GRASSベクターデータモデル

The GRASS vector data model

It is important to understand the GRASS vector data model prior to digitizing.<A NAME="240"></A> In general, GRASS uses a topological vector model.<A NAME="241"></A> This means that areas are not represented as closed polygons, but by one or more boundaries. A boundary between two adjacent areas is digitized only once, and it is shared by both areas. Boundaries must be connected without gaps. An area is identified (labeled) by the centroid of the area. <P> Besides boundaries and centroids, a vector map can also contain points and lines. All these geometry elements can be mixed in one vector and will be represented in different so called 'layers' inside one GRASS vector map. So in GRASS a layer is not a vector or raster map but a level inside a vector layer. This is important to distinguish carefully. footnode.html#foot242 <P> It is possible to store more 'layers' in one vector dataset. For example, fields, forests and lakes can be stored in one vector. Adjacent forest and lake can share the same boundary, but they have separate attribute tables. It is also possible to attach attributes to boundaries. For example, the boundary between lake and forest is a road, so it can have a different attribute table. <P> The 'layer' of the feature is defined by 'layer' inside GRASS. 'Layer' is the number which defines if there are more than one layer inside the dataset, e.g. if the geometry is forest or lake. For now, it can be only a number, in the future GRASS will also support names as fields in the user interface. <P> Attributes can be stored inside the GRASS LOCATION as DBase or SQLITE3 or in external database tables, for example PostgreSQL, MySQL, Oracle, etc.<A NAME="244"></A> <P> Attributes in database tables are linked to geometry elements using a 'category' value.<A NAME="245"></A> 'Category' (key, ID) is an integer attached to geometry primitives, and it is used as the link to one key column in the database table.
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