[HOT] What's water?

Pierre Béland pierzenh at yahoo.fr
Mon May 4 13:59:53 UTC 2015

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 torrents. 
I've yet to successfully load the hiu layer anywhere - ongoing JOSM struggles

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 menu.
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

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