JP7619038B2 - Ground evaluation method - Google Patents

Description

The present invention relates to a ground evaluation method.

In recent years, heavy rainfall in urban areas has caused many human and property damages, and the need for flood risk assessment has increased. In response to this, local governments and other organizations have created and published their own flood hazard maps. Meanwhile, companies with bases (factories, branches, etc.) scattered throughout the country may compare risk levels to determine priorities when implementing flood prevention measures. However, since conditions such as damage estimates differ for each local government, it is necessary to investigate the risk levels individually, which is time-consuming. Therefore, a method has been proposed to assess risk levels in different regions using the same index. For example, Non-Patent Document 1 discloses a method for assessing the risk level of flooding by using the Laplacian, which is often used as an index of topographical unevenness, in order to simply assess the risk level of flooding (inland flooding and outside flooding) that the land inherently has using the same index nationwide.

Obayashi Corporation Technical Research Institute Report No. 79 2015 "Study on simplified risk assessment of urban flood damage using GIS data"

In the risk assessment method of Patent Document 1, the unevenness of the ground is calculated using elevation data when the area to be assessed is divided into meshes of a specified size (each side is 250 m long), but in this case, it is difficult to accurately grasp depressions of different scales. For example, when the average elevation data when the area to be assessed is divided into a relatively large scale of 250 m is used, it is difficult to reflect areas where the elevation difference changes locally in a narrow area of about 50 m in the assessment of unevenness. In other words, it is difficult to accurately assess the unevenness of the ground, and there is a possibility that a discrepancy will occur between the unevenness assessment and the actual flood risk.

The present invention was made in consideration of these problems, and its purpose is to accurately evaluate the unevenness of the ground.

The main invention for achieving the above-mentioned object is a ground evaluation method comprising the steps of: calculating a first Laplacian, which is an index of ground unevenness in a certain area, from elevation data for each division when the certain area is divided into a first mesh size; calculating a second Laplacian, which is an index of ground unevenness in a certain area, from elevation data for each division when the certain area is divided into a second mesh size different from the first mesh size; and evaluating the ground unevenness of the certain area using a computer based on the first Laplacian and the second Laplacian , wherein if the first Laplacian and the second Laplacian that overlap in a certain part of the certain area are both positive values, the certain part is evaluated as a depression.

Other features of the present invention will become apparent from the description of this specification and the accompanying drawings.

The present invention allows for accurate evaluation of ground unevenness.

FIG. 13 is a diagram illustrating a method for calculating the Laplacian. FIG. 13 is a diagram illustrating an example of a case where the Laplacian is calculated for a certain region with a mesh size of 250 m. FIG. 13 is a diagram illustrating an example of a case where the Laplacian is calculated for a certain region with a mesh size of 50 m. FIG. 13 is a diagram for explaining an area EA that is a target of unevenness evaluation. Fig. 5A is a diagram showing elevation data for each of the 3×4 meshes constituting the area M250. Fig. 5B is a diagram showing the Laplacian L calculated for each mesh based on the elevation data in Fig. 5A. This is a diagram showing the unevenness of the ground when the area EA is divided into a mesh size of 250 m. Fig. 7A is a diagram showing elevation data for each of the 5 vertical × 4 horizontal meshes that make up region M100. Fig. 7B is a diagram showing the Laplacian L calculated for each mesh based on the elevation data in Fig. 7A. This is a diagram showing the unevenness of the ground when the area EA is divided into a mesh size of 100 m. Fig. 9A is a diagram showing elevation data for each of the 6 vertical × 6 horizontal meshes that make up region M50. Fig. 9B is a diagram showing the Laplacian L calculated for each mesh based on the elevation data in Fig. 9A. This is a diagram showing the unevenness of the ground when the area EA is divided into a mesh size of 50 m. This figure shows the unevenness of the ground when the Laplacian obtained when the area EA is divided into a mesh size of 250 m is combined with the Laplacian obtained when the area is divided into a mesh size of 100 m. This figure shows the unevenness of the ground when the Laplacian obtained when the area EA is divided into a mesh size of 250 m, the Laplacian when it is divided into a mesh size of 100 m, and the Laplacian when it is divided into a mesh size of 50 m are combined.

