[postgis-users] TIGER/Line Shapefiles released

[Jonathan W. Lowe]

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 (jlowe at giswebsite.com) 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 (sfrost at snowman.net) 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
> > > _______________________________________________
> > > postgis-users mailing list
> > > postgis-users at postgis.refractions.net
> > > http://postgis.refractions.net/mailman/listinfo/postgis-users
> > 
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