I've been working on an amazing national resource, regional to me,
which I greatly enjoy both in real life and mapping in OSM: the
Ventana Wilderness (around Big Sur, California) of the Los Padres
National Forest (LPNF). I have discovered that Ventana is only the
northernmost of ten wilderness areas in this single national forest
alone, and have found missing from OSM and multiple/conflicting
sources for these wilderness boundary data, correct and current
versions of which I very much want to see in OSM.
Recently, I was pointed to the "official" (US Department of
Agriculture/Forest Service) data source for these wilderness areas,
in the form of a shapefile containing all wilderness areas in all
national forests. This shapefile is about 23 megabytes, which
becomes about seven times larger in .osm format, due to its xlm-ish
nature. However, before initiating the translation from .shp to
.osm, JOSM informed me:
"JOSM was unable to find a strict mathematical transformation between
GCS_North_American_1983 and WGS84."
And further warned that the polyconic spherical case for such a
transformation is not implemented, so the algorithm would fall back
to the (error-prone) elliptical equations. Such a lenient
transformation can yield up to 1 kilometer errors. I am explicitly
warned that choosing this methodology means the results should NOT be
uploaded to OSM.
Yet, I am not so easily defeated. I still need to "whittle away" the
entire national dataset to just the ten LPNF multipolygons relations
(and their members) representing the LPNF wilderness areas. Assuming
I do, I further believe that I can add to my JOSM editing a USGS
layer which often or always has a wilderness boundary on it,
potentially allowing me to engage in some smart edit-blending away of
the (up to 1 km) transformation error. In other words, I'm using
multiple data sources to mitigate the potential error.
Question #1: If the additional USGS layer showing wilderness
boundaries does indeed allow me to mitigate away the transformation
error with some careful edit adjustments where necessary (though, it
is also possible NO or VERY MINIMAL adjustments are necessary),
should I feel comfortable uploading these boundary data? This begs
the additional question of "why not just start with USGS rather than
USFS data..." but you'll agree it is easier to start with something
largely done and tweak it, than it is to start from nothing and
painstakingly take a much longer time and effort to make it exact
from scratch.
Question #2: If this "test case" proves successful in uploading
wilderness boundaries in central and southern California for the
LPNF, and I document the set of steps I took (largely already just
done), might the greater OSM community reach consensus that all
national wilderness areas (and other national treasures it can be
valuable to have in OSM, such as wild and scenic rivers) are suitable
candidates for importation and upload? Even with a potential error
from a "lenient transformation?"
Question #3: Is it possible to improve the JOSM's transformation
algorithm so it uses polyconic spherical instead of the error-prone
elliptical equations? Is the best way to do this with a trac bug
report/feature request?
Thank you for your feedback,
SteveA
California
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