A ground evaluation method comprising: calculating a first Laplacian, which is an index of ground unevenness in a certain area, from elevation data for each section when the certain area is divided into a first mesh size; calculating a second Laplacian, which is an index of ground unevenness in the certain area, from elevation data for each section when the certain area is divided into a second mesh size different from the first mesh size; and evaluating the unevenness of the ground in the certain area using a computer based on the first Laplacian and the second Laplacian.

According to this ground evaluation method, the unevenness of the ground is evaluated based on the first and second Laplacians calculated from elevation data of different mesh sizes, making it easier to reflect the effects of local depressions and wide-area depressions in the ground. Furthermore, if the first and second Laplacians are calculated using uniform elevation data across the country, regional variation is less likely to occur, and unevenness evaluation can be performed using the same index. This allows the unevenness of the ground to be accurately evaluated.

In such a ground evaluation method, it is desirable that the length of one side of the first mesh size is at least twice the length of one side of the second mesh size.

According to this ground evaluation method, by dividing the area to be evaluated into meshes with sides that differ in length by at least twice, it becomes easier to deal with depressions of different scales, such as depressions that are formed over a wide area or localized depressions, and the unevenness of the ground can be evaluated more accurately.

In such a ground evaluation method, it is desirable that the first mesh size and the second mesh size are each changeable.

This ground evaluation method allows for more accurate evaluation of unevenness by calculating the Laplacian by selecting an appropriate mesh size according to the scale of unevenness for each terrain, for example, densely populated urban areas or mountainous areas along mountains.

In such a ground evaluation method, it is desirable to calculate a third Laplacian, which is an index of the ground unevenness of the certain area, from elevation data for each division when the certain area is divided into a third mesh size different from the first mesh size and the second mesh size, and to evaluate the degree of concavity of the ground of the certain area using a computer based on the first Laplacian, the second Laplacian, and the third Laplacian.

According to this ground evaluation method, by using three types of Laplacians with different mesh sizes, unevenness at different scales can be more easily reflected. Therefore, a more accurate evaluation can be performed compared to evaluating the unevenness of the ground based on two types of Laplacians, the first Laplacian and the second Laplacian.

In such a ground evaluation method, it is desirable to calculate n types of Laplacians that each represent an index of the ground unevenness of the certain area from the elevation data for each division when the certain area is divided into n types of mesh sizes, and to evaluate the degree of ground concavity of the certain area using a computer based on two or more of the n types of Laplacians.

According to this ground evaluation method, by calculating multiple Laplacians with different mesh sizes, it becomes easier to deal with unevenness of different scales, so by calculating multiple types (n types) of Laplacians and evaluating the ground unevenness based on two or more of these Laplacians, it becomes easier to perform an accurate evaluation according to the actual ground unevenness.

In such a ground evaluation method, it is desirable that the length of one side of the mesh size is 5 m or more.

According to this ground evaluation method, if the mesh size is 5m or more, the Laplacian can be calculated based on highly consistent and stable data published by the Ministry of Land, Infrastructure, Transport and Tourism and other organizations. Furthermore, by setting the mesh size to 5m or more, the data is less likely to become complicated compared to when the mesh is too fine, and it is easier to suppress the generation of noise, making it easier to accurately evaluate the unevenness of the ground.

In such a ground evaluation method, it is desirable to evaluate the risk of flooding in the certain area based on the evaluation results of the unevenness of the ground in the certain area.

According to this method of ground evaluation, the area where meshes evaluated as depressions overlap is likely to have a lower ground elevation than the surrounding area, and is highly likely to be subject to future flooding due to heavy rainfall, etc. Therefore, by taking into consideration the results of the evaluation of the unevenness of the ground using multiple Laplacians (e.g., the first Laplacian and the second Laplacian), it is possible to objectively evaluate the risk of future flooding, etc.

====Embodiment====
In general, damage caused by flooding is likely to occur in areas where the elevation of the ground is lower than that of the surrounding areas. In other words, in areas where the ground is a depression (depression), there is a risk that once flooding begins, the flooding will progress at a rapid rate. Therefore, in this embodiment, the Laplacian is used as an index for determining the unevenness of the terrain to evaluate the unevenness of the ground in a certain area and determine the flood risk of the area.
<Evaluation of ground unevenness using Laplacian>
First, the basic principle of ground unevenness evaluation using the Laplacian will be described. Conventionally, the Laplacian has often been used as an index of topographical unevenness, and by calculating the Laplacian based on elevation data of a certain area (ground), it is possible to numerically express the fluctuation of unevenness in that area (ground).

