Shadow Detection Check (osmlab#103)

* check building parts

* convert heights; check buildings

* unit tests

* catch enclosed parts; config

* use walker for floating parts

* reflag single parts

* use buildings in walker

* use walker on all; update instruction

* fix num errors

* add relation support

* negative is ground

* add relation instruction

* handle building relations

* add building filter

* comments and docs

* make converter static

* fix typo

* code cleanup

* Long BaseCheck

* comment clean up

* sonar

config/configuration.json

@@ -277,6 +277,14 @@
       "tags":"highway"
     }
   },
+  "ShadowDetectionCheck": {
+    "challenge": {
+      "description": "Verify the height and level tags to make sure the building does not float in 3D",
+      "blurb": "Fix floating buildings",
+      "instruction": "Open your favorite editor and check the height and placement of the building or building part for 3D alignment.",
+      "difficulty": "NORMAL"
+    }
+  },
   "SharpAngleCheck": {
     "threshold.degrees": 97.0,
     "challenge": {

docs/checks/shadowDetectionCheck.md

@@ -0,0 +1,37 @@
+# Shadow Detection Check
+
+This check flags buildings that are floating in 3D, casting abnormal shadows on the base map when rendered.
+
+In OSM, 3D buildings are achieved by assigning `height` and/or `level` tags to ways or relations that have a `building` or `building:part` tag.
+The [Simple 3D Buildings](https://wiki.openstreetmap.org/wiki/Simple_3D_buildings) osm wiki page documents much of this tagging scheme.
+Important to this check is the use of `building:min_level` and `min_height` tags to define the bottom z level of a part of a building.
+When improperly used these tags will cause building parts to become disconnected and float.  
+
+#### Live Examples
+
+1. Way [id:260580125](https://www.openstreetmap.org/way/260580125) has a `building:min_level` tag that is > 0, but their is nothing below it to connect it to the ground.
+2. Way [id:462577211](https://www.openstreetmap.org/way/462577211) has a `building:min_level` tag of 5, but the part below it, 
+way [id:462577208](https://www.openstreetmap.org/way/462577208), has a `level` tag of 4. This causes half the building to float one level above the other half.
+
+#### Code Review
+
+In [Atlas](https://github.com/osmlab/atlas), OSM elements are represented as Edges, Points, Lines, Nodes, Areas & Relations; in our case, we’re are looking at
+[Edges](https://github.com/osmlab/atlas/blob/dev/src/main/java/org/openstreetmap/atlas/geography/atlas/items/Edge.java), and
+[Relations](https://github.com/osmlab/atlas/blob/dev/src/main/java/org/openstreetmap/atlas/geography/atlas/items/Relation.java).
+
+The first process in this check is to validate incoming objects, to see if they have the potential to be floating building parts.  
+This is done by testing that each object:
+
+* is an Area or multipolygon Relation
+* has a `building` or `building:part` tag
+* has a `min_height` or `building:min_level` tag
+
+Once an object has been validated it has its `min_height` or `building:min_level` tag checked to see if it is > 0, indicating it is off the ground.
+If it is, a BFS walker is used to find connected building parts that form a path to the ground (see the `getFloatingParts` method).
+The walker uses geographic indices and polygon overlap calculations to locate parts that overlap in 2D (see the `neighboringPart` method), 
+and then checks height and level tags to determine 3D overlap (see the `neighborsHeightContains` method). 
+When the walker is unable to find a 3D path to the ground, all the collected building parts are flagged as floating. 
+
+To learn more about the code, please look at the comments in the source code for the check.  
+[ShadowDetectionCheck](../../src/main/java/org/openstreetmap/atlas/checks/validation/areas/ShadowDetectionCheck.java)
+