Vector FAQ GRASS6

From GRASS-Wiki
Jump to: navigation, search

Q:GRASS 6 vector architecture - FAQ: Why on earth a topological model!? It introduces in fact many problems. Nontopological GIS is easier to use and all popular proprietary GIS desktops like ArcView or Mapinfo use nontopological model. But there are some advantages of topological model over nontopological. In addition, OpenGIS specification is not topological, so GRASS may never conform Open GIS once topological model is used.
A: Well, we just prefer quality.

Q: What are the programming details?
A: See here: http://grass.osgeo.org/programming7/vlibFAQ.html

Q: Everything is going RDBMS. The only modern and good enough solution is to store all data in relational database. Why do you spend all that time working on grass vector format if you can use PostGIS/Postgres?
A: Its' a matter of choice. And GRASS GIS support PostGIS/PostgreSQL. You can choose the database backend, and in GRASS 7 a direct integration has been implemented. Note that GRASS's vector format is topological, which opens the doors to all sorts of analyses which a non-topological format (like SHAPE) does not. And you can link a vector to an external RDBMS with v.db.connect or use directly a PostGIS data source with v.external.

Q: Why can't I have layers called 1516?
A: 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_]*). Reason: GRASS uses SQL which does not permit names starting with a number or symbol.

Q: Can you save color rules in vector files?
A: Yes, see GRASSRGB column in the d.vect manual page and v.colors

Q: It is well known that SHAPE files may contain topological errors. What can I do?
A: GRASS helps you: the v.in.ogr import cleans most problems, furthermore there are v.build which offers an error map as well as v.clean and v.edit for special problems.

Q: Why do we need support for so many formats and databases? I think that one format + one database is enough.
A: Again it is a matter of choice. What's wrong with supporting several formats?

Q: Is it possible to write/edit non native formats like shapefile, coverage or PostGIS?
A: yes: v.out.ogr and from GRASS 7 onwards, v.external

Q: Why are two line types (line and boundary) needed? Everything is easier with just one type, otherwise thousands of hours must be spent to assign the right type to each line.
A: If you have lines which are part of areas and lines which are not part of area (Example: roads, rivers etc) how could you distinguish them?

Q: Is it possible to generate TIN from raster DEM or 3D points?
A: More or less. Not exciting yet, but TINs are also not too exciting. See also Addons

Q: Are there volumes or other 3D objects supported?
A: 3D faces yes (e.g. CAD-style 3D buildings etc), use v.in.ogr or v.in.dxf to import or v.extrude to extrude from 2D.

Q: Are there multiobjects like multipoint or multiline supported?
A: Yes - v.build.polyline

Q: Does any support for overlay/analyses exist in GRASS 6?
A: yes - v.overlay and v.select

Q: Why do we need more categories attached to one element?
A: why not? Maybe you have different attributes to assign to one vector

Q: Is it supported Microsoft SQL Server / Oracle / Informix?
A: Oracle via OGR, Microsoft via ODBC, see sql

Q: Are there images and other arbitrary documents supported as attributes?
A: no (not yet?). Perhaps add blob support?

Q: How can be nontopological formats read by topological GIS without conversion?
A: pseudotopology is built - v.external

Q: Are there routes like in ArcInfo supported?
A: yes (see d.path, v.net.*, v.lrs.*)

Q: Will be possible to read attributes from ArcInfo coverages?
A: yes, use v.in.ogr

Q: Where can I find a description of the new vector format?
A: Try http://www.ing.unitn.it/~grass/conferences/GRASS2002/proceedings/proceedings/pdfs/Blazek_Radim.pdf and http://grass.osgeo.org/programming7 ("GRASS GIS Vector Architecture")