Figure 1 is a diagram explaining how to calculate the Laplacian. When calculating the Laplacian, first, the area to be evaluated is divided into meshes of a specified size, and the elevation data of the terrain is obtained for each mesh. In Figure 1, a certain area E is divided into nine meshes R1 to R9 (3 vertical x 3 horizontal). The elevation data for each mesh (R1 to R9) can be, for example, the elevation data published nationwide by the Ministry of Land, Infrastructure, Transport and Tourism (Geospatial Information Authority of Japan Basic Map Information: https://fgd.gsi.go.jp/download/menu.php).

In Figure 1, of the 3x3 meshes, if the elevation of the central mesh R5 is u(m,m), the Laplacian L(m,n) can be calculated by the following formula (1) using the elevations (m-1,n), u(m+1,n), u(m,n-1), and u(m,n+1) of the four meshes (R4, R6, R2, R8) surrounding R5.

If the Laplacian L(m,n) calculated by formula (1) is a positive value (L>0), it indicates a concave terrain, if it is zero (L=0), it indicates a flat terrain, and if it is a negative value (L<0), it indicates a convex terrain. Furthermore, the larger the absolute value of Laplacian L(m,n), the greater the unevenness compared to the surrounding ground. Therefore, by calculating Laplacian L(m,n) for each region, it is possible to evaluate the unevenness of that region.

On the other hand, when performing unevenness evaluation using such a Laplacian, the unevenness evaluation may differ depending on the mesh size (mesh spacing) setting, even for the same region. Figure 2 is a diagram that explains an example of a case where the Laplacian is calculated for a certain region with a mesh size of 250 m. Similarly, Figure 3 is a diagram that explains an example of a case where the Laplacian is calculated for a certain region with a mesh size of 50 m.

Figure 2 shows the relationship between the actual ground unevenness and the calculated Laplacian when the Laplacian is calculated with a relatively large mesh size (250 m). Note that in Figure 2, for the sake of simplicity, only the variation in unevenness in the horizontal direction (east-west direction) is shown. The ground to be evaluated has an uneven shape as shown in Figure 2A. Specifically, when three regions divided by a mesh size of 250 m are designated as X0, X1, and X2, there is a large depression A in the entire area of region X1 and in part of X2. Furthermore, in region X1, depression A has a local depression B, which is a locally depressed portion, and a flat portion C, which is a flat portion corresponding to the bottom of depression A.

When the Laplacian L is calculated for such uneven ground using a 250m mesh, as shown in FIG. 2B, the Laplacian L is positive (L>0) in region X1, and is zero or less (L≦0) in regions X0 and X2. In other words, region X1 is evaluated as being a depression. As described above, region X1 has depressions A over a wide area, so the evaluation that region X1 is a depression is generally correct. However, when viewed locally, the elevation difference between the local depression B and the flat portion C is not reflected in the evaluation, and the unevenness of the ground cannot be accurately evaluated. This is because the Laplacian L is calculated based on the average elevation data of the region divided into meshes of a predetermined size, so even if an elevation difference (unevenness) occurs within the same mesh, the elevation difference is not detected.

Next, Figure 3 shows the relationship between the actual ground unevenness and the calculated Laplacian for area X1 of the same ground as in Figure 2, where the Laplacian is calculated using a smaller mesh size (50 m) than in Figure 2. In other words, it shows the relationship between the unevenness and the Laplacian in areas X11 to X15 (50 m intervals), which are obtained by dividing area X1 (250 m intervals) in Figure 2 into five meshes (see Figure 3A).

When the Laplacian L is calculated for a 50m mesh, as shown in Figure 3B, Laplacian L is positive (L>0) in areas X11 and X12, and is zero or less (L≦0) in areas X13 to X15. That is, areas X11 and X12 are evaluated as depressions. In this case, local depression B, which is locally depressed in area X12, is evaluated as a depression. On the other hand, flat area C formed in areas X13 to X15 is evaluated as a convex or flat land, even though it is part of depression A (depression), and it is not reflected that the area is a depression as a whole. This is because Laplacian L is calculated based on the difference in elevation between adjacent meshes, and therefore areas with a small difference in elevation between adjacent meshes, such as flat area C, are not judged to be depressions. Therefore, in this case too, the unevenness of the ground cannot be accurately evaluated.

