Forwarded to the list upon request from T Laronde.
On Fri, Dec 4, 2009 at 3:27 PM, <[email protected]> wrote: > Hello, > > I stumbled upon the "new ideas" for vector of GRASS 7 and wanted to give > hints. I'm not subscribed to grass-dev (and don't want to) so I'm not > sure if the message will be accepted or not by the program managing the > list. > > FWIW, here are my comments. You can do whatever you feel with them. > > Since it is clear that the ones commenting---and they say it...---don't > understand the vectorial stuff and are even not developers, it will be > IMO a very bad idea to let people adding at haphazard mess in this > area... > > Cheers, > -- > Thierry Laronde (Alceste) <tlaronde +AT+ polynum +dot+ com> > http://www.kergis.com/ > Key fingerprint = 0FF7 E906 FBAF FE95 FD89 250D 52B1 AE95 6006 F40C > > > ---------- Forwarded message ---------- > From: tlaronde > To: [email protected] > Date: Fri, 4 Dec 2009 15:15:27 +0100 > Subject: ticket #542: vector for GRASS 7 > [Please CC me on replies, since I'm not subscribed.] > > FWIW, having given a look at GPL GRASS plans for 7.0, I stumbled upon > ticket #542 about "new ideas" for vector. > > May I suggest: don't do that! It seems that, since Radim Blazek > leaved, no one has really grasped the vector "philosophy"---at least if > you want GRASS to continue to be topological. > > IMO, the first thing you should do is to create a lexicon, that is a > list of normalised words for GRASS and their meaning; this will avoid > the confusion between "lines" and "arcs" for example. Particularly, > when, for GRASS, two things are clearly distinct, even if in current > speaking there are fuzzy, you must impose two distinct words. Example: > arcs are the primitives of the vector definition (level 0). "Lines" are > geometrical objects deduced from primitives, as are "points", "faces" > and, perhaps, "volumes". Nodes are not geometrical objects : they are > topological creations. Centroïds are not geometrical objects neither, but > just a topological mean to _store_ attribute information : this is > attributed to a _geometrical_ object (hence: a "point", a "line", a > "face" or a "volume") if the point is equal to a point, along a line, on > a face or inside a volume. Note: I use special words, distinct words for > the topological relations so that in the _code_ saying "equal" means > point; saying "along" means line; saying "on" means face and so on. > Always distinct words for distinct meanings, and when possible, the > first letter, or the two first letters are sufficient to tell the > complete word. > > The level 0 is just the storage of the _arcs_. Even geometrical elements > "points" are stored as _arcs_ (two vertices identical). > > The level 0 is needed for the definition of the primitives and for the > construction of the topology. It is not needed for a huge part of the > applications when you do not need _metrics_. Topology gives _order_ ; > coordinate system, definitions, arcs give you _metrics_. So improving > the reading of the _arcs_ is only needed from time to time. > > The topology gives you direct access (offsets) to the metrics if needed. > So there is already information to speed things. > > May I say that the main problems are not to "improve" your current (new) > vector handling, but to clean the problems introduced. Specifically, > merging the Sites as Points in the vector was, IMO, an error. There is a > need for the grid (also called: raster but this is a bad name), > singularities (Sites) and topological vector (arcs). > > Secondly, the introduction of 3D in vector is not perhaps a great idea > if you remember what a GIS is mainly for. For the coordinates systems, > there is a surface of reference: a geoïde, simplified as an ellipsoïde. > The grid is a mean, along with a connexion of Sites to described a more > complexe surface relative to the geoïde (via the ellipsoïde). > Generally, in cartography, the vector lines are drawn on the surface. > So the 3D is given by the grid or the triangulation of Sites > underneath. Imagine a corrugated iron as a surface. You can draw > two "straight lines" (view from above) ; view from the side, they > are not straight at all. But since they are linked to the surface, > you can describe them by a starting and an ending vertice; you do > not need, at every moment, to drag a whole Bezier description. Only > when you need the 3D, you combine the surface with the straight > lines. And if you change the description of the surface, you do > not need to change the description of the vector: it is "just" > drawn on the surface linked, whatever it is. You need 3D only for > metrics; and if the vectors objects are not on the same surface, > you describe them in distinct layers. > > Just to give you the incentive to think twice about the vectorial stuff. > It is really an interesting part, and the CERL GRASS strength is the > topological stuff. Take the time to understand this deeply first, before > making "something" just because "new" is an advertising word. > > HTH > -- > Thierry Laronde (Alceste) <tlaronde +AT+ polynum +dot+ com> > http://www.kergis.com/ > Key fingerprint = 0FF7 E906 FBAF FE95 FD89 250D 52B1 AE95 6006 F40C > > _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
