The generic key location=* is used in different contexts to define the location of a feature which may be above the ground or buried. Used with features such as man_made=pipeline and power=cable which may be buried in the soil.
For feature which are contained within a self-supporting tunnel, which may also be used for other purposes or provided for maintenance purposes, then use tunnel=* instead. A buried cable is normally surrounded by earth. A cable in a tunnel is normally surrounded by air in with a larger self-supporting structure.
Use of the layer=* tag is optional and only useful where there is a benefit to describing the relative vertical arrangement of a number of underground (or overgound) features. It is assumed that features that cross which are tagged with 'location' which do not share nodes will not actually interact with other features which also use 'location' (unlike roads/ railways and rivers etc where they need to be on different layers if they do not intersect).
- location=underground Used where a feature is underground (or under water).
- location=underwater An alternative tag for features that are under water.
- location=overground Used where a feature such as a pipeline or cable is overground.
Specifically for power substation features a number of additional values for location has been defined, see Substation#Location. These values such as location=indoor may also be useful for other features.