In this paper, the real building and road data are used to test the proposed BDMPGA. The experimental data are located in the middle part of the topographic map of Shenzhen city, China, which contains a total of 121 buildings and is located in six different blocks. The original data are shown in
Figure 4. The original scale of these data is 1:10,000. Due to the widening of the road to a certain width, spatial conflicts occur because the distance between buildings and roads decreases. In addition, the proposed method is implemented in the MATLAB (MathWorks, Natick, Mass, USA Version 2016a) programming language, and ArcGIS software (ESRI, Redlands, CA, USA, Version 10.2) is used to display and compare the displacement results.
4.1. Parameter Setting
Before the experiment, some parameters need to be determined, such as the population size, maximum generation number (MaxGen), crossover probability and mutation probability, where the population size and maximum generation number (MaxGen) are related to the number of buildings and conflicts in each block. If this parameter value is set too small, then it will lead to a small number of chromosomes and too few iterations in each population, which will not be sufficiently evolved to find the optimal solution. If this parameter value is set too large, then increased running costs will ensue. Therefore, these two parameters are determined by Wilson’s research [
17], which uses experimentally determined heuristics, as shown in Equations (9) and (10).
where parameter
is the population size,
is the maximum generation number, and
and
represent the number of buildings and the number of conflicts in each block, respectively.
In the GA, the genetic operations mainly include crossover and mutation. The frequency at which the genetic operations occur is controlled by the crossover and mutation probability, which determine the global search capability and local search capability of the algorithm. Many scholars recommend choosing a larger crossover probability and a smaller mutation probability. However, in practical applications, the specific value of the parameters is generally obtained through experiments. In this study, to compare with other algorithms, the crossover probability and mutation probability in GA and IGA are 0.8 and 0.08, respectively. Since the BDMPGA has multiple populations, the parameter values assigned to each population are different. According to the suggestion of scholars Schaffer et al. [
40], the crossover probability of each population is randomly assigned within the range of [0.7, 0.9], and the mutation probability of each population is randomly assigned within the range of [0.001, 0.05]. In addition, the number of populations and the number of minimum generations of the optimal individual are all set to 10 in the BDMPGA. Because the conflict is mainly caused by the widening of the road, the principle of displacement is to maintain the accuracy of the position of the buildings as much as possible while resolving all possible conflicts. Therefore, the objective function weights are set in principle as follows
. The specific value is the same as Sun’s work [
27], that is, the
is set to 30,000, the
is set to 25,000 and the
is set to 1. The values of the parameters used in these three methods are shown in
Table 1. The symbol – indicates that the corresponding method does not contain that parameter.
4.2. Analysis and Evaluation of Experimental Results
In this experiment, we use the proposed method to resolve spatial conflicts by displacing buildings. The weighted distance of the triangulation-based skeleton is used to detect spatial conflicts, which considers the characteristics of buildings and is more cognitively appropriate than the minimum distance [
24].
Figure 5a shows the displacement results of the proposed BDMPGA.
Figure 5b shows a comparison of the original data and displacement results based on the BDMPGA, where the grey polygons are the original building data and the displacement results are represented by blue polygons. The basic statistical information of the displacement results based on the BDMPGA is listed in
Table 2. From
Figure 5 and
Table 2, we find that the proposed BDMPGA can effectively resolve all possible spatial conflicts between buildings and buildings and between buildings and roads in each block and as much as possible to ensure the positional accuracy of buildings.
To better evaluate the displacement results of the proposed method, we implemented two other intelligent optimization algorithms to perform comparative experiments. The first method is the GA, which was used by Wilson [
17] to displace buildings. The other method is the IGA, which was proposed by Sun [
27] to improve the GA. The experimental results of the BDMPGA, GA, and IGA are compared using two evaluation indices: the number of unresolved conflicts and the total displacement distances of the buildings. The number of unresolved conflicts is the main evaluation index of the algorithm performance, indicating the ability of the algorithm to resolve spatial conflicts. The smaller this value is, the stronger the ability of the algorithm to resolve conflicts is. The total displacement distance indicates the disruption degree of the positional accuracy and spatial relationships. The smaller the value is, the better the positional accuracy and spatial relationship are.
Figure 6a shows the displacement results of the GA.
Figure 6b shows a comparison of the original data and displacement results based on the GA, where the grey polygons are the original building data, and the displacement results are represented by purple polygons. The basic statistical information of the displacement results based on the GA is listed in
Table 3. From
Table 3, it can be seen that the GA can resolve all the conflicts between buildings and buildings. However, it cannot resolve all the conflicts between the buildings and roads. There are three unresolved conflicts in block 2 and two unresolved conflicts in block 4.
Figure 7a shows the displacement results of the IGA.
Figure 7b shows a comparison of the original data and displacement results based on the IGA, where the grey polygons are the original building data, and the displacement results are represented by cyan polygons. The basic statistical information of the displacement results based on the IGA is listed in
Table 4.
Table 4 shows that the IGA had five unresolved conflicts between buildings and roads, located in block 2, block 4, and block 5. More seriously, the IGA generates secondary conflicts. Through comparison, it is found that the BDMPGA has a stronger ability to resolve spatial conflicts.
