Used to mark the vertical relationship between two features. The
layer=* tag is used to mark the vertical relationship between
two different features. It provides no information about the
absolute height or the absolute vertical spacing between the
features for which ele=* should be used. If two ways intersect
then they must have the same layer, if one passes above another
then they should have different layers. In general it is assumed
that any object that has no layer tag is at the natural, or
permanently changed ground level (which can also be considered
to be at layer=0). Positive values up to layer=5 are used for
elements that pass above features at ground level and negative
values down to layer=-5 for features that are below ground. It
is sometimes appropriate to tag some features, such as
man_made=pipelines and power=cables to be tagged with
location=overground or location=underground which indicates that
they are in the air or buried without providing any information
about relative heights where pipes