missing in the puzzle, but it doesn't sound like the issue is with the
PostGIS conversion part of the process, so, thanks for the help to this
point -- I'll check with the OpenStreetMap community next.
>
> Thanks,
>
> Stephen
>
> > On Fri, 2008-04-04 at 01:07 +0100, Jonathan W. Lowe wrote:
> > > Stephen,
> > >
> > > My initial testing has been on Alameda County (California) TIGER data.
> > > The two attached image files show an overlay of US Census 2000 Blocks
> > > over an area south of the UC Berkeley campus. The offset is the same
> > > for both Google and OpenStreetMap (OSM). This suggests that I've made a
> > > mistake somewhere, because the OSM tiles in the United States are all
> > > rendered from TIGER linework, so the TIGER census blocks should match
> > > exactly.
> > >
> > > For the same source shapefile (tabblock00.shp), there's a nearly perfect
> > > match between block boundaries and streets in the area just South of
> > > Oakland's Lake Merritt. It smells like a datum conversion issue...
> > >
> > > The conversion path was from shapefile to PostGIS using shp2pgsql. I
> > > used a custom projection of 32767 rather than 4269 because the existing
> > > srtext for 4269 had a degree value as 0.01745329251994328, but the US
> > > Census metadata listed a degree value of 0.017453292519943295. Perhaps
> > > not significant? My spatial_ref_sys entries for 4269 and 32767 are
> > > otherwise pretty similar:
> > >
> > > SRID: 4269
> > > SRTEXT: GEOGCS["NAD83",DATUM["North_American_Datum_1983",
> > > SPHEROID["GRS 1980",6378137,298.257222101,
> > > AUTHORITY["EPSG","7019"]],
> > > AUTHORITY["EPSG","6269"]],
> > > PRIMEM["Greenwich",0,
> > > AUTHORITY["EPSG","8901"]],
> > > UNIT["degree",0.01745329251994328,
> > > AUTHORITY["EPSG","9122"]],
> > > AUTHORITY["EPSG","4269"]]
> > > PROJ4TEXT: +proj=longlat +ellps=GRS80 +datum=NAD83 +no_defs
> > >
> > > SRID: 32767
> > > SRTEXT: GEOGCS["GCS_North_American_1983",
> > > DATUM["D_North_American_1983",
> > > SPHEROID["GRS_1980",6378137,298.257222101]],
> > > PRIMEM["Greenwich",0],
> > > UNIT["Degree",0.017453292519943295]]
> > > PROJ4TEXT: +proj=longlat +ellps=clrk66 +datum=NAD27 +no_defs
> > >
> > > To display census block data in OpenStreetMap, I extract it from PostGIS
> > > with a transform to EPSG 4326, although the coordinates don't seem to
> > > change as a result. (This seems correct, as datum=NAD83 and datum=WGS84
> > > are, for my purposes at least, are essentially identical.)
> > >
> > > Thanks,
> > > Jonathan
> > >
> > > 2 attachments: TIGER2007andOSM.png, TIGER2007andGoogle.png
> > >
> > >
> > > On Thu, 2008-04-03 at 19:18 -0400, Stephen Frost wrote:
> > > > Jonathan,
> > > >
> > > > * Jonathan W. Lowe ([EMAIL PROTECTED]) wrote:
> > > > > Have you yet tried overlaying TIGER 2007 linework or census
block/tract
> > > > > polygons over Google or OpenStreetMap tiles? I'm seeing a good
match in
> > > > > some areas but a significant shift (~50 meters) in others. Thought
it
> > > > > might be a datum conversion issue, but can't seem to find a match.
> > > >
> > > > I hadn't looked at the linework too much yet or tried to overlay it.
> > > > I'm curious where you're seeing the differences though because I know
> > > > that Census is only about half way through their MAF improvment project
> > > > and I actually have some info about what has been done so far and what
> > > > hasn't. It'd be interesting to see if it matches up.
> > > >
> > > > There are a few places (Guam, Hawaii islands) where they actually do
use
> > > > an SRID other than 4269, but my scripts don't yet handle that and I'm
> > > > guessing that's not what you're referring to anyway. :)
> > > >
> > > > Thanks!
> > > >
> > > > Stephen
> > > >
> > > > > On Thu, 2008-04-03 at 17:07 -0400, Stephen Frost wrote:
> > > > > > * Stephen Frost ([EMAIL PROTECTED]) wrote:
> > > > > > > I think they may have also upgraded their pipe.. I got about
1.41MB/s
> > > > > > > (11 Mb/s) for the whole transfer. It's about 22G all told. I'll
> > > > > > > probably be trying to load it up into PG on one of our servers
tomorrow.
> > > > > > > It was a bit over 4 hours for me to pull down off of their
> > > > > > > ftp2.census.gov ftp site.
> > > > > >
> > > > > > Just to update those who might be interested- I've finished the
data
> > > > > > load into one of our servers at work. It comes to ~60GB on disk in
> > > > > > PostgreSQL/PostGIS with appropriate indexes in most places and
whatnot.
> > > > > > Based on what I've seen so far, it looks *very* nice, especially
the
> > > > > > hydrogrophy ("areawater"). It also appears to be pretty consistant
> > > > > > across the layers, which is also good.
> > > > > >
> > > > > > If anyone's interested in the scripts used to load the data
(they're
> > > > > > pretty simple, really), I'd be happy to provide them.
> > > > > >
> > > > > > Enjoy,
> > > > > >
> > > > > > Stephen
> > > > > > _______________________________________________
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> > > > >
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> >
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