[talk-au] Use of DEM data to generate streams

[Andrew Harvey]

On Mon, 13 Jan 2025 at 18:43, Tom Brennan <website at ozultimate.com> wrote:

> The general process of deriving stream data from DEMs is documented at
> https://maps.ozultimate.com/wiki/qgis_hydrology
>
> In terms of detail, I have found that using the SAGA Channel Network tool,
> an Initiation Threshold of around 9.6-9.7 (based on D8 flow, log
> accumulation), along with a minimum length of around 60m, gives a similar
> level of detail to the existing DCS stream data. That's in my areas of
> interest, typically the Blue Mountains. A different threshold might apply
> in flatter country.
>
> At this stage I'm not proposing an import. I think there are too many
> potential problems around connectivity. I did the "import" (manually!) of
> the missing named streams for the Sydney Basin, and it involved a massive
> amount of work manually connecting things up, fixing broken connections etc.
>
> I have no idea how you would do an automated import of streams without a
> lot of manual work, though I'm happy to take suggestions/advice!
>
> I was mainly proposing to improve the existing streams with the aid of an
> additional source of data.
>

Sounds good, it would be good to have better stream data. It's a balance
between spending more time on the import code to have it automated vs doing
more manually. I think just generating the stream data automatically and
then manually cutting sections into JOSM works.

Either way I think it would be helpful for others to have the end-to-end
process documented on the wiki.

Where you're improving the geometry of an existing way I'd recommend (or
perhaps even insist) using the JOSM Replace Geometry tool to retain the
way's history instead of deleting what's in OSM and adding a new object.

I wonder if we can also map further details around traversing the creeks,
which sections need abseiling, which sections need a hand rope, which
sections are deep pools (I used water=stream_pool) and then complete all
the waterfall heights.
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