[Tagging] Irrigation: usage=irrigation vs irrigation=yes [Was Irrigation: ditches, canals and drains]

Joseph Eisenberg joseph.eisenberg at gmail.com
Wed Jun 5 01:54:58 UTC 2019


On 6/4/19, Fran├žois Lacombe <fl.infosreseaux at gmail.com> wrote:
> According to semantics of canals and ditches, a canal can't be used for
> drainage and a ditch/drain shouldn't be used for irrigation

Many mappers are using waterway=ditch for irrigation

I wrote a long analysis of the situation at this page;
https://wiki.openstreetmap.org/wiki/User:Jeisenbe/Waterway_Widths

Of the 4 tags that specify usage for irrigation
(service/usage=irrigation, irrigation=yes and canal/ditch=irrigation)
there are 10519 waterway=canal vs 3673 waterway=ditch specified - so
over 25% are ditches.

As far as drainage: in southern California and Arizona the large
drainage features are called canals, because they can be 10m wide or
more and have concrete lining.

My wife says they were always warned not to play in the canals, even
though it was fun to skateboard down the concrete embankments, because
they could suddenly fill with flash-flood water if it had been raining
in the mountains around Phoenix.

There is a separate system of canals that is dedicated to irrigating
farmland, in addition to a long-distance aqueduct from the Colorado
river.

75% of waterway=drain are 2m wide or less, so most mappers do not use
this tag for wide, canal-sized drainage waterways.

While it might be possible to encourage mappers to use waterway=drain
or waterway=ditch for drainage canals of all sizes, this would require
map renderers to check the width tag for these types of waterways as
well.

Right now waterway=canal is the only type of waterway that is
ambiguous as to size: drains and ditches are small, like streams, but
an irrigation canal can be as large as a river or as small as a minor
stream or drain, based on the understanding that "canal" is the
correct value for all sizes of irrigation features (and for things
like mill races).

Charts:

Natural waterway widths:
1) Cummulative percentage below a certain width
- https://user-images.githubusercontent.com/42757252/58748368-a5aaf380-84b2-11e9-8a14-05ad3ca3781b.png
2) Bar chart - adding up to 100%
- https://user-images.githubusercontent.com/42757252/58745525-06273a00-848d-11e9-8d5f-eac7daa81eb7.png

Artificial waterway widths:
3) Cummulative percentage below a certain width
- https://user-images.githubusercontent.com/42757252/58748367-a479c680-84b2-11e9-8ab8-9d57841c77af.png
4) Bar chart - 100%
- https://user-images.githubusercontent.com/42757252/58745526-08899400-848d-11e9-9b90-79ec753ab735.png

More charts at https://github.com/gravitystorm/openstreetmap-carto/issues/3795

And eventually I'll add them to
https://wiki.openstreetmap.org/wiki/User:Jeisenbe/Waterway_Widths

- Joseph



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