[Qgis-user] getting started for a complete GIS beginner
micha at arava.co.il
Mon Nov 26 16:49:09 EST 2007
tom sgouros wrote:
> Thank you again for your help (in my new neighborhood). I guess I
> somehow had the idea that data corresponding to a town *was*
> geo-referenced, since I also have the coordinates of that town's
> outline, and maybe that was where I was driving myself crazy.
What do you mean when you say you "have the coordinates of the town's
outline"? Surely each town's outline must be hundreds of coordinates?
> I understand the idea of editing the dbf file to get the data I want in
> there, and that certainly helps me begin to clear the air. But let me
> ask something else, since my task is simple, but I expect to have to do
> it quite often.
> The towns that I have are divided. Some are non-contiguous and there
> are lots of islands (I'm on the shore). Some towns make up twenty or
> thirty entries. There are only a few dozen towns, but nonetheless,
> editing the spreadsheet is more of a pain than it sounds (39 towns, 311
> entries). I know that doing a join is possible in the spreadsheet, but
> it is, shall we say, somewhat inelegant, and like I said, this is going
> to happen a lot. Is this the GIS-approved way to go, or is there
> something better I could learn about?
Editing a shapefile's dbf using a spreadsheet is definitely *not* the
GIS approved way. It was suggested as a "quick and dirty fix" for the
problem you described. At first it sounded like: "I have a nail and two
boards. What tool can I use to connect them?" But now it turns out you
that you actually plan to build a house. So you'll surely want to
resupply your toolbox!
For example, to solve the towns on islands problem, GRASS has a module
v.reclass, that can take individual polygons with some common attribute
value, and "merge" them into a multipart polygon. However I'm not sure
that's the proper approach here. If you merged all the islands into a
single town feature, you'd "loose" the area and any other data
particular to each individual island. You might best choose to give a
town code (or town name) to each section that is part of a town, and use
that code or name to link to a database table of other town data. This
would most efficiently be done in PostGIS, of course.
> Say I have another pile of shape data, like zip-code outlines or census
> tracts. If I want to map those outlines to my town shapes, and come up
> with a list of zip codes or tracts that overlap each town, what would I
> use? I don't mind being pointed at something with a shallow learning
> curve, so long as I know that it will, in fact, have what I need
> somewhere up the slope. My problem so far has only been that from as
> far down here as I am, I can't tell what's on which slope.
Again, these are spatial queries that QGIS alone can't do. The GRASS
v.overlay module would do the trick.
Dive in and try, and if you stumble over specific problems, come back
and ask again...
> Again, I thank everyone for their indulgence of such basic questions.
Arava Development Co, Sapir, Israel
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