[OSM-talk] OSM TIGER-based map quality

Christopher Schmidt crschmidt at metacarta.com
Fri Apr 4 12:05:20 BST 2008


On Fri, Apr 04, 2008 at 08:29:56AM +0000, Jonathan W. Lowe wrote:
> When overlaying OSM with the recently available TIGER 2007 shapefile
> data for Census Blocks in Alameda County (California), I'm encountering
> both an offset and difference in relative position of the linework.  In
> short, OSM's data looks a lot more accurate and consistent -- streets
> that should be straight actually look straight in OSM, but often zig-zag
> in the TIGER 2007 edges and tabblock shapefiles.  For a visual, visit:
> http://www.giswebsite.com/demos/tiger_overlays.html

Um, isn't this the whole point of OSM? The TIGER data was imported so
that it could be improved manually by users. This doesn't include just
the geometry either: attributes have been changed as well. (See
http://crschmidt.net/osm/history.html?type=way&id=21456366)

> Any observations or ideas about where the misalignment might come from?

This misalignment is common in TIGER: It's designed for 1:100000
accuracy. Anything more than that (you're at about 1:7500 there) is not
going to be accurate. (Or at least likely to not be.)

> Though it smells partially like a datum problem, that path hasn't
> yielded any solutions yet.

Doubtful. You'd have a more significant shift.

This is the real power of OSM at work. See it, and marvel in its glory.
:)

Regards,
-- 
Christopher Schmidt
MetaCarta




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