[HOT] What's water?
ol.john.el at gmail.com
Mon May 4 14:35:53 UTC 2015
I loaded hiu and even looked at it some before my recurring memory error
started messing up my JOSM experience. [awhile ago I added the suggested
fix but either did it wrong or I have some 64-bit type issue]
What I noticed could have been partially due to alignment, but the stream
was generally marked slightly to the east of what is represented in the
imagery. I think your instincts regarding its path not actually going
through the treetops is correct. For the reaches in the immediate area, it
appears highly constrained with little to no floodplain.
As far as revising its location, I wouldn't unless the mapped channel
conflicted with a settlement, an important transportation route, or a
bridge or something like that -- and even then probably just in the
immediate area of the conflict. ... Gotta get to work...
On Mon, May 4, 2015 at 6:59 AM, Pierre Béland <pierzenh at yahoo.fr> wrote:
> I agree.
> In JOSM, it helps sometimes to switch to the Opencyclemap layer to see
> elevations when we have not a clear image, shadow or other obstacle.
> Shadow will be a problem in narrow valleys. I have seen this looking at
> the Langtang valley surrounded by high summits.
> *De :* Pat Tressel <ptressel at myuw.net>
> *À :* john o'l <ol.john.el at gmail.com>
> *Cc :* hot <hot at openstreetmap.org>
> *Envoyé le :* Lundi 4 mai 2015 9h36
> *Objet :* Re: [HOT] What's water?
> John --
> Having mapped a number of streams over the past years, I guess the first
> thing to say is that it is surprisingly difficult. Mapping the same
> watercourse at different scales can have very different results and the
> stream channel itself can change considerably with any given flood event.
> Aerial, satellite, and topographic maps each have their own strengths and
> weaknesses as the basis for stream/river delineation. As a mapping
> consolation, since they move around so much and vary so much over time,
> approximations at the scales we are using should be good enough.
> One of the things to keep in mind is the reason we are mapping
> watercourses. In our current context, when I map one, or even study one in
> high resolution photos, I usually think of them as a potential source of
> water for nearby inhabitants, as travel obstacles potentially restricting
> crossings to particular locations or structures, and as a pathway along
> which very destructive events can occur - from floods to mudflows to debris
> I've yet to successfully load the hiu layer anywhere - ongoing JOSM
> Imagery -> Image Preferences
> Click the TMS button on the lower right. In the popup:
> Enter the above URL in box 1., but ==> take off the "tms:" <==, because it
> will add that for you.
> Enter a name for the imagery in box 4 -- this is what will appear in the
> Click OK.
> but after a quick check of the coordinates you gave in Google Earth I can
> say unequivocally that it is stream or for OSM purposes a river.
> There's definitely a river -- I'm trying to pin down whether the water is
> in the narrow channel at the west side of the dark region in the above
> image, or if there is actually water flooding out over a large area.
> I suspect what you are seeing is mostly a result of shadow, which often
> makes aerial and satellite imagery challenging - streams are often shadowed
> given they occupy the lowest areas in a landscape and mountains/steep
> valley walls cast great shadows.
> True, the shadow could be from the angle of the land, not just the trees.
> That would hint that there should be less shadow where the river is flowing
> toward the southeast, as the sun is shining from south-southeast. If the
> above procedure works, have a peek.
> btw, for a cool feature, about 1 km to the SSW is an alluvial fan!
> -- Pat
> HOT mailing list
> HOT at openstreetmap.org
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