[Tagging] Watershed or Drainage Basin relation draft proposal
osm at imagico.de
Thu Sep 13 11:22:04 UTC 2018
On Thursday 13 September 2018, Joseph Eisenberg wrote:
> So you believe the ridges are verifiable (and the network of
> waterways, I assume), but potentially parts of the watershed would
> not be verifiable because eg. terrain is too flat?
There are many reasons why a the watershed structure can be practically
non-verifiable on the ground. Relatively flat topography is only one
of them. I would estimate that at least about 40 percent of the Earth
land surfaces have a drainage structure that is practically
non-verifiable (in the sense that independent estimates from several
mappers would not converge to the same geometry). The percentage would
be even higher if you want it to be verifiable through remote mapping.
> I was thinking
> that in fairly flat areas it is still possbile to see which way water
> flows in drainage channels, and it's often possible to find the
> highest line throught he terrain when surveying.
Nothing speaks against mapping physically observable features like
drainage channels but mapping additional abstract geometries derived
from them in OSM makes little sense IMO.
Note in flat areas with artificial drainage channels the actual drainage
structure can be extremely complex.
> Would these examples be verifiable?
Not having first hand knowledge of these cases means i can't really
tell. A waterway is a geometry directly physically observable on the
ground. The watershed divide is an abstract geometry OTOH. You see a
ridge and *assume* that water from one side of the ridge flows into a
certain drainage system and on the other side it flows into a different
one. But you don't observe this. You might have a simple case where
this seems obvious but the fundamental problem is still there. In
humid climate areas you can make the mapping more reliable by first
diligently mapping the waterway network in the whole area but then what
is the point in mapping the watershed divides in addition?
Also note in priciple watersheds form an infinitely deep hierarchy of
geometries. To be able to practically map these you would have to
define a discretization system in this hierarchy which would be an
arbitrary up-front decision with no basis in the physical world.
> I value your opinion Christoph, because I hoped this relation might
> encourage more complete mapping of ridges, watersheds and drainage
> basins, thus making it easier to render good maps, eg
If you want to facilitate better maps w.r.t. hydrography the best way
would be to put all the time and energy you might put into mapping
watersheds into mapping the waterway network. Priorities should be:
* correct connectivity
* correct distinction onto artificial and natural
* correct flow direction
* completeness and unifomity in detail of mapping
Where these conditions are fulfilled processing the waterbody data for
accurate maps is orders of magnitude easier than elsewhere. Compared
to that the would-be gain of having watershed geometries available in
addition would be relatively small.
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