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

GRASS GRASS user
1

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

2

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
_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user

--
Dr. Benjamin Ducke, M.A.
{*} Geospatial Consultant
{*} GIS Developer

  benducke@fastmail.fm

3

On Wed, Mar 5, 2014 at 12:08 PM, Moritz Lennert
<mlennert@club.worldonline.be> 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.

Nothing to add to Ben's reply from a user perspective.

From a conceptual perspective, it depends on the definition of "vector

map". Considering the number of vector formats (76 supported by OGR),
any definition must be independent of a specific vector format.

In OGR, a vector is defined as a data source which contains any number
of layers which in turn contain vector geometry objects. A vector
geometry object in turn can be a collection of vector geometry objects
of possibly different types. This property of a geometry object and
geometry collection conforms to the OGC simple feature specification,
a software and vector format independent (!) standard. That means that
even a single geometry object in a single layer in a single data
source can contain multiple feature types.

In my experience, a common task (probably the most common task) is to
convert GIS data of any format to the format of your choice. Thus I
would be surprised if regular GIS users are not familiar with the
diversity you find out there in the wild.

Markus M

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
_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user

4

Markus Metz-3 wrote

On Wed, Mar 5, 2014 at 12:08 PM, Moritz Lennert
&lt;

mlennert@.worldonline

&gt; 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.

Nothing to add to Ben's reply from a user perspective.

From a conceptual perspective, it depends on the definition of "vector
map". Considering the number of vector formats (76 supported by OGR),
any definition must be independent of a specific vector format.

In OGR, a vector is defined as a data source which contains any number
of layers which in turn contain vector geometry objects. A vector
geometry object in turn can be a collection of vector geometry objects
of possibly different types. This property of a geometry object and
geometry collection conforms to the OGC simple feature specification,
a software and vector format independent (!) standard. That means that
even a single geometry object in a single layer in a single data
source can contain multiple feature types.

In my experience, a common task (probably the most common task) is to
convert GIS data of any format to the format of your choice. Thus I
would be surprised if regular GIS users are not familiar with the
diversity you find out there in the wild.

Markus M

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
_______________________________________________
grass-user mailing list

grass-user@.osgeo

http://lists.osgeo.org/mailman/listinfo/grass-user

_______________________________________________
grass-user mailing list

grass-user@.osgeo

http://lists.osgeo.org/mailman/listinfo/grass-user

I concur with all Ben and Markus have said.

some personal experiences as vegetation scientist who has done a lot of
habitat mapping for national administrations and agencies:

habitat entities can be points (e.g. old trees, ...), lines (e.g. small
running waters, ...) or polygones (hay meadows, ...); in all the cases the
GIS representations of points, lines and polygones were kept in different
GIS datasets (shapefiles), although all these entities are often related to
each other and share often the same database schema.

it was always a pain to keep three different GIS dataset updated or to amend
the database three times...

this is caused by historical technical reason, but for habitat mapping IMHO
this should be avoided in future because technical constraints have be
changed ...

-----
best regards
Helmut
--
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5

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