<Roughness evaluation using multiple Laplacians>
The evaluation of ground unevenness in this embodiment will be specifically described below. As described in Fig. 2 and Fig. 3, when evaluating the unevenness of the ground using the Laplacian, depending on how the mesh size is set, there is a risk that the unevenness cannot be accurately evaluated. In other words, there is a risk that the risk of flooding cannot be accurately determined. Therefore, in this embodiment, multiple types of Laplacians are calculated while changing the mesh size for a certain area, and the unevenness of the ground is evaluated based on the multiple types of Laplacians calculated.

Figure 4 is a diagram illustrating an area EA that is the subject of unevenness evaluation in this embodiment. Area EA is an area of a certain size that is cut out from an actual region, and is the area surrounded by the outermost frame line in Figure 4. Within area EA, multiple flooded areas P are located, which are indicated by hatching. These flooded areas P indicate parts of area EA where flood damage occurred in the past. The following explanation focuses on flooded areas P1, P2, and P3 out of the multiple flooded areas P.

In this embodiment, the Laplacian is calculated for three different mesh sizes: 250 m, 100 m, and 50 m. First, within the area EA, the range in which the Laplacian is calculated is set for each mesh size. For example, when calculating the Laplacian for a 250 m mesh, an area M250 surrounded by long dashed lines is set so that the flooded areas P1 and P2 are located near the center. Area M250 is an area divided into 3 vertical x 4 horizontal meshes spaced 250 m apart (see Figure 6, described later).

Similarly, when calculating the Laplacian using a 100m mesh, a region M100 surrounded by a thick solid line is set so that flooded areas P1 and P2 are located near the center. Region M100 is a region divided into 5 vertical x 4 horizontal meshes at 100m intervals (see Figure 8, described below). When calculating the Laplacian using a 50m mesh, a region M50 surrounded by a short dashed line is set so that flooded areas P1 and P2 are located near the center. Region M50 is a region divided into 6 vertical x 6 horizontal meshes at 50m intervals (see Figure 10, described below).

Next, the Laplacian L is calculated for each of the set areas M250, M100, and M50 according to the method described in Figure 1 and formula (1), and the unevenness of the ground is evaluated.

Figure 5A shows elevation data for each of the 3 x 4 meshes that make up region M250. The elevation data can be obtained, for example, from the Geospatial Information Authority of Japan's basic map information mentioned above. Figure 5B shows the Laplacian L calculated for each mesh based on the elevation data in Figure 5A. In this way, the Laplacian L, which is an index of ground unevenness, is obtained for each mesh that divides region M250.

Figure 6 shows the unevenness of the ground when the area EA is divided into meshes of 250 m size. In Figure 6, the meshes are colored in five levels according to the magnitude of Laplacian L obtained in Figure 5B, to visualize the parts of the area EA where the ground is lower (parts with large concaves). Note that in Figure 6, Laplacian L is also calculated for the area outside area M250, and the meshes are colored based on the results. Also, for meshes where the Laplacian L value is zero or negative in Figure 5B (i.e. meshes where the ground is evaluated as flat or convex), the Laplacian L is uniformly set to "less than zero" in Figure 6, and the meshes are left uncolored. The same applies to Figures 8 and 10 below.

In Figure 6, the Laplacian L of the mesh in which the flooded areas P1 to P3 are located is below zero. In other words, the ground in the area in which the flooded areas P1 to P3 are located is evaluated as being flat or convex. In other words, when the area EA is divided into mesh sizes of 250 m and the Laplacian L is calculated, none of the flooded areas P1 to P3 are evaluated as being depressions with a high risk of flooding. In this case, it is difficult to predict the risk of flooding in the flooded areas P1 to P3.

In the same manner, Laplacian L is calculated for each mesh in region M100 to evaluate the unevenness of the ground. Figure 7A shows elevation data for each of the 5 x 4 meshes that make up region M100. Figure 7B shows Laplacian L calculated for each mesh based on the elevation data in Figure 7A. Figure 8 shows the unevenness of the ground when region EA is divided into meshes with a size of 100 m.

