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?
Thanks, Dave
--
On Mon, Oct 24, 2016 at 9:04 AM, Dave Roberts <droberts@montana.edu> 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. Depends on the particular context I guess, could be related to
topology too. 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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On Mon, Oct 24, 2016 at 4:07 PM, Anna Petrášová <kratochanna@gmail.com> wrote:
On Mon, Oct 24, 2016 at 9:04 AM, Dave Roberts <droberts@montana.edu> 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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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 <kratochanna@gmail.com> wrote:
On Mon, Oct 24, 2016 at 9:04 AM, Dave Roberts <droberts@montana.edu> 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
--
--
On Tue, 25 Oct 2016, Dave Roberts wrote:
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.
Dave,
If I may contribute a thought or two to this thread: there are two
definitions of 'geometry' which we all use. One definition is coordinate
geometry which can be described as where points on a geographic surface are
located; the x, y values.
Then there's object geometry. When setting a dinner table it's common to
place plates, glasses, and flatware in designated positions relative to each
other. Same with the controls in a vehicle: the accelerator is on the right,
the brake in the center and the clutch on the left (at least here in the
US; it's been decades since I drove in the UK).
If we think of GRASS' geometry as object geometry, the placement of vector
objects relative to each other it might lessen any confusion.
Rich
Dear Dave,
On Tue, Oct 25, 2016 at 9:25 PM, Dave Roberts <droberts@montana.edu> wrote:
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.
Adding a new layer with e.g. v.category does change the geometries
because the combination of (category, layer) is stored in the coor
file together with the geometry can be of type point, centroid, line,
boundary.
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.
"Geometry" refers first of all to points, lines, and areas. Centroids
and boundaries are special cases. Nodes are not a geometry feature,
but an internally used structure that connects lines and boundaries.
From a user perspective, nodes are important for vector network
analysis, otherwise nodes are used internally to construct areas.
Obviously that's more operational than conceptual so I was wondering where
the concept of geometry is actually defined.
Can you provide an example of a GRASS manual where the usage of the
term "geometry" is not clear?
Markus M
Thanks, Dave
On 10/24/16 14:39, Markus Metz wrote:
On Mon, Oct 24, 2016 at 4:07 PM, Anna Petrášová <kratochanna@gmail.com>
wrote:On Mon, Oct 24, 2016 at 9:04 AM, Dave Roberts <droberts@montana.edu>
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
----
Thanks Rich. Comments interspersed.
On 10/25/16 13:33, Rich Shepard wrote:
On Tue, 25 Oct 2016, Dave Roberts wrote:
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.Dave,
If I may contribute a thought or two to this thread: there are two
definitions of 'geometry' which we all use. One definition is coordinate
geometry which can be described as where points on a geographic surface are
located; the x, y values.
Right, this is what most of us think of immediately.
Then there's object geometry. When setting a dinner table it's common to
place plates, glasses, and flatware in designated positions relative to
each
other. Same with the controls in a vehicle: the accelerator is on the
right,
the brake in the center and the clutch on the left (at least here in the
US; it's been decades since I drove in the UK).If we think of GRASS' geometry as object geometry, the placement of
vector
objects relative to each other it might lessen any confusion.
But here again is the confusion. The position of objects relative to each other is independent of the number of layers associated with each object. And so it seems to me that when GRASS users talk about "geometry" they mean space AND layers combined because they are inseparable in the implementation. Perhaps we need a new word for spatial realization and attributes combined rather than geometry.
Thanks, Dave
On Tue, 25 Oct 2016, Dave Roberts wrote:
But here again is the confusion. The position of objects relative to each
other is independent of the number of layers associated with each object.
And so it seems to me that when GRASS users talk about "geometry" they
mean space AND layers combined because they are inseparable in the
implementation. Perhaps we need a new word for spatial realization and
attributes combined rather than geometry.
Dave,
Well, I consider layers to be the third dimension in GRASS vector object
geometry. They are not independent geometrically if we think of them as
'themes,' each mapping objects of the same type, and yet that theme/layer
has positional relationhips (geometry) with other layer which is usually
where our interest lies.
Rich
On Tue, Oct 25, 2016 at 10:23 PM, Rich Shepard <rshepard@appl-ecosys.com> wrote:
On Tue, 25 Oct 2016, Dave Roberts wrote:
But here again is the confusion. The position of objects relative to each
other is independent of the number of layers associated with each object.
And so it seems to me that when GRASS users talk about "geometry" they
mean space AND layers combined because they are inseparable in the
implementation. Perhaps we need a new word for spatial realization and
attributes combined rather than geometry.Dave,
Well, I consider layers to be the third dimension in GRASS vector object
geometry. They are not independent geometrically if we think of them as
'themes,' each mapping objects of the same type, and yet that theme/layer
has positional relationhips (geometry) with other layer which is usually
where our interest lies.
I agree, nice description! Thanks, Rich.
Markus M