Precision loss when applying `geof:longitude` or `geof:latitude`

[hannahbast]

The following query on the OSM data https://qlever.cs.uni-freiburg.de/osm-planet/xCerw9

SELECT ?osm_id ?geometry ?lng ?lat WHERE {
  osmrel:62768 ogc:sfContains ?osm_id .
  ?osm_id rdf:type osm:node .
  ?osm_id geo:hasGeometry/geo:asWKT ?geometry .
  BIND (geof:longitude(?geometry) AS ?lng) .
  BIND (geof:latitude(?geometry) AS ?lat) .
}
LIMIT 3

returns the following result:

| ?osm_id | ?geometry | ?lng | ?lat |
| osmnode:10003156787 | POINT(7.854637 48.017093) | 7.85464 | 48.0171 |
| osmnode:10004601576 | POINT(7.830281 48.002005) | 7.83028 | 48.002 |
| osmnode:10005201820 | POINT(7.821752 48.003969) | 7.82175 | 48.004 |

@ullingerc Do you understand where the precision loss comes from? In my understanding the POINT(... ...) is already rounded due to QLever's limited precision for points. But why is that rounded result rounded again when applying geof:longitude or geof:latitude?

PS: It seems that the precision of all our doubles is only four digits after the dot https://qlever.cs.uni-freiburg.de/wikidata/6n45iu

[hannahbast]

@ullingerc OK, I think it's simply this line in our code:

https://github.com/ad-freiburg/qlever/blob/master/src/engine/ExportQueryExecutionTrees.cpp#L335

The default precision is 6 digits (including digits before the dot).

I think the problem is clear now, but the fix is not so clear. For example, see https://godbolt.org/z/9KKe33vGs

[ullingerc]

@hannahbast As you have already pointed out, this is unrelated to the geof:latitude and geof:longitude functions. They actually inflate not round the precision from the 30 bit representation in the GeoPoint ValueId to the 60 bit representation in the Double ValueId.
I had similar problems when implementing GeoPoint's string representation. The GeoPoint class applies std::to_string which renders doubles with 6 digits behind the dot - this is what happens when the POINT(...) literal is exported (see GeoPoint.cpp#L100-L104). An easy fix at least for this case would be to also use std::to_string in ExportQueryExecutionTrees.cpp#L335.