Add "is_valid" filter option

[mcauer]

Use Case Description

An is_valid filter option would give the opportunity to:

• avoid summing up negative areas from broken polygons
• analysing the amount of bad geometries as an quality measure
• give a user the possibility to extract only "usable" features for further usage in other contexts

Request Description

It would be very convenient to write a filter like:
fitler=geometry:polygon and geometry:is_valid and building=*
or
fitler=geometry:valid_polygon and building=*

2nd option would even be more precise, because some cases (touching inner rings) are invalid for polygons but valid for osm-relations.
On the other hands the first one is easier to negate and count all not is_valid objects.

Additional Information

This is quite important for hex.ohsome.org because recently we computed a negative area aggregation value due to broken building geometries in the history.

[tyrasd]

I've added this to the corresponding ticket over at the oshdb repository: GIScience/oshdb#311

[tyrasd]

I think we can agree that the is_valid filter would work on the principles/definitions of the simple feature standard.

There is one additional aspect we have consider here: There is not always a 1:1 correspondence between invalid and valid OSM objects and the corresponding geometries produced by the OSHDB's geometry builder. This is because the OSHDB does perform a "best effort" to produce the most reasonable (and valid if possible) geometry output for every OSM input and also needs to perform some geometry "fixing" in cases where it is unavoidable. All of this also needs to be balanced with performance aspects.

input \ output	valid geometry	invalid geometry
valid OSM object	✔️	"broken" by OSHDB geometry builder1
invalid OSM object	"fixed" by OSHDB geometry builder2	✔️

Footnotes

1. one example would be the non-handling of nested outer-inner-outer-inner situations – there are rare but actually valid OSM objects which can result in invalid geometries. ↩

2. one example is that in order to associate inner rings with their corresponding outer part of a multi-polygon with more than one outer ring, the OSHDB needs to do an containment check which automatically removes all inner rings outside of any outer ring. This can produce a valid geometry for an actually invalid OSM object. ↩