will not appear in the synthetic aperture radar image. In order to estimate the degree of wear of the manhole cover 1000, it is thought that a resolution of at least ½ or less, and preferably ¼ or less, of the diameter of the manhole cover is required.

[0022] In the above method, the detectable wear depth is determined by the observed wavelength λ of the incident wave, the difference in the unevenness of the manhole cover, and the incident angle θ of the incident wave. Therefore, it is not possible to determine the desired wear height that an operator wants to set as a maintenance standard. As an example that solves this problem, a manhole cover 2000 structure is proposed, as shown in Figure 2. The manhole cover 2000 includes a first layer 2110, which is a microwave absorbing layer exposed above the convex portion 2100 of the manhole cover 2000. The microwave absorbing layer may be a layer composed of a material with a high loss coefficient, which absorbs microwaves and converts them into thermal energy. The absorption principle utilizes microwave-absorbing materials such as magnetic materials such as ferrite, carbon-based materials such as carbon nanotubes and graphene, or conductive polymers. The first layer 2110 may also be composed of other microwave-absorbing materials. Microwaves may be absorbed by incorporating other microwave-absorbing materials into the metal matrix. Furthermore, a paint containing the above material may be applied to the metal substrate. The first layer 2110 may be provided only on a portion of the convex portion 2100.

[0023] Below the first layer 2110 is a second layer 2120 that has a lower microwave absorption rate and a higher microwave reflectance rate than the first layer 2110. The material of the second layer 2120 may be the same as the material of the manhole cover 1000. The second layer 2120 may be made of, for example, cast iron.

[0024] The configuration of the manhole cover 2000 other than the convex portion 2100 may be the same as that of the conventional manhole cover 1000.

[0025] When the first layer 2110 is present, the incident waves are absorbed by the first layer 2110, and no scattered reflection occurs. Therefore, in synthetic aperture radar observation images, the manhole cover 2000 having the first layer 2110 appears black (high brightness). Then, as wear progresses and the wear progresses beyond the height of the underside of the first layer 2110, the first layer 2110 disappears, and the second layer (bare material) 2120 appears ( FIG. 3 ). At this time, the second layer 2120, which is made of a metal that easily reflects the incident waves, becomes exposed, and scattered reflection occurs, as with a typical manhole cover 1000. Therefore, in synthetic aperture radar observation images, the manhole cover with the second layer 2120 exposed appears white (low brightness). That is, when the manhole cover 2000 is in good condition (with little wear), it appears black in the synthetic aperture radar image, but when it is worn, it appears white. This makes it possible to measure the degree of wear using the synthetic aperture radar image. This method is based on the premise that the following relationship is satisfied: H1>H2 (Equation 2). Here, H1 is the wear reference height (see FIG. 2). When the convex portion 2100 of the manhole cover 2000 falls below the wear reference height, it is recommended that the manhole cover 2000 be replaced. The height of the underside of the first layer 2110 may be H1. H2 is the minimum value of the difference in height between the convex and concave portions that satisfies Equation 1. By periodically observing the manhole cover 2000 with a synthetic aperture radar, it is necessary to detect a change in the synthetic aperture radar image from black to white. This detection may be performed by comparing the brightness of the image with a threshold value.

[0026] An embodiment of the monitoring device will now be described with reference to the drawings.

[0027] In each drawing, the same or corresponding parts are denoted by the same reference numerals. In the description of this embodiment, the description of the same or corresponding parts will be omitted or simplified as appropriate.

[0028] The configuration of the monitoring device 100 according to this embodiment will be described with reference to FIG.

[0029] The monitoring device 100 includes a control unit 10 and an output unit 40. The monitoring device 100 may also include a database 20 and an input unit 30. The monitoring device 100 is, for example, a server. The monitoring device 100 may be located in a data center or a management office in a manhole.

[0030] The input unit 30 acquires an image 31 including a manhole cover, obtained by a synthetic aperture radar (step S1 in FIG. 5 ). The image 31 may be captured by a synthetic aperture radar satellite 1. The synthetic aperture radar satellite 1 may send the captured image 31 to the antenna 2. The input unit 30 may receive the image 31. Note that a user of the monitoring device 100 may connect a medium on which the image 31 is stored to the input unit 30.

[0031] The image 31 may include location information. The location information is, for example, latitude and longitude. The image 31 may include a shooting scale. Digital elevation model data may be applied to the image 31. As shown in FIG. 2 , the image 31 may include information indicating whether the manhole cover includes a first layer 2110 exposed upward and a convex portion 2100 below the first layer 2110, the convex portion 2100 including a second layer 2120 having a microwave absorption rate lower than that of the first layer 2110 and a microwave reflectivity higher than that of the first layer 2110. Hereinafter, this information will be referred to as "microwave-absorbing cover information." The image 31 may include the wavelength λ of the incident wave of the synthetic aperture radar that captured the image 31 and the incident angle θ of the incident wave.