Although these methods can be used to displace buildings and resolve spatial conflicts, they yield different displacement results (e.g., displacement distance and displacement direction) at some local regions. To better illustrate the differences in the displacement results of the various methods, we selected some typical regions to display the corresponding detailed information, such as the regions marked with red circles in
Figure 5,
Figure 6 and
Figure 7.
Table 5 shows the detailed information of the displacement results in these typical regions using the proposed BDMPGA method and the two comparative methods (i.e., GA and IGA).
Table 5 indicates the following:
Compared with the GA and the IGA, the proposed BDMPGA can generate more reasonable displacement results. For example, as shown in local area No.A in
Table 5, when a building is located at an acute corner and is conflicted with multiple roads, the BDMPGA can better solve the conflict by moving a longer distance, and the displacement direction is the resultant direction of multiple conflicting roads. When a building is located at an obtuse corner and is conflicted with multiple roads, as shown in local area No.B in
Table 5, the BDMPGA will make the building move a short distance to resolve the conflict. However, the GA cannot resolve multiple conflicts and even aggravate the conflicts. The IGA method only resolves some of the conflicts.
The proposed BDMPGA has a better ability than the GA and IGA to maintain building positional accuracy. For example, as show in local area No.C in
Table 5, the IGA tends to move long distances to resolve the conflict. However, the proposed BDMPGA only makes the conflict building move a shorter distance and satisfies the constraint condition of ensuring the positional accuracy of the building as much as possible while resolving the spatial conflicts.
Table 6 shows the basic statistical information of the displacement distance for each method. As seen from the statistical information, the BDMPGA has a minimum displacement distance of 0.0 m, meaning that if a building does not conflict with surrounding objects, then it is not moved. The maximum displacement distance of the BDMPGA is 3.99 m, which meets the specified maximum displacement distance threshold (e.g., 0.5 mm). The total displacement distance of the BDMPGA is 133.19 m, less than 140.31 m of the GA and 149.71 m of the IGA, indicating that the positional accuracy of the BDMPGA is maintained well.
Successful displacement should not only solve all possible spatial conflicts and avoid generating secondary conflicts after displacing some buildings but also preserve the important arrangement and distribution patterns of buildings. To better demonstrate the differences between the proposed BDMPGA and the other two comparative methods in maintaining the arrangement and distribution pattern of buildings, some typical examples are chosen, as shown in
Figure 8 and
Figure 9, which indicate the following:
Compared with the GA and IGA, the proposed BDMPGA can maintain the relative position between buildings during the displacement. For example,
Figure 8a shows the displacement results of the BDMPGA, and the displacement results of the GA and the IGA are
Figure 8b,c, respectively. By comparison, it can be seen that although both the GA and IGA can resolve spatial conflicts, the distance relations among buildings are changed after the displacement so that some buildings become closer (with the GA), while some become farther away (with the IGA). However, the proposed BDMPGA exhibits a good ability to avoid changing the relative position of the buildings because the conflicting buildings are all moving in the same direction and distance. Meanwhile, as shown in
Figure 8d–f, two adjacent buildings are in conflict with the road, the GA and the IGA only move one building, and the conflict between the other buildings remains unresolved. At the same time, the GA increases the distance between buildings and the IGA produces a more serious secondary conflict. However, the proposed BDMPGA moves two buildings simultaneously and resolves all conflicts, which can keep the relative positions between buildings unchanged.
Compared with the GA and IGA, the proposed BDMPGA can better maintain the linear arrangement and distribution pattern of buildings. For example, as shown in
Figure 9, the comparative methods, including the GA and IGA, only move some buildings in the building group, which not only fails to solve all the spatial conflicts but also fails to maintain the arrangement and distribution pattern of the building group. However, the proposed BDMPGA has a better ability to overcome the shortcomings of the comparative methods because the BDMPGA takes the least preserving generation of the optimal individual as the termination condition, and the method will constantly optimize the shortest displacement distance after resolving spatial conflicts to ensure that the arrangement and distribution pattern of buildings do not change.
Furthermore, to statistically evaluate the stability and convergence rate of the proposed BDMPGA,
Figure 10 shows the iterative process of different methods in each block. From
Figure 10, we can see that the proposed BDMPGA is very stable and converges very fast. This method only needs a few iterations to reach the optimal value.
4.3. Limitations
When using the BDMPGA to resolve spatial conflicts, there must be enough displacement space. If the density of buildings in the block is too high, there is not enough displacement space around the conflicting buildings, then other generalization operations, such as elimination, aggregation, and typification, are required to resolve the spatial conflicts. As show in
Figure 11a, when the road is symbolized, it creates overlap conflicts with the adjacent buildings. In this case, the displacement operator cannot resolve the spatial conflicts, and the size reduction operation can be used to change the size of the buildings to resolve the spatial conflicts, as shown in
Figure 11b. As shown in
Figure 11c, when a group of buildings in regular arrangement have spatial conflicts with the road, the typification operation can be adopted to replace them with fewer buildings to resolve the spatial conflict on the premise of maintaining the spatial arrangement pattern of the buildings, as shown in
Figure 11d. Future research aims to extend the implementation of our proposed approach by combining it with other contextual generalization operations to resolve more complex conflict cases.