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− | <H2>
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− | The GRASS vector data model
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− | </H2>
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− | <P>
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− | It is important to understand the GRASS vector data model prior to
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− | digitizing.<A NAME="240"></A> In general, GRASS uses a topological
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− | vector model.<A NAME="241"></A> This means that areas are not represented
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− | as closed polygons, but by one or more boundaries. A boundary between two
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− | adjacent areas is digitized only once, and it is shared by both areas.
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− | Boundaries must be connected without gaps. An area is identified (labeled)
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− | by the centroid of the area.
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− | <P>
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− | Besides boundaries and centroids, a vector map can also contain
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− | points and lines. All these geometry elements can be mixed
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− | in one vector and will be represented in different so called 'layers' inside
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− | one GRASS vector map. So in GRASS a layer is not a vector or raster map but a
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− | level inside a vector layer. This is important to distinguish carefully.
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− | <P>
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− | It is possible to store more 'layers' in one vector dataset. For example,
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− | fields, forests and lakes can be stored in one vector. Adjacent
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− | forest and lake can share the same boundary, but they have separate attribute
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− | tables. It is also possible to attach attributes to boundaries. For example,
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− | the boundary between lake and forest is a road, so it can have a different
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− | attribute table.
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− | <P>
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− | The 'layer' of the feature is defined by 'layer' inside GRASS. 'Layer' is the
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− | number which defines if there are more than one layer inside the dataset, e.g.
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− | if the geometry is forest or lake. For now, it can be only a number, in the
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− | future GRASS will also support names as fields in the user interface.
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− | <P>
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− | Attributes can be stored inside the GRASS LOCATION as DBase or
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− | SQLITE3 or in external database tables, for example PostgreSQL, MySQL,
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− | Oracle, etc.<A NAME="244"></A>
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− | <P>
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− | Attributes in database tables are linked to geometry elements using
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− | a 'category' value.<A NAME="245"></A> 'Category' (key, ID) is an
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− | integer attached to geometry primitives, and it is used as the link to one
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− | key column in the database table.
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− | <BR>
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− | <P>
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