[postgis-users] new to gis, trying to get started

[Chris Hermansen]

Assuming you've done all the googling you can stand, looking for "world 
city shapefile" and strings like that...

In my experience of dealing with data, and recognizing the tools that 
are available for cleaning things up, if I were you I would start by 
converting the datasets you find interesting into a common format that 
you can edit, say shapefiles.

For this conversion process you might use ogr2ogr or you can try 
camp2camp.  By the way, though I say this without any personal 
experience, I would imagine that compiling ogr2ogr for the Mac should 
not be horribly difficult.

As you begin to collect your data in shapefiles, you can use OpenJUMP to 
edit it, for instance by selecting the data of interest, copying and 
pasting it into a new layer, adjusting the tabular schema, and saving as 
a new shapefile.  There are other alternatives to OpenJUMP, such as 
QuantumGIS, but I've had some good experiences lately cleaning up data 
with OpenJUMP.  If a substantial number of datafiles exist with the same 
schema, and the schema is suitable to you, you can perhaps just pull out 
the information of interest with the ogr2ogr selection capability and 
avoid all the pointing - and - clicking.

When you're done with the edit / cleanup step, you should have a great 
number of reduced / edited / restructured datasets that fall into groups 
with similar attributes, that you've made as similar as possible through 
your editing / cleanup.

At this point you can use ogr2ogr or camp2camp or shp2pgsql to import 
your datasets into PostGIS.

Your last job would be to write some SQL that would massage your data 
into a final coherent structure.

This is not a trivial project!  Unless you get very lucking in your 
search for starting datasets.

If I were contemplating this project, I would be wondering about things 
like "how valid are these city polygons I'm finding, anyway - do they 
represent the actual 'city limits', where such things exist? or were 
they some visual boundary compiled by the map maker?  or something else 
entirely?"  Think about big, free-styling cities like Mexico City or 
Lagos in this context.  And remember that urbanization doesn't usually 
halt at the 'city limits'.

Anyway, good luck.  You can write to me off-list if it seems that your 
queries are getting too distant from the subject matter of this list.

anders conbere wrote:
> I'm trying to do some work with cities, but struggling to figure out
> first how to get polygon data about the worlds cities and second how
> to patch together many different formats and files into a singular
> postGIS database
>
> Say for instance all the Tiger/Line data...
>
> I would be more than happy to build a US set of data based on that for
> city polygons, but they're not in shapefile format (and ogr2ogr isn't
> available on my mac, though I can convert them on my linux box at
> home), and I'm not sure I understand who the shapefile importer works
> in terms of clobbering data already there etc.
>
> anyway... I guess I'm lost and looking for ways to deal with all the
> vast different paths doing google searches for GIS seams to take you.
>
> ~ Anders
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>   


-- 
Regards,

Chris Hermansen · mailto:clh at timberline.ca
tel:+1.604.714.2878 · fax:+1.604.733.0631
Timberline Natural Resource Group · http://www.timberline.ca
401 · 958 West 8th Avenue · Vancouver BC · Canada · V5Z 1E5

C'est ma façon de parler.