[Talk-us] Better urban WMS layers for JOSM

Alan Millar am12 at bolis.com
Wed Mar 12 00:30:37 GMT 2008

I've been trying to find some easier ways to clean up the Tiger data for
my area around Portland, Oregon.  I've been corresponding with a few
others also trying to do the same thing.

I'll skip to the punchline first:  Better JOSM layers for aerial photos at


and public street maps from the county tax office at


Here's the long version of the story:

For my county, I've found excellent position consistency among all data
sources other than Tiger.  My GPS readings match the Yahoo aerial photos,
Google Earth, Washington County tax maps, Beaverton city maps, and the
USGS urban area photo maps at terraservice.net.   The Tiger data is
broadly correct with pretty much all the right streets in the right
relationships, but it is often shifted out of place by varying amounts.

So everybody agrees except Tiger.  This is good, in that it gives me
confidence in the other sources, meaning I don't have to travel every
single street with my GPS since my tax dollars already did so.  I figure
that if I get a large rectangle by GPS which matches the aerial photos,
then the area within the rectangle can be aligned with just the photo. 
I've been looking for the most efficient way to do that.  Sometimes the
Yahoo aerial photos are not as clear as I would like, such as being
obscured by trees.

Although I like the simplicity of Potlatch, I found I really need the
multiple-node selection and movement capability in JOSM for realigning the
Tiger data.  The key, then, has been to find better sources of public data
which I can use as WMS layers in JOSM.

The first was better aerial photos.  I don't copy from Google Maps or
Earth, but I do occasionally cross-check things there.  I noticed in
Google Earth that the on-screen credits do change, and seem to reflect the
actual source of your current view.  I found Portland, OR credited for
close-up aerial street photos.  Digging around some more, I found that
this data is being distributed by the USGS as part of its urban area focus
project, and I stumbled into this page:


showing how to use it as a WMS layer in JOSM.  Item number one (better
photos) solved.

Item number two took longer to figure out.  I've used the Washington
County tax maps at http://washims.co.washington.or.us for several years
for personal use, before I found OSM.  I have seen references on the
county web site to ESRI and ARC software, so I knew there was a decent
base underneath.  The one thing I could not find was any WMS services.

I spent some time with PDF maps, and hit a dead-end with MetaCarta's
currently-broken map rectifier :-(   This sent me hunting again back
closer to the source.  I eventually Googled into a Navy project which is a
WMS server for many GIS sources, at


With some WMS experimentation in Google Earth, I was able to find what I
was seeking:  decent street maps from the county tax assessor at varying
levels of resolution.  Quite excellent resolution, in fact; the details
increase as you zoom in closer to the street and lot level.  Problem two

Although I'm focusing on one particular county, it appears that the Navy
project has hundreds or thousands of data sources available.  If your
county data is there, it may be worth a look.

I understand that we need to be very cautious about copying other people's
intellectual property into OSM.  In this case, since the data is based on
public records from my tax assessor's office, I don't expect there is a
problem in using it as a verification base for the Tiger data.

The only minor problem I am having is that the Portland tax maps have
light grey lines on a white background for the streets, and the mappaint
mode in JOSM is hard to see on top of it.  Switching to wireframe mode is
an adequate workaround for now.

- Alan

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