[0032] The database 20 may store information about the manhole cover. More specifically, the database 20 may store roughness data 21 about the manhole cover. The roughness data 21 indicates the initial surface roughness σ of the manhole cover. As described above, the surface roughness σ may be the standard deviation of the irregularities from a reference plane (datum plane) in which the average height of the top surface of the manhole cover is set to 0. The database 20 may also store positional information about the manhole cover.

[0033] The database 20 may store images including manhole covers acquired by the synthetic aperture radar in the past as past images 22. The database 20 may store multiple past images 22 of the same manhole cover acquired by the synthetic aperture radar, for example, on a yearly basis. The past images 22 may be images 31 accepted by the input unit 30.

[0034] The database 20 may include, for example, a semiconductor memory or an optical memory. The database 20 may include an electromagnetic storage medium such as a magnetic disk. The database 20 may include a communication module configured to be able to communicate with the control unit 10.

[0035] The control unit 10 estimates the degree of wear of the manhole cover based on the brightness of the image 31 input to the input unit 30 (step S2 in FIG. 5). The control unit 10 then generates estimated data 11 indicating the degree of wear of the manhole cover (step S3). The brightness of the image 31 corresponds to the intensity of the reflected wave. The range of the intensity of the reflected wave represented by the brightness of the image 31 is, for example, 0 to 255 decibels.

[0036] The degree of wear of the manhole cover may be expressed by the amount of reduction in the height of the convex portion of the manhole cover. The degree of wear may also be expressed by the surface roughness σ of the manhole cover.

[0037] The control unit 10 may estimate the degree of wear of the manhole cover based on the presence or absence or degree of scattering on the surface of the manhole cover. If the brightness of the image 31 is equal to or greater than a predetermined first threshold, it may be determined that scattering is occurring on the surface of the manhole cover (see FIG. 1 ), and it may be inferred that the wear of the manhole cover is small. If the brightness of the image 31 is less than the predetermined first threshold, it may be determined that scattering is not occurring on the surface of the manhole cover (almost all specular reflection is occurring), and it may be inferred that the wear of the manhole cover is large. The control unit 10 may estimate the wear of the manhole cover in more detail based on the brightness of the image 31.

[0038] If the image 31 includes the wavelength λ and the angle of incidence θ of the incident wave of the synthetic aperture radar, the control unit 10 may further estimate the surface roughness σ of the manhole cover at the time the image 31 was acquired. For example, if the control unit 10 estimates that scattering is occurring on the surface of the manhole cover, the control unit 10 may estimate that the surface roughness σ is σ > λ / (8 cos θ). The control unit 10 may also obtain the surface roughness σ in more detail.

[0039] The control unit 10 may acquire roughness data 21 indicating the initial surface roughness of the manhole cover, which is stored in the database 20. The control unit 10 may further estimate the degree of wear of the manhole cover based on the roughness data 21.

[0040] The control unit 10 may generate an image showing the degree of wear of the manhole cover at the position of the manhole cover as the estimated data 11. When generating the image, the position information of the image 31 may be used. The image 31 may be overlaid on an aerial photograph or a map to generate the image as the estimated data 11. The generated image allows the user to more easily visually identify the position of the worn manhole. The output unit 40, which will be described later, may include geographic information system software.

[0041] The control unit 10 may acquire past images 22 including the manhole cover that were previously acquired by the synthetic aperture radar and stored in the database 20. The control unit 10 may estimate the degree of wear of the manhole cover based further on the brightness of the past images 22.

[0042] For example, suppose the brightness of past image 22 was less than a predetermined first threshold value and the brightness of image 31 was equal to or greater than a predetermined first threshold value. In this case, control unit 10 may infer that first layer 2110 was present in manhole cover 2000 at the time past image 22 was obtained and that first layer 2110 had disappeared from manhole cover 2000 at the time image 31 was obtained. Furthermore, control unit 10 may make the above-described inference in the above case when the microwave-absorbing cover information for image 31 indicates that manhole cover 2000 is provided with convex portion 2100 below first layer 2110, the convex portion including second layer 2120 having a microwave absorption rate lower than that of first layer 2110 and a microwave reflectivity higher than that of first layer 2110. By having control unit 10 infer the degree of wear based further on the microwave-absorbing cover information, the degree of wear of the manhole cover can be accurately inferred, for example, even if the inference at the time past image 22 or image 31 was obtained was incorrect.

