AREAXTAB

Description

AREAXTAB generates an area cross-tabulation report containing correlation coefficients (Chi Square coefficients). These coefficients gauge the extent of correlation between two raster layers.

Parameters

areaxtab(file, dbic1, dbic2, dbib, units)

Name	Type	Caption	Length	Value range
FILE *	str	Input file name	1 -
DBIC1 *	List[int]	Input raster channel for rows	1 - 1
DBIC2 *	List[int]	Input raster channel for columns	1 - 1
DBIB	List[int]	Input bitmap	0 - 1
UNITS	str	Area units	0 - 17	Square Meters, Square Kilometers, Hectares, Square Miles, Acres, Square Feet Default: Hectares

* Required parameter

Parameter descriptions

FILE

Specifies the name of the PCIDSK database file that contains the input raster layers.

DBIC1

Specifies the raster data channel to use in generating part of the report. This data is used in the table rows. The raster can be of any data type.

DBIC2

Specifies the raster data channel to use in generating part of the report. This data is used in the table columns. The raster can be of any data type.

DBIB

If specified, the report is generated only for pixels under the bitmap.

UNITS

Specifies the area units to use in the report.

The default value is Hectares.

Details

AREAXTAB generates an ASCII report that enumerates each pixel value along with its name, the area percentage, the row percentage, and the column percentage covered by the pixel value.

Optionally, the report can be generated for only the pixel values that fall under a specified bitmap.

Pixels with a NO_DATA_VALUE are omitted from the report.

Chi Square coefficients measure the degree of correlation, association, or dependencies of a sample. Because the contingency table varies in dimension, three different estimates are generated, as described below.

Contingency Coefficient

The Contingency Coefficient (C) theoretically lies between 0.0 and 1.0, but does not always reach 1.0, even though the rasters seem completely associated.

Tschuprow's T

Tschuprow's T varies between 0.0 (for independence) and 1.0, but can only attain the maximum value in square tables.

Cramer's V

Cramer's V corrects some of the defiencies of C and T in that it achieves its maximum in assymmetic arrays. The V is always at least as large as T.

Example

Calculate the Area Cross-Tabulation Report for raster channel 1 and 2 of irvine.pix with 'Hectares' area units.

from pci.areaxtab import areaxtab

file     =  "irvine.pix"
dbic1    =  [1]	# Use raster channel 1 as input
dbic2    =  [2]	# Use raster channel 2 as input
dbib     = [] 	# Not using DBIB
units    = "" 	# Defaults to 'Hectares'

areaxtab( file, dbic1, dbic2, dbib, units )
            

References

Reynolds, H. T., The Analysis of Cross-Classifications, New York, The Free Press, 236 pages.