# GRASS Help System

## SNAPPING THRESHOLDS: AN INTRODUCTION

Map data are digitized in vector format. The edges of map feature are traced with a digitizer. These edges consist of a series of line segments (arcs) connected at their end-points (nodes). The geographic coordinates of these line segments and nodes are stored in a vector data file.

If the user is digitizing the perimeter of a closed, connected surface (e.g., an installation boundary), all of the line segments representing the perimeter must join together (like tinker-toys) in the data file.

A line segment that is attached to a node is said to be "snapped" to that node. To join two (or more) line segments, the user must "snap" these lines to the same geographic node point.

A snapping threshold determines what digitized points are assigned to the same node. If two nodes fall within the snapping threshold distance of one another, they will be forced to use the same node point. The result will be a single node rather than the original two, and the two line segments previously connected to two different nodes will be joined at the single node retained.

The snapping threshold makes it possible for those digitizing to connect lines without posessing superhuman abilities. The machine will assign any two points falling within the snapping threshold to the same geographic coordinate node point, even were they (unintentionally) entered at very slightly different spots.

Read The Snapping of Nodes for information on how snapping is conducted.

Please refer to Setting the Snapping Threshold Value for important information on setting the snapping threshold.

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: Sat Jan 8 06:50:26 2005