In Figure 8, the mesh in which flooded area P2 is located has the largest Laplacian L value, and flooded area P2 is evaluated as a large depression. The mesh in which part of flooded area P3 (the part on the right) is located also has a relatively large Laplacian L value and is evaluated as a depression. On the other hand, the Laplacian L of the mesh in which flooded area P1 is located is below zero, and flooded area P1 is not evaluated as a depression. In other words, if area EA is divided into mesh sizes of 100 m and Laplacian L is calculated, flooded areas P2 and P3 are evaluated as depressions with a high risk of flooding, but flooded area P1 is not evaluated as a depression, and it is difficult to predict the risk of flooding in flooded area P1.

Furthermore, for area M50, Laplacian L is calculated for each mesh in a similar manner to evaluate the unevenness of the ground. Fig. 9A shows elevation data for each of the 6 vertical x 6 horizontal meshes that make up area M50. Fig. 9B shows Laplacian L calculated for each mesh based on the elevation data in Fig. 9A. Fig. 10 shows the unevenness of the ground when area EA is divided into meshes with a size of 50 m.

In Figure 10, the value of Laplacian L of the mesh in which flooded areas P1 and P2 are located is large, and flooded areas P1 and P2 are evaluated as depressions. On the other hand, the Laplacian L of the mesh in which flooded area P3 is located is below zero, and flooded area P3 is not evaluated as a depression. In other words, if area EA is divided into mesh sizes of 50 m and Laplacian L is calculated, flooded areas P1 and P2 are evaluated as depressions with a high risk of flooding, but flooded area P3 is not evaluated as a depression, and it is difficult to predict the risk of flooding in flooded area P3.

As shown in Figures 6 to 10, when dividing a certain area into mesh sizes of a predetermined size, calculating the Laplacian, and evaluating the unevenness of the ground in that certain area based on the calculated Laplacian, it may not be possible to perform an accurate evaluation.

In contrast, by combining multiple types of Laplacians calculated for different mesh sizes, the unevenness of the ground can be evaluated more accurately. Figure 11 shows the unevenness of the ground when the Laplacian (first Laplacian) obtained when the area EA is divided into a mesh size of 250 m is combined with the Laplacian (second Laplacian) obtained when the area EA is divided into a mesh size of 100 m.

Figure 11 is obtained by combining (adding) the data of Laplacian L (first Laplacian) in the 250 m mesh size used in creating Figure 6 and the data of Laplacian L (second Laplacian) in the 100 m mesh size used in creating Figure 8 using a known information processing device such as a computer. As a result, in Figure 11, the flooded areas P2 and P3 are evaluated as depressions. Therefore, it can be seen that the evaluation of the unevenness is more accurate than when the evaluation of the unevenness is based only on the Laplacian (first Laplacian) in the 250 m mesh size. In this way, by evaluating the concavity of the ground based on multiple Laplacians calculated from the elevation data of meshes of different sizes, the effects of local depressions and wide-area depressions in the ground are more easily reflected, and a more accurate evaluation of the unevenness of the ground can be performed. In addition, by using nationally unified elevation data published by the Geospatial Information Authority of Japan, etc., regional variations are less likely to occur, and the unevenness of the ground can be evaluated using the same indicator.

In addition, if two different Laplacians L (the first Laplacian and the second Laplacian) that overlap in a certain part of the area EA are both positive values (evaluation of a depression), there is a high possibility that the part is actually a depression. Therefore, by using multiple different Laplacians to evaluate the unevenness of the ground, it is possible to easily predict the risk of flooding.

Figure 12 shows the unevenness of the ground when the Laplacian (first Laplacian) obtained when the area EA is divided into a mesh size of 250 m, the Laplacian (second Laplacian) when it is divided into a mesh size of 100 m, and the Laplacian (third Laplacian) when it is divided into a mesh size of 50 m are combined. Figure 12 was obtained by further combining (adding up) the data of the Laplacian L (third Laplacian) for the 50 m mesh size used in creating Figure 10 with Figure 11.

In Figure 12, all of the flooded areas P1 to P3 are evaluated as depressions. For example, flooded area P1, which was not evaluated as a depression in Figure 11, is evaluated as a depression in Figure 12. Furthermore, in Figure 12, almost all of the parts of area EA where flooded areas P other than flooded areas P1 to P3 are located are evaluated as depressions. In other words, all of the parts of area EA with a high risk of flooding (areas where flooding has actually occurred) are evaluated as depressions.

