Output Grid
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Figure 16: A Euclidean distance function computes the distance from the nearest source cell
In the above example, the measurement of the distance from any cell must include the entire source grid; therefore this analytical procedure is a global function.
Figure 17 provides an example of the cost distance function. The source grid is identical to that in the preceding illustration. However, this time a cost grid is employed to weigh travel cost. The value in each cell of the cost grid indicates the cost for traveling through that cell. Thus, the cost for traveling from the cell located in the first row, second column to its adjacent source cell to the right is half the cost of traveling through itself plus half the cost of traveling through the neighboring cell.
Figure 17 provides an example of the cost distance function. The source grid is identical to that in the preceding illustration. However, this time a cost grid is employed to weigh travel cost. The value in each cell of the cost grid indicates the cost for traveling through that cell. Thus, the cost for traveling from the cell located in the first row, second column to its adjacent source cell to the right is half the cost of traveling through itself plus half the cost of traveling through the neighboring cell.
Figure 17: Travel cost for each cell is derived from the distance to the nearest source cell weighted by a cost function
Another useful global function is the cost path function, which identifies the least cost path from each selected cell to its nearest source cell in terms of cost distance. These global functions are particularly useful for evaluating the connectivity of a landscape and the proximity of a cell to any given entities.
SOME IMPORTANT RASTER ANALYSIS OPERATIONS
In this section some of the important raster based analysis are dealt:
• Renumbering Areas in a Grid File
• Performing a Cost Surface Analysis
• Performing an Optimal Path Analysis
• Performing a Proximity Search
Area Numbering: Area Numbering assigns a unique attribute value to each area in a specified grid file. An area consists of two or more adjacent cells that have the same cell value or a single cell with no adjacent cell of the same value. To consider a group of cells with the same values beside each other, a cell must have a cell of the same value on at least one side of it horizontally or vertically (4connectivity), or on at least one side horizontally, vertically, or diagonally (8connectivity). Figure 18 shows a simple example of area numbering.
Figure 18. Illustrates simple example of Area numbering with a bit map as input. The pixels, which are connected, are assigned the same code. Different results are obtained when only the horizontal and vertical neighbors are considered (4connected) or whether all neighbors are considered (8connected)
Figure 18. Illustrates simple example of Area numbering with a bit map as input. The pixels, which are connected, are assigned the same code. Different results are obtained when only the horizontal and vertical neighbors are considered (4connected) or whether all neighbors are considered (8connected)
One can renumber all of the areas in a grid, or you can renumber only those areas that have one or more specific values. If you renumber all of the areas, Area Number assigns a value of 1 to the first area located. It then assigns a value of 2 to the second area, and continues this reassignment method until all of the areas are renumbered. When you renumber areas that contain a specified value (such as 13), the first such area is assigned the maximum grid value plus 1. For example, if the maximum grid value is 25, Area Number assigns a value of 26 to the first area, a value of 27 to the second area, and continues until all of the areas that contain the specified values are renumbered.
Cost Surface Analysis: Cost Surface generates a grid in which each grid cell represents the cost to travel to that grid cell from the nearest of one or more start locations. The cost of traveling to a given cell is determined from a weight grid file. Zero Weights option uses attribute values of 0 as the start locations. The By Row/Column option uses the specified row and column location as the start location.
Optimal Path: Optimal Path lets us analyze a grid file to find the best path between a specified location and the closest start location as used in generating a cost surface. The computation is based on a cost surface file that you generate with Cost Surface.
One must specify the start location by row and column. The zeros in the input cost surface represent one endpoint. The specified start location represents the other endpoint.
Testing the values of neighboring cells for the smallest value generates the path. When the smallest value is found, the path moves to that location, where it repeats the process to move the next cell. The output is the path of least resistance between two points, with the least expensive, but not necessarily the straightest, line between two endpoints. The output file consists of only the output path attribute value, which can be optionally specified, surrounded by void values.
Performing A Proximity Search: Proximity lets you search a grid file for all the occurrences of a cell value or a feature within either a specified distance or a specified number of cells from the origin.
You can set both the origin and the target to a single value or a set of values. The number of cells to find can also be limited. For example, if you specify to find 10 cells, the search stops when 10 occurrences of the cell have been found within the specified distance of each origin value. If you do not limit the number of cells, the search continues until all target values are located.
The output grid file has the usertype code and the datatype code of the input file. The girdcell values in the output file indicate whether the grid cell corresponds to an origin value, the value searched for and located within the specified target, or neither of these.
The origin and target values may be retained as the original values or specified to be another value.

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