How Tolerance (Coverage) works

Tolerances define the resolution of coordinates in a coverage. The Tolerance tool can be used to set these tolerances. For single precision coverages, the tolerance values are stored in the tol.adf file, while for double-precision coverages, they're stored in the par.adf file. Both files can be found in the coverage directory.

The FUZZY tolerance of a coverage is the minimum distance between arc coordinates in that coverage. The DANGLE length is the shortest acceptable length allowed for arcs that have the same polygon to their left and right sides. Both the FUZZY tolerance and DANGLE length are specified as parameters to some geoprocessing Coverage tools (for example, Clean and Buffer) and overlay tools (for example, Clip and Identity).

The TIC_MATCH tolerance is the maximum allowable RMS error when a map of the coverage is registered on a digitizer. The TIC_MATCH tolerance is given in coverage units.

EDIT, NODESNAP, WEED, GRAIN, and SNAP tolerances are used in editing sessions in the ArcInfo Workstation ArcEdit Module. EDIT distance controls the search distance for selecting features; NODESNAP distance controls how close a node on an added arc has to be to an existing node to be snapped to that node; WEED tolerance controls the minimum allowable distance between vertices on an added arc; GRAIN tolerance controls the distance between vertices when you add an arc with a curve in it, add a circle, or spline an arc during ADD; and SNAP distance controls how close features must be to snap together.

These edit tolerances set up the edit environment for the coverage. They can be changed as necessary, and their new values are updated in the TOL file.

When a tolerance is set for FUZZY, DANGLE, or TIC_MATCH with the Tolerance tool, it does not immediately reflect the actual resolution of the coverage. The coverage must be processed using the new tolerance before it will accurately represent the coverage coordinates. Therefore, each tolerance has a "verified" indicator. If a coverage has been processed with the specified tolerance, then that tolerance is a verified tolerance; otherwise, it's an unverified tolerance. To see which tolerances have been set, and which are Verified, open the Coverage Properties page and go to the Tolerances tab. To do this, right click the coverage name in ArcCatalog and click Properties. Tolerances for EDIT, NODESNAP, WEED, GRAIN, and SNAP are verified as soon as they have been set with the Tolerance tool.

The tolerance values in the tolerance files are automatically created or updated for any coverage that has been processed by any tool that uses these values.

At ARC/INFO Version 7 or greater, the items Database ID and Transaction ID appear at the end of the tolerance list. The values for these items are zero unless the coverage was created as a result of a CHECKOUT from an ArcStorm database. The values cannot be reset on the command line.