In this way, in FIG. 12, in addition to the data of the first and second Laplacians, the data of the third Laplacian is further combined to evaluate the unevenness of the ground based on three types of Laplacians. In this case, local depressions that could not be extracted by the first Laplacian with a large mesh size (250 m) are more likely to be extracted by the third Laplacian with a small mesh size (50 m). Conversely, depressions covering a wide area that could not be extracted by the third Laplacian with a small mesh size are more likely to be extracted by the first Laplacian with a large mesh size. In other words, by using three or more types of Laplacians, depressions of different scales are more likely to be reflected. Therefore, a more accurate evaluation can be performed compared to the case where the unevenness of the ground is evaluated based on two types of Laplacians (the first Laplacian and the second Laplacian) as in FIG. 11.

Furthermore, it is possible to combine Laplacians (for example, a fourth Laplacian, not shown) obtained when the area EA is divided into mesh sizes different from the first to third Laplacians, and evaluate the unevenness of the ground based on four or more types of Laplacians. The more mesh sizes there are, the higher the chance that depressions of different scales will be extracted, making it easier to improve the accuracy of the unevenness evaluation. However, care must be taken because increasing the number of Laplacian types used for the unevenness evaluation too much (i.e., increasing the size of the mesh dividing the area too much) may result in noise that worsens the accuracy of the unevenness evaluation.

It is also possible to calculate n types (n=2, 3, 4...) of Laplacians by dividing the area EA into n different mesh sizes, and evaluate the unevenness of the ground based on two or more of the n types of Laplacians. As described above, calculating multiple Laplacians with different mesh sizes makes it easier to deal with depressions of different scales (makes it easier to extract depressions), so it is desirable to select a Laplacian with an appropriate mesh size depending on the unevenness of the terrain to be evaluated and perform the unevenness evaluation. In this way, an accurate evaluation can be performed according to the actual unevenness of the terrain.

In addition, when dividing the area EA into different mesh sizes and calculating multiple types of Laplacians, it is desirable that the size of one mesh is at least twice the size of the other mesh. In other words, when dividing the area EA into two types of meshes, the first mesh and the second mesh, it is desirable that the length of one side of the first mesh is at least twice the length of one side of the second mesh. In this embodiment, three types of Laplacians are calculated by dividing the area EA into three mesh sizes of 250 m, 100 m, and 50 m (see Figures 5 to 10), and the length of one side of one mesh is at least twice the length of one side of the other mesh. In this way, by adjusting the mesh size, it becomes easier to respond to the actual unevenness of the terrain to be evaluated. For example, it becomes easier to respond to depressions of different scales, such as a depression (depression A) formed over a wide area as described in Figure 2 and a local depression (local depression B) as described in Figure 3. Therefore, a more accurate unevenness evaluation can be performed.

In addition, in this embodiment, the Laplacian was calculated based on three different mesh sizes of 250 m, 100 m, and 50 m, but the mesh size can be changed appropriately depending on the actual terrain. For example, in densely populated urban areas on plains, localized unevenness may be formed in a small area of land, so it may be better to calculate the Laplacian by dividing the area into mesh sizes as small as possible. On the other hand, in mountainous areas along the mountains, unevenness may be formed on a large scale, so it may be better to divide the area into mesh sizes as large as possible and calculate the Laplacian. In this way, by selecting a mesh size appropriate to the terrain and calculating the Laplacian, it is possible to perform a more accurate unevenness evaluation.

However, in this embodiment, the minimum mesh size is 5m. In other words, the length of one side of the mesh when dividing the area is 5m or more. Since the elevation data of the Geospatial Information Authority of Japan described above is displayed at 5m intervals, if the mesh is 5m or more, the Laplacian can be calculated based on the highly consistent and stable data published by the Geospatial Information Authority of Japan. Furthermore, if the evaluation target area is divided into very fine mesh sizes such as 1m, the data becomes complicated and there is a high possibility of noise generation, which may make it difficult to accurately evaluate the unevenness of the ground. Therefore, when performing the ground unevenness evaluation in this embodiment, it is desirable to use a mesh size of 5m or more.

