Add geonearestpoint operator

editions/geospatialdemo/tiddlers/cities/Winchester.tid

@@ -0,0 +1,8 @@
+title: cities/Winchester
+tags: $:/tags/GeoMarker
+caption: Winchester
+lat: 51.0632
+long: -1.308
+alt: 0
+
+This is Winchester!

plugins/tiddlywiki/geospatial/docs.tid

@@ -53,6 +53,24 @@ The `geodistance` operator calculates the distance between two points in [[GeoJS
 </$let>
 ```
 
+!! `geonearestpoint` operator
+
+The `geonearestpoint` operator determines the point in a list that is nearest to a target point. Each input list item is interpreted as a [[GeoJSON Point Feature]] comprising the candidate points. The target point is specified as the first operand in [[GeoJSON Point Feature]] format.
+
+```
+<$let
+	oxford={{{ [geopoint[51.751944],[-1.257778]jsonset[id],[Oxford]] }}}
+	winchester={{{ [geopoint[51.0632],[-1.308]jsonset[id],[Winchester]] }}}
+	new-york={{{ [geopoint[40.730610],[-73.935242]jsonset[id],[New York]] }}}
+>
+
+<$text text={{{ =[<oxford>] =[<winchester>] +[geonearestpoint<new-york>jsonget[id]] }}}/>,
+<$text text={{{ =[<oxford>] =[[Not a point]] +[geonearestpoint<new-york>jsonget[id]] }}}/>,
+<$text text={{{ =[[Not a point]] +[geonearestpoint<new-york>jsonget[id]] }}}/>
+
+</$let>
+```
+
 !! `geounion` operator
 
 The `geounion` operator calculates the union between two or more [[GeoJSON Polygon Features|GeoJSON Polygon Feature]].

plugins/tiddlywiki/geospatial/operators/measurement.js

@@ -28,4 +28,20 @@ exports.geodistance = function(source,operator,options) {
 	return [JSON.stringify(turf.distance(from,to,{units: units}))];
 };
 
+exports.geonearestpoint = function(source,operator,options) {
+	var target = geotools.parsePoint(operator.operands[0]),
+		points = [];
+	source(function(tiddler,title) {
+		var point = geotools.parsePoint(title);
+		if(point) {
+			points.push(point)
+		}
+	});
+	if(points.length > 0) {
+		return [JSON.stringify(turf.nearestPoint(target,turf.featureCollection(points)))];
+	} else {
+		return [];
+	}
+};
+
 })();

plugins/tiddlywiki/geospatial/tests/operators/geonearestpoint.tid

@@ -0,0 +1,24 @@
+title: $:/plugins/tiddlywiki/geospatial/tests/geonearestpoint
+description: geonearestpoint operator
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\whitespace trim
+<$let
+	oxford={{{ [geopoint[51.751944],[-1.257778]jsonset[id],[Oxford]] }}}
+	winchester={{{ [geopoint[51.0632],[-1.308]jsonset[id],[Winchester]] }}}
+	new-york={{{ [geopoint[40.730610],[-73.935242]jsonset[id],[New York]] }}}
+>
+
+<$text text={{{ =[<oxford>] =[<winchester>] +[geonearestpoint<new-york>jsonget[id]] }}}/>,
+<$text text={{{ =[<oxford>] =[[Not a point]] +[geonearestpoint<new-york>jsonget[id]] }}}/>,
+<$text text={{{ =[[Not a point]] +[geonearestpoint<new-york>jsonget[id]] }}}/>
+
+</$let>
+
++
+title: ExpectedResult
+
+<p>Winchester,Oxford,</p>