[GRASS-dev] terminology issues in grass7

[Dylan Beaudette]

On Monday 11 August 2008, Paul Kelly wrote:
> On Mon, 11 Aug 2008, Maciej Sieczka wrote:
> > Paul Kelly pisze:
> >> I think though, that connecting multiple layers to different tables is
> >> the main application for layers? Are they much use for anything else? In
> >> which case, calling them tables makes things clearer. Perhaps even table
> >> would be enough - each vector map can be connected to multiple tables,
> >> each vector map can have multiple tables, each vector map can have
> >> multiple table links... is there a big difference in meaning between
> >> those different sentences? I feel removing the word "link" improves the
> >> clarity of the meaning without adding any additional ambiguity.
> >
> > I don't agree with Paul. In GRASS vector terminology the term "table"
> > already has a very well defined meaning and it must not be used for
> > anything else.
> >
> > (A "table" is an object in the database that stores the given "layer"'s
> > attributes, and the "table" and "layer"'s geometrical features are
> > linked using "key column" in which the "categories" are stored inside
> > the "table".)
>
> Agree 100%. In fact, that was exactly my point. In the interests of
> clarity/simplicity, I was proposing to consider a table as "belonging" to
> a particular map, e.g. the table containing a key column (or key field)
> whose values correspond to the categories in layer 1 of vector map x
> could be considered as table 1 of vector map x. The fact that tables may
> belong to multiple vector maps, and indeed exist independently outside of
> GRASS, doesn't IMO impede on the clarity of this relationship between a
> vector map and multiple tables: the same table may belong to different
> vector maps and be associated with a different table number in each.
>
> > Regarding Moritz's remark I indeed missed the fact that the vector map
> > having 0 or more "layers" does not directly imply it has the same number
> > of data "tables". Given that, "table link" to replace "layer" as I
> > suggested is bad. If we are to change the term, we should do it right.
> > How do you like "category set" then, "catset" in short? Together with
> > with replacing term vector "map" with vector "layer" it would yield:
> >
> > Each vector "feature" (line, point etc.) can have 0 or more "categories"
> > in a vector "layer". Each "category" belongs to only 1 "category set".
> > Each "category set" of a vector "layer" can be connected or not with a
> > single database "table". The "key column" in that "table" stores the
> > "categories" of "features" present in the given "category set".
> >
> > Any good?
>
> I think we are thinking the same :) IIUC, a category set is quite
> meaningless unless the table containing more information on each category
> in the set is also known. My proposal is simply to leave the concept that
> a category set can be meaningful in the context of more than one linked
> table to advanced users, and associate the table directory with the vector
> map. This could lead to the situation where, e.g. table 0 of vector map x
> and table 1 of vector map y refer to the same table.

After some more thought--

There may be an obvious reason that I am missing, but what about labeling 
geometry primitives (points, line, boundaries, etc.) with a "feature id" 
instead of a "category". I understand that the 'cats' have been a fundamental 
part of GRASS for a long time, but replacing the construct now may lead to 
simpler verbage downstream:

Each geometry primitive (feature) has an ID that links that feature to 1 or 
more records in a table-- we could even call this the "FID" for short, 
or "GID" (geometry ID) if "FID" is off-limits. 

I tend to think about the entire "layer" notation as if it were a sheet with 
IDs that has been draped over the geometry. Changing the "layer" allows one 
to change the relationship to records in some table.

Some thoughts on how to convey these concepts:

cat -> FID/GID

layer -> FID interface 1,2,3,...
layer -> table interface 1,2,3,...
layer -> feature to table relationship 1,2,3...

... I don't really like any of these, but you probably get the idea.

Cheers,

Dylan

> My question is still open though - are there any other practical
> applications for layers, other than enabling vector maps to be connecyed
> to multiple tables?
>
> > Talking about layers (in their current meaning) - there is no convenient
> > tool to report the number of layers in a vector map. There is only
> > v.category opt=report. Could v.info be extended in this regard? Oh, and
> > the regular v.info already reports number of "dblinks" (which I guess
> > might be renamed to "table links", but I won't insist), while v.info -t
> > doesn't. Could this be addressed too please?
> >
> >> With regard to calling maps something different though, I think that
> >> would be very confusing and not a good idea (especially if they were
> >> renamed to layers). Map has IMHO a much clearer meaning than layer.
> >> There is the issue of ambiguity with a printed map I suppose, but use of
> >> the word in that context is kind of non-technical I feel. The use of the
> >> word map has a clearly defined historical meaning in GRASS (and
> >> influences other words too, e.g. a mapset = a collection of maps -
> >> should this be renamed a layerset?) and I feel that it should stay.
> >
> > Paul has points here. Yet I *guess* I'd prefer to trade legacy for
> > clarity anyway. Calling GRASS "maps" "layers" would improve clarity
> > IMHO, especially for newcommers. Word "map" has been in use for
> > centuries and the word immediately brings a nice picture with north
> > arrow, legend and stuff to my mind. "Layer" is *the* GIS word for a set
> > of features that can be represented graphically as a map, as well as a
> > table or a set of statistic properties etc.
>
> Well I have no idea here; all I can do is point out that I learned GIS
> from GRASS and have no experience of any other GIS, and the concept that
> each raster and vector file is actually a real map was really helpful to
> me when learning. :)
>
> Paul
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-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341