Ok. Did some digging. The type is defined at https://meta.wikimedia.org/wiki/Wikidata/Data_model#Geographic_locations

a precision (decimal, representing degrees of distance, defaults to 0, 9 digits after the dot and three before, unsigned, used to save the precision of the representation)

After some digging I found http://git.wikimedia.org/blob/mediawiki%2Fextensions%2FDataValues.git/727ccd66b03f42e1f5458ea204ca15b315cf5f04/DataValues%2Fresources%2FglobeCoordinate.js%2Fsrc%2FglobeCoordinate.GlobeCoordinate.js

function isValidPrecision( precision ) {

    var precisions = globeCoordinate.GlobeCoordinate.PRECISIONS;
 
    for( var i in precisions ) {
        if( Math.abs( precision - precisions[i] ) < 0.0000001 ) {
            return true;
        }
    }
 
    return false;
}
 
/**
 * Precisions a globe coordinate may feature.
 * @type {number[]}
 */
GlobeCoordinate.PRECISIONS = [
    10,
    1,
    0.1,
    1 / 60,
    0.01,
    0.001,
    1 / 3600,
    0.0001,
    1 / 36000,
    0.00001,
    1 / 360000,
    0.000001,
    1 / 3600000
];

So it looks like only these values (with a minor deviation of 0.0000001) are considered valid. Is this true? Is this intentional? Where the hell is this documented? If this is true, why does the api accept invalid precisions?

Looks like the JS cant render anything that isn't explicitly defined as okay!?

http://git.wikimedia.org/blob/mediawiki%2Fextensions%2FDataValues.git/727ccd66b03f42e1f5458ea204ca15b315cf5f04/DataValues%2Fresources%2FglobeCoordinate.js%2Fsrc%2FglobeCoordinate.GlobeCoordinate.js#L180

https://www.wikidata.org/w/index.php?title=Q3969823&oldid=113061813
has a precision value of 2.0e-5 but displays as undeserializable
with JS off its okay.

• This bug has been marked as a duplicate of bug 64887 ***