GIS buffer application and Algorithm Implementation

Geospatial Relationship Analysis of geographic information mainly includes the degree of closeness
(Proximity) analysis, overlay analysis, network analysis, etc. Buffer Zone Analysis is a kind of closeness analysis. A buffer zone is used to identify the influence of a geographical entity or spatial object on the surrounding objects and establish a band area with a certain width around it. As an independent data layer, the buffer zone can be used for Spatial Analysis of roads, rivers, environmental pollution sources, residential areas, and radiation sources to provide a scientific basis for a certain purpose, different professional models can play an important role in areas such as life, military, and urban-rural planning.

For different geometric types of points, lines, and surfaces, the method of creating a buffer varies with each other. It is relatively simple to create a point buffer, that is, taking a certain point element as the center and taking the buffer radius
R is used as a circle to obtain the buffer zone of the point element. The buffer zone of the line element is a line axis,
R is a parallel line between the two sides. Two semi-circular arc segments are built on both ends of the line to form a buffer zone together with parallel lines. The establishment of the surface buffer zone is a parallel line based on the boundary of the surface elements, the area in the parallel line and baseline is the area buffer. In addition, you can create a buffer zone for the Raster Data and create a dynamic buffer zone based on different model equations. The basic methods for creating a buffer zone for any data are similar.

There are two buffer implementation algorithms: vector method and Raster Method. The vector method has a small amount of data, and the method is relatively mature. The Raster image requires Boolean calculation between the raster pixels. When the buffer zone is large, it will bring heavy computing load, which has certain limitations in practical use. Generally, Vector Algorithm follows the following steps:

Point: Determine the center point-centered on the center point,
R generates a circle for the radius -- obtains the boundary of the buffer zone.

Line and plane: Determine the axis-distance
R: generate a parallel curve of the central axis -- process the angle arc section -- perform intersection and merge operations on the generated arc section -- generate a buffer boundary

Common Vector Data Center Line expansion algorithms:

Angular line division method

Basic Idea: the "simple parallel line method" is used to create parallel lines on both sides of the axis, form a tip in the rotor, and form an arc segment at both ends to form a buffer zone.

Defect: it is difficult to ensure the equal width of the left and right sides of the buffer at the tip. The correction process is complex and mainly reflected in the large and small axis angle. The algorithm model is complex, it is mainly because many exceptions need to be handled during the geometric generation process.

Convex Arc Method

Basic Idea: As the name implies, an arc is used to replace the sharp angle on the outer side of the corner. The sharp angle method is still used on the inner side to generate a buffer.

Implementation steps:

1. Determine the linearity and determine whether the adjacent three points are in the same line;

2. Determine the convex and concave sides of the fold point to determine which side of the corner uses a straight line for intersection and which side uses an arc connection;

3. Embedding of Convex Arc, that is, the arc formed on the outside of the corner is connected to the line segments on both sides;

4. Edge relationship identification and processing. Island polygon participate in the composition of the buffer boundary, and overlapping polygon do not participate in the composition of the buffer boundary;

5. The formation of the buffer boundary is to merge the overlapping areas and draw peripheral edges, including the contour of the island polygon, to form the final buffer boundary.

In the buffer algorithm, it is important to note thatOverlapping and merging of buffer Polygon, Including the overlap of the same element buffer and the overlap of multiple elements between the buffer. The raster in the raster data buffer has a value corresponding to its influence degree. If the overlapping area has the same influence degree, any value is taken. If it is different, a large influence degree is taken instead of a small influence degree. There are three processing algorithms for Vector Data: mathematical calculation, vector-grid conversion, and vector-grid mixing.