[0043] Furthermore, suppose that the brightness of past image 22 obtained at a first time point is less than a predetermined first threshold, the brightness of past image 22 obtained at a second time point after the first time point is equal to or greater than the predetermined first threshold, and the brightness of image 31 is less than the predetermined first threshold. In this case, in addition to the above-mentioned inference, control unit 10 may infer that no scattering was occurring on the surface of manhole cover 2000 at the time image 31 was obtained. In other words, control unit 10 may infer that first layer 2110 had disappeared and second layer 2120 had also worn away at the time image 31 was obtained. In this case, control unit 10 may issue a warning that convex portion 2100 of manhole cover 2000 is significantly worn away.

[0044] The control unit 10 may align the past image 22 obtained at the first time point with the past image 22 or image 31 obtained at the second time point, and then calculate the difference. For example, the control unit 10 may calculate the difference in brightness between these images. The control unit 10 may apply a color corresponding to the magnitude of the difference to the image as the estimated data 11. The control unit 10 may highlight the difference based on the magnitude of the difference. For example, a manhole cover with a large difference may be displayed in red as a manhole cover that requires attention. On the other hand, a manhole cover with a small difference may be displayed in white. These configurations allow the user to more easily check the degree of wear of the manhole cover.

[0045] The control unit 10 may include a database management system that manages the database 20 .

[0046] The control unit 10 includes at least one processor, at least one programmable circuit, at least one dedicated circuit, or any combination thereof. The processor is a general-purpose processor such as a CPU or GPU, or a dedicated processor specialized for specific processing. "CPU" is an abbreviation for central processing unit. "GPU" is an abbreviation for graphics processing unit. An example of the programmable circuit is an FPGA. "FPGA" is an abbreviation for field-programmable gate array. An example of the dedicated circuit is an ASIC. "ASIC" is an abbreviation for application specific integrated circuit.

[0047] The output unit 40 outputs the inferred data 11 generated by the control unit 10 (step S4 in FIG. 5). The output unit 40 includes at least one output interface. The output interface is, for example, a display. The display is, for example, a liquid crystal display (LCD) or an organic electroluminescent (EL) display. The output unit 40 may be connected to the monitoring device 100 as an external output device. As a connection method, any method such as a universal serial bus (USB), a high-definition multimedia interface (HDMI) (registered trademark), or Bluetooth (registered trademark) can be used.

[0048] The control unit 10, the input unit 30, and the output unit 40 may each individually or collectively include at least one communication module. The communication module is, for example, a module that complies with a wired LAN communication standard such as Ethernet (registered trademark) or a wireless LAN communication standard such as IEEE 802.11. "LAN" is an abbreviation for local area network. "IEEE" is an abbreviation for Institute of Electrical and Electronics Engineers.

[0049] The monitoring device 100 of the present disclosure can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided over a network. For example, the functions of the monitoring device 100 are realized by executing a program according to this embodiment on a processor serving as the control unit 10. That is, the functions of the monitoring device 100 are realized by software. The program causes a computer to execute the operations of the monitoring device 100, thereby causing the computer to function as the monitoring device 100. That is, the computer functions as the monitoring device 100 by executing the operations of the monitoring device 100 in accordance with the program.

[0050] The program can be stored on a non-transitory computer-readable medium. Examples of the non-transitory computer-readable medium include flash memory, magnetic recording devices, optical disks, magneto-optical recording media, and ROMs. The program can be distributed by selling, transferring, or lending portable media such as SD cards, DVDs, or CD-ROMs that store the program. "SD" is an abbreviation for Secure Digital. "DVD" is an abbreviation for digital versatile disc. "CD-ROM" is an abbreviation for compact disc read only memory. The program can also be distributed by storing it in the storage of a server and transferring it from the server to another computer. The program can also be provided as a program product.

[0051] A computer temporarily stores a program stored on a portable medium or transferred from a server in its main storage device. The computer then reads the program stored in the main storage device with its processor and executes processing in accordance with the read program. The computer may also read the program directly from the portable medium and execute processing in accordance with the program. The computer may also execute processing in accordance with the received program each time a program is transferred from the server to the computer. Processing may also be executed using a so-called ASP-type service that realizes functions simply by issuing execution instructions and obtaining results, without transferring the program from the server to the computer. "ASP" is an abbreviation for application service provider. A program is information used for processing by a computer and includes anything equivalent to a program. For example, data that is not a direct instruction to a computer but has properties that define computer processing falls under the category of "something equivalent to a program."

