[HOT] What's water?

john o'l ol.john.el at gmail.com
Mon May 4 12:16:49 UTC 2015

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
struggles - 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. 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. btw, for a cool feature,
about 1 km to the SSW is an alluvial fan!



On Mon, May 4, 2015 at 2:09 AM, Pat Tressel <ptressel at myuw.net> wrote:

> Hi, Suzan!
>   I mapped dry streams. Can someone experienced check my work yesterday?
>> I also saw waterways in areas where no water could run, as in forests or
>> over land without any waterway. I also questioned some paths could be
>> wsterways. Good to check Newbie work!
> It could also be that they were using different imagery that is
> misaligned.  There are two email threads about alignment right now...  The
> imagery I'm using is definitely not correctly aligned :-( though that's not
> the concern here, which is more about how to interpret what I'm seeing.
> Your question about distinguishing a path from a streambed is similar, and
> I've been wondering about that too.  Maybe we should find and post some
> examples.
> -- Pat
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