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How To: Understand how coverage precision affects calculation of fuzzy tolerance

Procedure

When running CLEAN, a request to specify a fuzzy tolerance appears. Instructions provided describe how to understand coverage precision and how it affects calculation of fuzzy tolerance.

Answer:

The following is what happens when CLEAN checks the fuzzy tolerance:

1. ArcInfo verifies that the number is greater than zero (and assigns a value of -1 if the user chose the default option). A fuzzy tolerance of 0 is not accepted by CLEAN.

2. If the fuzzy tolerance has not been specified, it is read from the coverage's tolerance (TOL) file.

3. If there is no TOL file present, the fuzzy tolerance is set to 1/10,000 of the width or height of the map extent (BND). If the BND is undefined, the fuzzy tolerance is set to 0.001.

4. The "Sorting..." process utilizes the fuzzy tolerance value to weed out unnecessary points.

5. The fuzzy tolerance is copied to another variable that is used in the "Intersection..." process. If on return from "Intersection..." the variable is not equal to the original fuzzy tolerance, the coverage tolerance is reset to the "Intersection..." tolerance.

6. In the "Intersection..." process a distinction is made between single and double precision. For double precision, the input tolerance is accepted without any further checking. For single precision, a much more complex process is initiated. Separate "tolerances" are computed for the X and Y directions. These tolerances are a function of the X or Y range (difference between left and right, or the upper and lower map limits), as well as the maximum value of these limits. The range and the maximum value, together with the number of single precision digits (7), determines the number of increments that need to be processed. The range is divided into "acceptable" increments, which in turn define the fuzzy tolerance. The minimum acceptable tolerance is defined as the maximum of the X and Y tolerances. If the minimum acceptable tolerance is larger than the input tolerance, the input tolerance is reset and the following message appears:

"Specified tolerance xxxxx is below the minimum resolution for this data -
intersections will be detected with a tolerance of yyyyyyyy instead."

7. The CLEAN command proceeds with "Assembling..." arcs or polygons.

Notes:

The explanation of fuzzy tolerance in the ArcInfo Data Model: Key Terms and Concepts is obsolete and has been rewritten for ArcInfo Version 7.x.