Michael, thank you very much for such in-depth analysis. I would conclude the following from it.
There are distinctions between these two terms, otherwise they would not be defined separately. In simple words, a ditch is a small open-air man-made or self-formed channel in the ground for absolutely any purpose, both lined or unlined entirely or partially. If the purpose of a ditch is to carry away suporflous water be it industrial discharge or rain water or wet land water or any other liquid it can be called a drain (or drainage ditch). Tagging such drains with e.g. waterway=ditch+usage=drainage would be normal. But it would sound nonsense if you call an open-air overground drainage construction a "ditch", e.g. http://landscapenashville.com/STA75559.JPG I would tag such things as man_made=* though. Drainage overground or buried pipes can be tagged with the existing man_made=pipeline tag. Cheers, Eugene вс, 3 февр. 2019 г. в 04:14, Michael Patrick <geodes...@gmail.com>: > A survey of international and some national lexicons indicates that the > two terms 'ditch' and 'drain' are equivalent used in the context of liquids > from the smallest to largest scales. > > The term 'drain' however seems mostly to apply at the interface where the > water transitions from the substrate ( soil ) to free running water, down > flow from that the water is 'channeled' through ditches, fluves, shutes, > spillways, canals, and a multitude of functional confinements. One of the > earliest ( 1920 ) legal references to British and American law notes this > equivalence, and the following an extract from a 2017 global standard > saying basically the same thing. > > UNESCO-WMO International Glossary of Hydrology at > https://unesdoc.unesco.org/ark:/48223/pf0000221862 -World Meteorological > Organization, United Nations Educational, Scientific and Cultural > Organization: > "...will be useful to national hydrological services as well as > educational and research institutions throughout the world – especially for > those who require more than one language for understanding or communicating > information about the field of hydrology. In establishing recognized > international equivalents of hydrological terms, our goal is also to > minimise misinterpretations and consolidate the foundation for stronger > international cooperation." > > 407 ditch see also drain > Man-made small open channel constructed > through earth or rock for the purpose of lowering > and/or conveying water > > 415 drain see also ditch > Conduit or small open channel by which water is > removed from a soil or an aquifer, by gravity, in > order to control the water level or to remove > excess water. > > Ditto with the USGS and the UK Ordnance Survey: > > For example, OS MasterMap Topography Layer User guide - "Water - Water > features are defined as features that contain, delimit or relate to > real-world objects containing water. The physical water features shown in > OS MasterMap Topography Layer include: ... drains and ditches; ... Dam, > ditch, dock, double, down, drain D, Double ditch or drain DD" ... a look > see at a lot of OS web map products show the same thing. In the case of the > UK, a vast amount of property lines are encoded as these ditches and > drains, so they formalized this equivalence to accommodate whatever the > locals called them. > > There is no dependence on the size, width, depth, etc. A perhaps extreme > example ( due to heavily mechanized agriculture in the U.S. ), but still > illustrative is that the USDA construction guidelines make the following > distinctions: > Small ditches ( maximum top width 15 feet ) > Medium-sized ditches ( top width 15 to 35 feet ) > Large ditches ( more than 35 feet top width) > In SE Asian rice production, their largest ditches probably would be in > the 'small' category compared to the U.S. I don't read Chinese, > Japanese, Korean, etc. but I'm sure they have a couple thousand years of > established vocabulary for their field water handing. > > The modern agricultural water handling industry ( what you would get if > you asked somebody to install a 'ditch' or a 'drain' in a field makes a > distinction as follows ( echoing the 'interface' idea above ): > > Ditch — A man-made, open drainage-way in or into which excess surface > water or groundwater drained from land, stormwater runoff, or floodwaters > flow either continuously or intermittently > Drain — A buried slotted or perforated pipe or other conduit (subsurface > drain) or a ditch (open drain) for carrying off surplus groundwater or > surface water. > > Ditches aren't restricted to water use. Sometimes they are there because > the material was sued to form an embankment, or used for road surface ( > 'borrows' in the USA ), animal control barriers, access control, boundary > marking, spill prevention and control of loose soils and aggregate slides. > And in all the water literature, in the U.S, and U.K., they pretty much > also freely used 'drainage ditch', not just simply 'ditch. > > Predominantly, if the cut is not further improved from the native > material, it seems to be called a ditch, if structure is added like > concrete lining, wooden bank sides, maybe it will get a more specific term. > Economics dictates that for the most part these enhancements only occur > over limited lengths for flow control, erosion, obstacles, evaporation, etc. > > Drainage structure means a device composed of a virtually non-erodible > material such as concrete, steel, plastic or other such material that > conveys water from one place to another by intercepting the flow and > carrying it to a release point for water management, drainage control or > flood control purposes. > > Looking at the aerial photography majority of 'drains' in the OS based web > maps, they are pretty much 'swales' ( > https://en.wikipedia.org/wiki/Swale_(landform) ), without the distinctive > edges of a 'ditch'. > > In conclusion: > > For legacy tagging, ditch/drain should be left alone because of > equivalence. > > For new tagging, ditch or drainage_ditch, ditch:drainage, or 'whatever' > scheme should indicate it is a ditch for conduction of water, and > 'drain_open' or some such to distinguish it from subterranean drains ( > despite being buried, these are actually sometime more visible than the > ditches on aerial / sat photography ). Anything 'drain' should be confined > to where the ground interfaces with a open channel. But a singular 'ditch' > would suffice. > > Because of global variety and local conditions, there should be no 'size' > distinction, or distinction because of structural presence, materials, etc. > > Local terminology takes precedence, at the highest level it is available. > > While a dictionary might be a useful start for determining a meaning, > there is almost always some better source of definitions in a specific > domain, culture, and region, and location. The U.N., E.U., U.K., Scotland, > and down to Renfrewshire all have documentation of what terms mean in those > local contexts, for example. > > Almost always, a single word will be immediately overloaded when used > world wide.Human languages have compound words, adjectives, verbs and > adverbs for a reason, and tagging schemes have equivalents. > > Michael Patrick > Data Ferret > > > > _______________________________________________ > Tagging mailing list > Tagging@openstreetmap.org > https://lists.openstreetmap.org/listinfo/tagging >
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