In addition, in this embodiment, the possibility of flood damage in the ground can be evaluated based on the evaluation result of the ground unevenness using the Laplacian as described above. In Fig. 11 and Fig. 12, the area EA is divided into two or three mesh sizes, and the ground unevenness is evaluated by calculating the Laplacian for each divided mesh. In this case, it is highly likely that the elevation is actually lower in the area where areas divided into multiple mesh sizes overlap and are evaluated as a depression. For example, in a certain part of Fig. 12, if the first to third Laplacians are all evaluated as a depression, it shows that the elevation is lower than the surrounding area when viewed at mesh intervals of 250 m, 100 m, and 50 m. In this case, in the overlapping areas evaluated as a depression, the colored meshes overlap and are displayed in a dark color (in Fig. 12, meshes displayed with hatching or diagonal lines are displayed overlapping each other), making it easy to visually recognize the depression.

In other words, the area where meshes evaluated as depressions overlap is likely to have a lower ground elevation than the surrounding area, and there is a high probability of flooding due to heavy rain in the future. Furthermore, according to the above formula (1), the mesh with a larger Laplacian value is more likely to have a larger elevation difference from the surrounding area, and therefore there is a risk of flooding in the area where such meshes overlap. Therefore, for example, by using a computer to extract areas where meshes with large Laplacian values overlap, or areas where the magnitude of the overlapping Laplacian is larger than a specified value, it is possible to predict ground that is prone to flooding. In this way, in this embodiment, the risk of future flooding can be objectively evaluated based on an evaluation of the unevenness of the ground.

===Other embodiments===
The above-mentioned embodiment is for facilitating understanding of the present invention, and is not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit of the present invention, and it goes without saying that the present invention includes equivalents. In particular, the following embodiments are also included in the present invention.

In the above embodiment, the basic map information of the Geospatial Information Authority of Japan was used as the elevation data for calculating the Laplacian, but other elevation data may be used as long as it is standardized nationwide and has a certain degree of reliability.

A: recess; B: local recess; C: flat portion;
X0~1 area, X11~15 area EA area,
P flooded area, P1 flooded area, P2 flooded area, P3 flooded area,
M250 area (mesh size 250m),
M100 area (mesh size 100m),
M50 area (mesh size 50m)

Claims (7)

Calculating a first Laplacian, which is an index of ground unevenness in a certain area, from elevation data for each section obtained by dividing the certain area into a first mesh size;
Calculating a second Laplacian, which is an index of ground unevenness of the certain area, from elevation data for each section obtained by dividing the certain area into a second mesh size different from the first mesh size;
evaluating the unevenness of the ground in the certain area using a computer based on the first Laplacian and the second Laplacian;
having
A ground evaluation method, characterized in that if the first Laplacian and the second Laplacian that overlap in a certain part of the certain area are both positive values, the certain part is evaluated as a depression . Calculating a first Laplacian, which is an index of ground unevenness in a certain area, from elevation data for each section obtained by dividing the certain area into a first mesh size;
Calculating a second Laplacian, which is an index of ground unevenness of the certain area, from elevation data for each section obtained by dividing the certain area into a second mesh size different from the first mesh size;
evaluating the unevenness of the ground in the certain area using a computer based on the first Laplacian and the second Laplacian;
having
A ground evaluation method, characterized in that the mesh of the first mesh size and the mesh of the second mesh size are meshes of different sizes and intersect with each other . The ground evaluation method according to claim 1 or 2,
A ground evaluation method, characterized in that the first mesh size and the second mesh size are each changeable. The ground evaluation method according to any one of claims 1 to 3,
A third Laplacian, which is an index of ground unevenness of the certain area, is calculated from elevation data for each division when the certain area is divided into a third mesh size different from the first mesh size and the second mesh size;
A ground evaluation method, comprising: evaluating a degree of depression of the ground in the certain area using a computer based on the first Laplacian, the second Laplacian, and the third Laplacian. The ground evaluation method according to any one of claims 1 to 4,
A ground evaluation method comprising the steps of: dividing a certain area into n different mesh sizes, calculating n types of Laplacians that each represent an index of the ground unevenness of the certain area from elevation data for each division; and evaluating the degree of ground concavity of the certain area using a computer based on two or more of the n types of Laplacians. The ground evaluation method according to any one of claims 1 to 5,
A ground evaluation method, characterized in that the length of one side of the mesh size is 5 m or more. The ground evaluation method according to any one of claims 1 to 5,
A ground evaluation method, comprising: evaluating the risk of flooding in a certain area based on the evaluation results of the unevenness of the ground in the certain area.

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