[Talk-us] dubious church node

[Mark Wagner]

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