Proposed features/Water bodies

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Water bodies
Status: Draft (under way)
Proposed by: *
Tagging: water=*
Applies to: relation
Definition: General tagging of water bodies
Rendered as: Blue, various
Drafted on:
Proposed on: 2009-02-19

This is a work in progress. If you have suggestions you can help either directly or in the discussion page.

Rationale

There are scattered efforts to improve how water bodies are represented in the database. For now we have the waterway and some of the natural tags.

Discussed issues cover :

There is no specific tag for wadis, ponds, oasis, waterfalls Proposed_features/Waterfall, underground rivers or lakes, probably more…

This proposal aims to regroup and support all these features in a coherent manner.

Tagging

A complete water limit is the contour of water at a specific height. If the limit is too long or contains holes individual ways can be collected into a multipolygon relation. Otherwise a single way is used as is. A complete water limit accepts the following tags :

Existing tags

key values meaning use with
natural water,wetland
wetland grass, trees, mangrove,…

New proposed tags

key values meaning use with
water river, lake, lake, pond, saltylake, sea,… Used to differentiate features semantically. water=*
bank sand, rocks, gravel, concrete, urban, trees… this kind of surface is to be expected on the banks (outside) of the area delimited by that way. Maybe make provisions to specify different surfaces on the right and left bank for linear features. water=* ; waterway=river,stream ; natural=wetland ; standalone
dry_surface sand, rocks, gravel, concrete… Surface when there is no water. Only useful if the distance between two water limits is very large.
extent permanent,high,low,mean,flood,changing,… Explains what type of water limit this way denotes. High marks the position of water when the water is high. water=* ; waterway=river,stream ; natural=wetland ; standalone
when (or oftness) season, tide, intermittent, exceptional,… indicates on which occasions the level is reached water=* ; waterway=river,stream ; natural=wetland ; standalone
sail yes,no (default depends on particular feature) indicates that you can sail on the water when it is at this level water=* ; waterway=river,stream ; natural=wetland ; standalone

The most significant feature will be tagged with the relevant tags above.

The alternate water levels will be tagged only with bank=*, extent=*, when=* or sail=*

Then, a relation is used to link the alternate levels to the most significant one:

key/role values meaning use with
type water_regime
core [most significant feature]
alternate_regime [alternate regimes]*

If the alternate regime is already using a relation (multipolygon) because of its size or its complexity, simply reuse that relation and put the related core feature in rhe core role.

Explanation of key values

key value meaning rendering example picture
bank sand
bank rocks
bank gravel
bank salt
bank mud
extent,when permanent The water level does not change. This is the default value and should not be specified. solid blue
extent high The water raises to that level on highwater events intermingle '+' symbols with the dashes from the when key / area hashed blue
extent low The water lowers to that level on lowwater events intermingle '∣' symbols with the dashes from the when key
extent mean The water is usually at that level. To be used on features with non-exceptional variable regimes. solid blue
extent flood The water can reach that level on flood events intermingle '++' symbols with the dashes from the when key
extent changing When the water is at lower levels, it flows through random paths. Very thin crisscrossing lines along the mean feature path. Crisscross.png
when season The level is reached at particular moments of the year
when tide The level changes in accordance to tidal cycles
when exceptional The level is reached on exceptional events (floods, storms) dotted blue
when controlled The level is reached due to human control (e.g. because of a dam) long blue dashes which indicate the map user should look for additional info.
when intermittent The level is reached on unpredictable events such as rain dashed blue
when periodical The level is reached on regular or predictable events. long blue dashes which indicate the map user should look for additional info.

Interaction between extent and when tags

These two tags have some level of dependency, and defining two sets of orthogonal rendering schemes will probably make the map too complicated. Below are some rendering suggestions based on value combinations

extent=(unspecified) extent=low extent=mean extent=high extent=flood [3] extent=changing
when=(unspecified) solid stroke / solid fill X solid stroke / solid fill X X very thin criscrossing blue lines
when=season [2] long dashes [1] X long dashes / light fill or hash dotted
when=tide [2] dotted [1] X dashed darker stroke / light fill or hash dotted
when=exceptional [2] dotted white [1] X dotted / light fill or hash dotted
when=controlled [2] long dashes [1] X long dashes / light fill or hash long dashes
when=intermittent [2]  ?? X dashed / light fill or hash X [4]
when=periodical [2] long dashes [1] X long dashes / light fill or hash long dashes

[1] The lowness should be recognisable thanks to the background of the corresponding mean regime.

[2] Same as extent=high

[3] Flood characteristic is recognised because it doesn’t lie against any particular background. Care must be taken not to mistake this with linear features al lower scale. Multiple flood lines can be used, with the date of the maximal event drawn aside the limit.

[4] There should not be no such thing as intermittent floods. Either use highwater extent or exceptional oftenness.