AREAOFINF

Description

Parameters

Parameter descriptions

FILI

Specifies the name of the file that contains the input vector segment to process, and the optional bitmap mask segment.

FILO

Specifies the name of the output file to which the raster image is written. If FILO is specified but does not exist, it will be created.

DBVS

Specifies the input vector point segment to be converted to a raster layer.

DBIB

Specifies the bitmap segment to sample data from the input vector point data.

A bitmap layer restricts the region when generating the weighted average. The results are generated inside the bitmap layer (value 1), and pixels falling outside the bitmap (value 0) are given a "No Data" value. The bitmap layer, if used, also defines the georeferencing of the output raster.

DBOC

Specifies the output raster channel that receives the raster layer derived from the point layer.

This parameter is used only if FILO already exists.

The georeferencing will either be determined from the georeferencing information, the bitmap layer, or calculated using the point layer extents and the pixel X/Y size.

The raster type is either 8U, 16U, or 32R, depending on the number of points used.

FLDNME

The input point data layer should contain a column that assigns a value to each point as an indication of its "attractiveness" or its intended contribution to the final output. This is referred to as a weight; the higher the weight value, the greater the influence exerted. If no attributes exist, the "z" value of the points is used for the weight.

EXPONEN

Controls the effect of the distance decay. The higher the value, the greater the effect of the distance decay. The default value is 1.

SRCATTR

Specifies whether the attributes are copied or joined. If an invalid value is specified, this parameter defaults to COPY.

PIXRES

This parameter applies only when a new output file (FILO) is being created. It allows you to specify a pixel resolution or size (in meters) for the output file. By default, the pixel size the same as the input file. The pixel size determines the number of pixels and lines in the output file; therefore, decreasing the pixel size results in a larger output file and increased computation time.

Valid values: x > 0.0, y > 0.0

PIXRES=400.0, 400.0 | Use a pixel size of 400m x 400m

REPORT

Specifies where to direct the generated report.

Available options are:

• TERM: generates a report on the terminal (default)
• DISK: generates a report on file "IMPRPT.LST"
• OFF: turns off report generation
• <filename>: appends a report to the specified file

MONITOR

The program progress can be monitored by printing the percentage of processing completed. A system parameter, MONITOR, controls this activity.

Available options are:

• ON: Turns the monitor ON (default)
• OFF: Turns the monitor OFF (recommended if running in batch or background mode)

Details

The attractiveness of a point in a gravity model is inversely proportional to distance and is directly related to the perceived attractiveness of the point itself. The point data must include weight values representing the attractiveness of each point. A weight is a numeric value assigned to each point as an indication of its importance or its intended contribution to the final output. You must also specify a distance-decay factor that calculates the diminishing attractiveness with the increasing distance. The greater the distance to the point, the less attractive the point becomes.

Using this information, AREAOFINF defines a catchment area around each point. A catchment area is an area surrounding a point in which any location is more attracted to that point than to any other point. This results in a one-to-one relationship: one area to one point. The higher the density of points, the smaller the catchment area. Conversely, data with a low density of points creates large catchment areas.

This function differs from THSNPOLY in that it uses a weight and distance-decay factor to define the catchment areas.

One pixel value is assigned to each point.

Example

A raster layer is built based on the segment 25 in irvine.pix and is saved to channel 8 in oirvine.pix.

EASI>FILI = "irvine.pix"
EASI>FILO = "oirvine.pix"
EASI>DBVS = 29
EASI>DBOC = 8
EASI>FLDNME  =			! Default "ATTRIBUTE"
EASI>EXPONEN =			! Default 1
EASI>SRCATTR =			! Default "COPY" 
EASI>PIXRES  =			! Ignored because oirvine.pix already exists

EASI>run areaofinf

References

Ken Jones and Jim Simmons, Location, Location, Location: Analyzing the Retail Environment (Methuen, Toronto, 1987),pp 114-116, 263-269.

Avijit Ghosh and Sara L. McLafferty, Location Strategies for Retail and Service Firms (Lexington Books, Toronto, 1987).