Proposed features/man made=polder
|Proposal status:||Draft (under way)|
|Definition:||Tag for polders|
|Rendered as:||free text|
A polder is an artificial low-lying area of land that is enclosed by dikes and subject to human hydrological control. 
Polders are tagged as areas/multipolygons with man_made=polder. Where the polder is surrounded by water, the outline of the polder follows the outline of the water area. Where the polder is not surrounded by water, the outline of the polder follows the centreline of the dike.
If a polder is (almost) completely surrounded by water, the same area/multipolygon can additionally be tagged with place=island/place=islet/natural=peninsula (whichever is appropriate). However, such an additional tag should only be used if the surrounding body of water is perceived as significant.
On the map, the name of the polder (name=*) can be displayed inside the area, similar to how islands/peninsulas are commonly rendered.
An area tagged with man_made=polder shall be referred to as a polder.
An area tagged with both man_made=polder and place=island shall be referred to as a polder island.
An area tagged with both man_made=polder and place=islet shall be referred to as a polder islet.
An area tagged with both man_made=polder and natural=peninsula shall be referred to as a polder peninsula.
OSM currently lacks a single consistent tagging scheme that can be used for all polders, which makes it impossible to easily represent and identify polder areas in the database.
Moreover, each of the various alternative tagging schemes currently in use has serious drawbacks:
- place=locality is intended for unpopulated locations, yet polders are frequently populated. Additionally, polders can be very large rural areas, making it awkward to treat them as a single "location".
- While landuse=grass is often the dominant land use within a polder, it is usually not the only land use. The non-grass parts of the polder should still be part of the polder area.
- Usage of place=island/place=islet/natural=peninsula is not always wrong, but these tags are unusable for the majority of polders. They are also awkward to use if the surrounding body of water is insignificant.
man_made=polder is preferred over place=polder for two reasons. Firstly, this neatly makes it possible for the same area to be simultaneously tagged as both polder and island/islet/peninsula. Secondly, this is consistent with natural=peninsula having been preferred over place=peninsula in the past.