[Talk-us] dubious church node

[Dave Swarthout]

"Second, many entries have their coordinates specified using the old NAD
27 datum, but somewhere along the line, that fact was lost and the
coordinates were assumed to be in either NAD 83 or WGS 84.  This
results in an offset that increases the further you go from central
Indiana; the offset in Alaska is upwards of a hundred meters to the

west."

Wow, thanks for that. If I understand what you're saying, this means many
of the old GNIS nodes will be positioned about 100 meters east of where
they should be? Or do I have your statement turned around?

The mine whose position I last adjusted, the Case Mine in the Chugach
Mountains on the Kenai Peninsula, was quite a distance from an area of bare
ground (visible only in ESRI) where an old mine site might have been. The
original position was to the east of that bare area. I didn't measure the
distance but will do that next time I come across such a mine.  The bare
spot also happens to be where the USGS Topo places the mine, consequently,
I felt moving it was justified.

I'm also guessing that the other Case Mine node,  the"duplicate" I
mentioned earlier, represents perhaps a second mine_entrance on the same
mining claim. However, there is nothing west of that node to provide any
clue to guide a repositioning, nor does it appear on theUSGS Topo map, so I
left it where it was.

On Sat, Sep 30, 2017 at 2:19 PM, Mark Wagner <mark+osm at carnildo.com> wrote:

> On Sat, 30 Sep 2017 06:56:31 +0700
> Dave Swarthout <daveswarthout at gmail.com> wrote:
>
> > Glad you mentioned that GNIS import, Ian.
> >
> > This isn't a pressing issue but I've been doing considerable mapping
> > in Alaska and encounter GNIS features constantly. Many of them are
> > nodes and refer to mines, usually abandoned mines, and contain
> > tagging that JOSM complains about, for example, using landuse=quarry
> > on a node. Sometimes I delete that tag and add man_made=mineshaft or
> > similar tagging but it's often not clear if the node is in the proper
> > location. The newer, high-resolution imagery will often suggest a
> > more likely spot for the node, and sometimes I'll move the node
> > there, but usually it isn't obvious. There are also duplicate nodes,
> > that is, mines having the same name but in a slightly different
> > position and carrying a different GNIS reference number.
> >
> > Can you provide some guidance about the accuracy of the positions, the
> > duplication, and perhaps weigh in on possible tagging scenarios?
>
> In my experience, there are two common sources of position error in
> GNIS:
>
> First, many GNIS entries are pulled off of old USGS topo maps.  These
> are of limited resolution, and you can't get a position more accurate
> than about a city block.  It's not much of an error, but when you're
> used to coordinates that will lead you to a specific door, it's
> something to keep in mind.
>
> Second, many entries have their coordinates specified using the old NAD
> 27 datum, but somewhere along the line, that fact was lost and the
> coordinates were assumed to be in either NAD 83 or WGS 84.  This
> results in an offset that increases the further you go from central
> Indiana; the offset in Alaska is upwards of a hundred meters to the
> west.
>
> For churches, hospitals, post offices, and other facilities in towns,
> it's not unusual for them to take the same coordinates as the center of
> the town.  This mis-positioning may be combined with one or both of the
> above.
>
> The other common error you'll encounter is that the tagging is only
> approximate as to type.  This is most obvious with medical facilities:
> everything from doctors' offices to retirement homes gets tagged as
> "amenity=hospital".  More common but less noticeable is that a wide
> range of vaguely recreation-related things get tagged as "leisure=park"
> -- in particular, watch out for historic markers tagged as such.
>
> Your quarries are subject to this same type-approximation: everything
> from a county road department's gravel pit to an extensive complex of
> mineshafts is tagged as "landuse=quarry", as are some mining-related
> industrial facilities.
>
> --
> Mark
>
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-- 
Dave Swarthout
Homer, Alaska
Chiang Mai, Thailand
Travel Blog at http://dswarthout.blogspot.com
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