Spatial Join

Properties#

• In SPARQL (GeoSPARQL, OpenStreetMap):
  • an object’s geometry is reached via geo:hasGeometry/geo:asWKT, a Property Path, and is a literal in WKT format (geo:wktLiteral), e.g. "POINT(7.842 47.998)"^^geo:wktLiteral;
  • the condition is a FILTER with geof:distance(g1, g2, unit), with the unit given as an IRI (unit:M for metres).
• Constant side. In the lecture example, one side is a single fixed point, so the “join” selects all buildings within 500 m of that point. With a variable on both sides, the same FILTER joins two sets of objects, like the DBIS SQL example bus_stops JOIN restaurants ON ST_Distance(…) < 500 (DBIS_VL05 > Join operation).
• ?building osmkey:building ?anything matches every OSM object with a building tag, whatever its value. The variable ?anything is only a placeholder.

Example#

PREFIX osmkey: <https://www.openstreetmap.org/wiki/Key:>
PREFIX geo: <http://www.opengis.net/ont/geosparql#>
PREFIX geof: <http://www.opengis.net/def/function/geosparql/>
PREFIX unit: <http://qudt.org/vocab/unit/>
SELECT ?building ?building_wkt WHERE {
  ?building osmkey:building ?anything .
  ?building geo:hasGeometry/geo:asWKT ?building_wkt .
  FILTER (geof:distance("POINT(7.842 47.998)"^^geo:wktLiteral,
                        ?building_wkt, unit:M) <= 500)
}
# Buildings in a 500 m radius around Freiburg Hauptbahnhof

With toy geometries: building $b_1$ at 200 m and $b_2$ at 800 m from the point give the result $\{b_1\}$. The FILTER removes $b_2$.

Sources#

• Slides 01, p. 24
• ADIS_VL01 > Spatial Join
• DBIS: DBIS Slides 05, p. 22 (spatial join in SQL)