| # GRASS Help System

## SETTING THE SNAPPING THRESHOLD VALUE

The default snapping threshold value should suit most occasions; however, the user can modify the threshold if necessary.

Digitizing in GRASS: When digitizing, the snapping threshold may be changed using the "v.digit" "Customize" menu. This method is useful when two nodes on a map being digitized need to be closer together than the snapping threshold allows. Lowering the snapping threshold should fix this. In such a case, the user should leave the snapping threshold at the lower value when the completed digitized file is saved.

Importing Data Files into GRASS: The snapping threshold may also need to be modified when importing a vector file produced by other software. Since different software handles snapping differently, it is possible that line segments in the file to be imported are not really connected to one another. GRASS requires that any shared nodes have exactly the same geographic coordinates. If the user is aware that nodes in to-be-imported file are not yet snapped, the user can force snapping to occur within a default distance while v.import is run.

If the user is unaware of the status of the file to be imported, it is recommended that the user first run v.import without requesting that nodes be snapped. If, after import to GRASS, the file appears to contain unsnapped nodes, the user can run v.support to correct this. Here, the user is offered the option of using a snapping threshold other than the default value.

Importing Scanned Files into GRASS: It has been observed that with machine-produced vector files (i.e., those electronically scanned), it is possible to have lines shorter than what might be expected from a manually digitized file. In this case, the snapping threshold should be set lower than the default when running v.support, to avoid losing such lines.

A NOTE OF CAUTION: v.support allows the user to assign a snapping threshold other than that used during digitizing. Users who assign a higher snapping threshold than this should be careful -- a higher snapping threshold may join line segments that the user did not intend to join, thereby corrupting the data file!!!

See "Cartographic Issues in the Development of a Digital GRASS Database" from which the information in this section was obtained, for further information on map development issues.

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Automatically created on: Sun Apr 10 10:50:16 2005