I wrote:
I examine these in JOSM right now. First they need to be unzipped,
and it looks like the (provided on that web page) PRJ file to change
from WGS 84 (default) to either NAD 27 or 83 projection is required.
I haven't done that to these data in the instant case, but I've
fiddled these before and I think it is doable.
OSM uses WGS-84 as its default, and I erred in not realizing these
data are in NAD 27, so a conversion does seem to be necessary. When
you open them as-is with JOSM (+ Shapefile plug-in), you get an alert:
"Unable to detect Coordinate Reference System. Would you like to
fallback to ESPG:4326 (WGS 84)?"
This implies that a "fix string" (perhaps via a .prj file, see below)
is required to get JOSM to "convert" (or "re-project") from the
native NAD 27 to WGS 84. This is truly required, I believe.
Dave Mansfield writes:
That brings up a good question. What datum is used by OSM? I would
assume it's WGS 84. NAD 83 is within about a meter of WGS 84. That's
closer than the GPS units most of us have so would not cause much of
an error if any. NAD 27 on the other hand could be off by as much as
180 meters.
The "Oak Ridge Page" http://www-cta.ornl.gov/transnet/RailRoads.html
does say this:
Geographic accuracy is roughly equivalent to a scale of 1:100,000,
but is object-specific (see documentation). Comparison to a set of
70,000 FRA inspection car GPS locations showed a lateral offset of 20
m root mean square, so a reasonable estimate of the geographic
accuracy of active lines in CONUS would be 25 m RMS. The geographic
coordinate system is decimal degrees, datum NAD27. No ESRI PRJ file
is included with the shapefiles, but if there were, it would look
like this (cut and paste if needed):
GEOGCS["GCS_North_American_1927",DATUM["D_North_American_1927",SPHEROID["Clarke_1866",6378206.4,294.9786982]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]
Any "magic incantation" that will just "do this" (for shapefile data)
is appreciated. I don't know if copy-pasting the above string into a
.prj in the shapefile folder will direct JOSM (via Shapefile plug-in)
to "adjust" the data to WGS 84, or if something different is
required. It is even possible that clicking on JOSM's alert button
of "Yes" is all that is required, but I'm not sure. Thank you in
advance.
SteveA
California
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