[GRASS-user] survey: use of different geometry types in same vector map

[Benjamin Ducke]

Hi,

There are many other GIS and file formats that have
multi-geometry support, such as ArcInfo and MapInfo.
Actually, I am not sure that shapefiles with their
single geometry limitation aren't the exception rather
than the rule!

There are uses for this. E.g. in GRASS 6, the original
network analysis work on multi-geom input, with network
links and nodes represented in one and the same map.
Clearly, this is just a design decision, but it does serve
to keep related data tightly together in one dataset.

For the same reason, multi-geometry layers are also
useful for topological data processing.
E.g. GRASS stores boundary line objects and centroid
point objects in one map. If you have e.g. Shapefiles
and want to run topology tests, then you always have
to juggle separate layers.

And why create, e.g. three spatial tables per theme
in a PostGIS DBMS, if you might as well keep everything
nicely together in one? Especially if you have common
attribute data schemas.

So, while I think that separating geometries into different
_layers_ makes data processing easier, the same is not
necessarily true for the actual data _sources_. There,
multi-geom support can make data storage and essential tasks
such as topological validation much more efficient.

Best,

Ben

On 05/03/14 12:08, Moritz Lennert wrote:
> Hello,
> 
> Recent discussions here with colleagues about GRASS' vector format and
> the teaching of vector handling in GIS have brought up a question about
> the fact that GRASS (contrary to some other well-known vector formats)
> allows a mix of geometry types (points, lines, polygons) in the same
> map, something which some GIS'ers consider quite unorthodox.
> 
> In order to enrich the reflection on that, I would like to ask GRASS
> users for use cases where this mix has been useful. Do you use such
> mixed geometry maps ? Which specific use cases do you use them for ?
> 
> In order not to flood the mailing list, you can also send me your
> response by private mail. I'll report back to the mailing list with the
> results.
> 
> Moritz
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-- 
Dr. Benjamin Ducke, M.A.
{*} Geospatial Consultant
{*} GIS Developer

  benducke at fastmail.fm