GRASS GIS Manual: Vector data processing in GRASS GIS

GRASS GIS vector map exchange between different locations (same projection) can be done in a lossless way using the v.pack and v.unpack modules. The naming convention for vector maps requires that map names start with a character, not a number (map name scheme: [A-Za-z][A-Za-z0-9_]*). Sometimes topology is not necessary and the additional memory and space requirements are burdensome to a particular task. Therefore two modules allow for working level 1 (non-topological) data within GRASS. The v.in.ascii module allows users to input points without building topology. This is very useful for large files where memory restrictions may cause difficulties. The other module which works with level 1 data is v.surf.rst which enables spatial approximation and topographic analysis from a point or isoline file. Area topology also holds information about isles. These isles are located within that area, not touching the boundaries of the outer area. Isles are holes inside the area, and can consist of one or more areas. They are used internally to maintain correct topology for areas. The v.type module can be used to convert between vector types if possible. The v.build module is used to generate topology. It optionally allows the user to extract erroneous vector objects into a separate map. Topological errors can be corrected either manually within wxGUI vector digitizer or, to some extent, automatically in v.clean. A dedicated vector editing module is v.edit which supports global and local editing operations. Adjacent polygons can be found by v.to.db (see 'sides' option). Many operations including extraction, queries, overlay, and export will only act on features which have been assigned a category number. Typically a centroid will hold the attribute data for the area with which the centroid is associated. Boundaries are not typically given a category ID as it would be ambiguous as to which area either side of it the attribute data would belong to. An exception might be when the boundary between two crop-fields is the center-line of a road, and the category information is an index to the road name. For everyday use boundaries and centroids can be treated as internal data types and the user can work directly and more simply with the 'area' type. Source.


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Last Modified: April 23, 2016 @ 1:03 am