[0052] Some or all of the functions of the monitoring device 100 may be implemented by a programmable circuit or a dedicated circuit as the control unit 10. In other words, some or all of the functions of the monitoring device 100 may be implemented by hardware.

[0053] The following additional notes are further disclosed regarding the above-described embodiments.

[0054] (Supplementary Item 1) A monitoring device comprising: a control unit that acquires an image including a manhole cover obtained by a synthetic aperture radar, estimates the degree of wear of the manhole cover based on the brightness of the image, and generates estimated data indicating the degree of wear; and an output unit that outputs the estimated data generated by the control unit. (Supplementary Item 2) The monitoring device according to Supplementary Item 1, wherein the control unit further acquires roughness data indicating the initial surface roughness of the manhole cover, and estimates the degree of wear further based on the roughness data. (Supplementary Item 3) The monitoring device according to Supplementary Item 1 or 2, wherein the control unit estimates the degree of wear further based on the brightness of an image including the manhole cover obtained by a synthetic aperture radar at an earlier time than the image. (Supplementary Item 4) The monitoring device of any one of claims 1 to 3, wherein, when the manhole cover includes a first layer exposed upward and a convex portion including a second layer below the first layer that has a lower microwave absorption rate than the first layer and a higher microwave reflectivity than the first layer, the control unit acquires multiple images including the manhole cover obtained by a synthetic aperture radar at multiple points in time, and estimates the degree of wear based on a change in brightness between the multiple images. (Supplementary Item 5) The monitoring device of claim 4, wherein the control unit further acquires information indicating whether the manhole cover includes a first layer exposed upward and a convex portion including a second layer below the first layer that has a lower microwave absorption rate than the first layer and a higher microwave reflectivity than the first layer, and estimates the degree of wear further based on the information. (Supplementary Item 6) The monitoring device of any one of Supplementary Items 1 to 5, wherein the control unit generates, as the estimated data, an image indicating the degree of wear at the position of the manhole cover. (Additional Item 7) A manhole cover with a convex portion, which includes a first layer exposed upward and a second layer below the first layer, the second layer having a microwave absorption rate lower than that of the first layer and a microwave reflectivity higher than that of the first layer.

[0055] The present disclosure is not limited to the above-described embodiments. For example, two or more blocks shown in the block diagram may be integrated, or one block may be divided. Two or more steps shown in the flowchart may be executed in parallel or in a different order depending on the processing capacity of the device executing each step, or as needed, instead of being executed in chronological order as described. Other modifications are possible without departing from the spirit of the present disclosure.

[0056] REFERENCE SIGNS LIST 1 Synthetic aperture radar satellite 2 Antenna 100 Monitoring device 10 Control unit 11 Estimated data 20 Database 21 Roughness data 22 Past image 30 Input unit 31 Image 40 Output unit 1000, 2000 Manhole cover 2100 Convex portion 2110 First layer 2120 Second layer

Claims

1. A monitoring device comprising: a control unit that acquires an image including a manhole cover obtained by a synthetic aperture radar, estimates the degree of wear of the manhole cover based on the brightness of the image, and generates estimated data indicating the degree of wear; and an output unit that outputs the estimated data generated by the control unit.

2. The monitoring device according to claim 1, wherein the control unit further acquires roughness data indicating the initial surface roughness of the manhole cover, and estimates the degree of wear based on the roughness data.

3. The monitoring device according to claim 1, wherein the control unit estimates the degree of wear based further on the brightness of an image including the manhole cover obtained by a synthetic aperture radar at a time earlier than the image.

4. The monitoring device of claim 1, wherein when the manhole cover has a convex portion that includes a first layer exposed upward and a second layer below the first layer that has a lower microwave absorption rate than the first layer and a higher microwave reflectance than the first layer, the control unit acquires multiple images including the manhole cover taken by a synthetic aperture radar at multiple points in time, and estimates the degree of wear based on changes in brightness between the multiple images.

5. The monitoring device described in claim 4, wherein the control unit further acquires information indicating whether the manhole cover has a convex portion that includes a first layer exposed upward and a second layer below the first layer, the second layer having a microwave absorption rate lower than the first layer and a microwave reflectance higher than the first layer, and further estimates the degree of wear based on the information.

6. The monitoring device according to any one of claims 1 to 5, wherein the control unit generates an image showing the degree of wear at the position of the manhole cover as the estimated data.

7. A manhole cover with a convex portion, which includes a first layer exposed upward and a second layer below the first layer, which has a lower microwave absorption rate than the first layer and a higher microwave reflectance rate than the first layer.

Images