[JENA-1915] spatial:greatCircle appears to be returning wrong answers

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Jena 3.13.1
• Fix Version/s: Jena 3.16.0
• Component/s: Spatial
• Assignee: Greg Albiston
• Reporter: Bryon Jacob
• Created: 12/Jun/20 15:54
• Updated: 13/Jul/20 15:51
• Resolved: 15/Jun/20 10:37

Description

the `spatial:greatCircle` function appears broken - sometimes returning negative numbers.  returning negative numbers can never be correct from a distance function - but I've tried to produce something more useful than that as a defect report and repro...

I was trying to "port" some SPARQL queries where I'd written the great circle function (Haversine formula) to use the spatial:greatCircle function - which means I had a pretty good test case to see where the issue arises.  I simplified my query down to a repeatable test that demonstrates the issue:

note that in my original query, I was using a custom jena function to compute `radians` - since that function won't be available to you in stock jena to try this out, I've precomputed the radians values and put them in the VALUES block next to lat/lon pairs. This should be runnable on any stock Jena instance with the jena-geosparql functions loaded. Note that for most of the distances computed, the two distances agree quite closely - but for the last two, the jena function returns a negative number, where the hand-computed value is a correct distance between those points

PREFIX m: <http://www.w3.org/2005/xpath-functions/math#>
PREFIX spatial: <http://jena.apache.org/function/spatial#>
# this is a namespace of custom functions, which won't be available in stock Jena - to reproduce
# this, I've pre-computed the radian values and included them in a VALUES block below...
# PREFIX f: <http://data.world/function/functions#>

SELECT ?lat1 ?lon1 ?lat2 ?lon2 ?φ1 ?φ2 ?Δφ ?Δλ ?d1 ?d2
 WHERE {
     VALUES (?lat1 ?lon1 ?lat2 ?lon2 ?φ1 ?φ2 ?Δφ ?Δλ) {
         # in my original query, I was computing radians using a custom radians function, which won't be available 
         # in stock Jena, so to make this reproducible I precomputed the radians values needed for the Haversine
        (41.2572	-95.9656	41.2592	    -95.9339    0.7201	0.7201	    0.0000	    0.0006)
        (41.2572	-95.9656	41.2482	    -96.072     0.7201	0.7199	    -0.0002	    -0.0019)
        (41.2572	-95.9656	41.5871	    -93.626     0.7201	0.7258	    0.0058	    0.0408)
        (41.2572	-95.9656	51.0472	    -113.9998   0.7201	0.8909	    0.1709	    -0.3148)
        (41.2572	-95.9656	40.7528	    -73.9876    0.7201	0.7113	    -0.0088	    0.3836)
        (41.2572	-95.9656	49.7237	    13.3422     0.7201	0.8678	    0.1478	    1.9078)
        (41.2572	-95.9656	-33.9065	18.4175     0.7201	-0.5918	    -1.3119	    1.9964)
        (41.2572	-95.9656	-33.8646	151.2099    0.7201	-0.5910	    -1.3111	    4.3140)
     }
    
    # calculate the "great circle" distance between the two (lat φ, long λ) points, in kilometers.
    # these are the function calls in my original query, commented out and replaced with VALUES above
    # BIND (f:radians(?lat1) AS ?φ1)
    # BIND (f:radians(?lat2) AS ?φ2)
    # BIND (f:radians(?lat2 - ?lat1) AS ?Δφ)
    # BIND (f:radians(?lon2 - ?lon1) AS ?Δλ)
    BIND (m:sin(?Δφ / 2) * m:sin(?Δφ / 2) + m:cos(?φ1) * m:cos(?φ2) * m:sin(?Δλ / 2) * m:sin(?Δλ / 2) AS ?a)
    BIND (2 * m:atan2(m:sqrt(?a), m:sqrt(1 - ?a)) AS ?c)
    BIND (6371 AS ?RadiusOfEarthInKm)
    BIND (?RadiusOfEarthInKm * ?c AS ?d1)

    # call the Jena function for comparison
    BIND(spatial:greatCircle(?lat1, ?lon1, ?lat2, ?lon2, <http://www.opengis.net/def/uom/OGC/1.0/kilometre>) AS ?d2)
}