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When mapping from aerial imagery, it's often quite hard to tell whether a particular waterway is a canal or drain or ditch. What should one look for to determine which tag is most appropriate? |
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I map primarily in Thailand. Thailand has irrigation canals, drains and ditches literally everywhere. It also has hundreds if not thousands of channelized streams that look like canals from above or from the ground. That makes it tricky to distinguish a canal from a channelized stream and it's virtually impossible to distinguish drains and ditches from one another using only aerial imagery. I imagine that with extremely high-resolution satellite imagery it might be possible but using the imagery we have presently, not. If you're on the ground and can see the waterway, the Wiki goes into detail about how to tag them. However, even the Wiki isn't the last word. For example, many small irrigation waterways here are cement-lined, like a ditch, while others are not. How would you tag a waterway if you don't know specifically what type it is?
(05 Apr '20, 07:19)
Zismac
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If I'm mapping a waterway using satellite imagery I make a guess, an educated guess, based on my familiarity with the area. I do live in Thailand and have seen and crossed countless waterways in my 10 years here. Many of the waterways I've mapped are ones I have seen on the ground but many are not. In my neighborhood of Chiang Mai, which produces a lot of rice and other "thirsty" crops, almost all rural roads have some sort of irrigation canal running alongside them. To me, what exact type of waterway it happens to be is less important than knowing one is there.
(05 Apr '20, 08:46)
AlaskaDave
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Hi Zismac, have a look here https://wiki.openstreetmap.org/wiki/Waterways yes, the Wiki, but start mapping in your own region and compare the results of your impressions out there. It is just a question of experience, just earned by doing it outside. Like Dave stated with a lot of experience go out and try. Ps use JOSM or any editor who’s able to save the results locally, just to prevent the lost or damage to the work of others. |
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A canal will have boats, buildings, docks and locks and some of those may have links to roads. A canal is usually kept clear of water weed and vegetation so they can be used. Often they will have a towpath or footpath by them. Drains and ditches are usually narrower and will often look wilder. although they will get cleared occasionally. @Andy - I agree with your definition but only in part. A canal is "an artificial waterway constructed to allow the passage of boats or ships inland or to convey water for irrigation." The large irrigation canals in my area do not have docks or boat traffic but do have locks and are often 30-40 m wide. As I said, the choice of tagging in such situations is tricky at best.
(06 Apr '20, 14:07)
AlaskaDave
Ok Dave. In GB i only know of the new river that, in the 1800s, was dug to supply water to London that is also used for leisure boat traffic now. see https://en.wikipedia.org/wiki/River_Lea
(06 Apr '20, 21:43)
andy mackey
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