[Tagging] Layers (was Eruvs etc.)

[Greg Troxel]

It would be good for OSM people to use qgis and get used to how layers
work in the GIS world.

Big points:

  Some of what we think of as tags are part of the layer definition.
  Instead of having tags that say "this is a road" on a way, there is a
  layer that is defined as road and it has lines.  But, some things like
  width/etc. are in a table that has rows where a row has a geometry
  object (line) and then attributes for name, width.  There is a
  regularization into table with columns rather than free form.

  What we think of as nodes within lines don't really exist in a
  first-class way.

  When editing, often people turn on "snapping" so that positioninng a
  point object or a vertex of a line/polygon will result in exact match
  of an object in some other layer.

  There is also a concept of topological constraints, which goes beyond
  snapping, but I'm not clear enough on it to describe it.

Really the big point is to have sets of features with part of the
info being at the set level.

I think it would be good if we avoided inventing GIS layers again,
somewhat differently, without doing so inteti[onally with compelling
rationale.
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