| # GRASS Help System

What Are Vector Data?

In GRASS vector format, area, linear, and point features are represented as the series of arcs forming their perimeters. An arc (aka, vector) is stored as a series of x,y coordinate pairs. The two end-points of an arc are called nodes. Two consecutive x,y pairs define an arc segment (aka, line segment).

Arcs, either singly or in combination with other arcs, form higher-level map features like linear features (e.g., roads or streams) and area features (e.g., soil types or timber cutting units). Arcs that form linear features are sometimes called lines, and arcs that outline areas are called area edges or area lines. Areas are also called polygons. In GRASS vector format, polygons are not stored explictly; they are reconstructed by finding the series of arcs that form their perimeters.

A GRASS vector data file is generally associated with other GRASS supporting files describing category values, category labels, and file topology. Together, these files constitute a vector map layer. Read the information under "GRASS Database Directory Structure," located back at the menu, if you wish further information on the location and content of vector data and supporting files.

Typically, data are digitized, imported, and exported in vector format and are analyzed in their raster format. Data can be displayed and output in either format.

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Automatically created on: Sat Oct 15 07:07:24 2005