On Thu, Feb 28, 2008 at 06:54:09PM +0100, we recorded a bogon-computron collision of the <[EMAIL PROTECTED]> flavor, containing: > On 28/02/08 16:39, Tom Russo wrote: >> I have been trying to wrap my brain around "multi-layered" GRASS vectors and >> have only succeeded in wrapping my brain into knots. Perhaps someone here >> with a solid understanding of this stuff can help me. >> > > [...] > >> >> Since I'm rambling now, I'll just cut to the chase and ask my question: >> >> Given a GRASS vector with two attached database tables, one of which (layer >> 2, attaching attributes to centrois) has a key field POLYID, how can I >> create a new layer using a third, much smaller table, that also has a >> POLYID field, such that the new layer only contains that subset of >> centroids where the third layer table's POLYID matches layer 2's POLYID >> for that centroid? >> > > > I think at this stage your best bet is probably to do something like this: > > v.category OldMap option=add layer=3 out=NewMap > > #find a link between the category numbers created and the original > # categories, this should work: > > v.build OldMap option=cdump > TempFile > > # You will have to edit TempFile to only take into account layer 2 and > # then find a way to import the results into your database
With the potential for tens of thousands of records, anything involving manually editing a dump of categories seems an unworkable solution. > v.db.connect -o NewMap layer=3 table=NewTable > > #to remove all centroids you do not want: > v.edit tool=delete > or > v.extract > > There might be an easier way, though... I sure hope so. > I think that you also might want to file a few wishes in the tracker, such > as: > > - extend v.category with an option to use a column linked to an existing > layer as the cat for a new layer > - allow v.extract to write the results to another layer of the same file > instead of a different file > - extend v.patch to allow to patch different maps into different layers of > the output map > - create a v.copy.layers to copy elements of one layer to another Once I actually understand the steps of what I really need to do, then perhaps I could intelligently formulate a real wish for the tracker. At this point I'm simply adrift in a sea of v. and db. commands and SQL queries with no compass. -- Tom Russo KM5VY SAR502 DM64ux http://www.swcp.com/~russo/ Tijeras, NM QRPL#1592 K2#398 SOC#236 AHTB#1 http://kevan.org/brain.cgi?DDTNM "And, isn't sanity really just a one-trick pony anyway? I mean all you get is one trick, rational thinking, but when you're good and crazy, oooh, oooh, oooh, the sky is the limit!" --- The Tick _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
