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

[Tom Brennan]

On 14/01/2025 10:50 am, Andrew Harvey wrote:
> 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.

To do an import - even where the connecting is done manually - you'd probably want a way of combining stream sections into larger streams. In my model, each stream segment (between junctions) is its own line. Which is fine if you're just using the data for tracing or adjusting, but not if you're copying it in directly. Way too many objects!!

One field that I have in my data that doesn't have an equivalent in OSM is a relevance/importance field. Essentially a number that gets bigger the larger the stream. NSW SS data also has a relevance field from 1?-9 (where 9 is the smallest). The main problem with streams in OSM at the moment is that they all suddenly appear at the same zoom level, even though in some cases the streams are larger than the nearby rivers that they flow into! It makes the map suddenly very cluttered. Yes, this is a rendering issue, but there's not much for the renderer to go on.

There are the width and intermittent tags, but they don't really give you a relative importance.

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

If I develop it further, then I'll add some more documentation. At the moment, I am just exporting the layer as GeoJSON and dragging it into ID. There are videos and tutorials on various things you can do with the DEM data at:
https://maps.ozultimate.com/wiki/nsw_lidar

> 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 don't think I knew about that tool, very handy to know.

> 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.

Tricky - one man's (or woman's) abseil is another man's downclimb! Same with pools. One year it's a shallow wade, the next it's a swim. But bigger waterfalls could be marked. They don't tend to change :)

cheers
Tom
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