Dear Markus and Anna,
I'd have to dig a little for man pages, but one interesting example is a
series of GRASS-user postings on "grass vector model, cats and layers
concept" from 6/13 which you were central to Markus. At the time, the
question was "why does adding a new layer to a map require creating a
new map?" And your answer (over two postings) was "Because you need to
modify vector geometries in order to add a new layer. Categories and
layers are first and foremost stored together with the geometries...
[Adding a new layer] ... changes the geometry directly."
Nikos Alexandris (and I) were baffled by this because it doesn't change
any vertices, nodes, or centroids.
Now, having studied the structure of the coor file I see that adding a
new layer to a vector object increases the length of that record in the
coor file, so that all records downstream of that record also have to be
re-written. It's simpler and safer to write a new vector object than to
modify the original. It would be possible to simulate the expected
behavior by writing the new map to "tmp", deleting the original map, and
renaming the tmp map to the original name, but obviously the programmers
elected not to do that. It could, however, be easily implemented in a
shell or python script.
So now when I see "geometry" in a manual page or GRASS-user posting I
interpret that to mean "vertices, nodes, centroids, and associated
categories ordered by the (not necessarily consecutive) layer numbers"
as that's the essential nature of a record in the coor file that manages
all this.
Obviously that's more operational than conceptual so I was wondering
where the concept of geometry is actually defined.
Thanks, Dave
On 10/24/16 14:39, Markus Metz wrote:
On Mon, Oct 24, 2016 at 4:07 PM, Anna Petrášová <[email protected]> wrote:
On Mon, Oct 24, 2016 at 9:04 AM, Dave Roberts <[email protected]> wrote:
Frequently in GRASS help files or emails in this list the term "geometry" is
used. Is geometry synonymous with a record in the coor file, or does it
have a broader meaning?
I would say geometry of features, such as points, lines, boundaries,
areas.
and centroids. In most cases, the vector features of interest are only
points, lines, and areas.
Depends on the particular context I guess, could be related to
topology too.
An example of the context, i.e. a particular manual, would be helpful
for clarification.
Markus M
I am actually not sure what is in the coor file, but you
are not supposed to access it directly anyway.
Anna
Thanks, Dave
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