Feature

A metric aggregation that aggregates a set of points as
a GeoJSON LineString ordered by some sort parameter.

specifics

A geo_line aggregation request would specify a geo_point field, as well
as a sort field. geo_point represents the values used in the LineString,
while the sort values will be used as the total ordering of the points.

the sort field would support any numeric field, including date.

sample usage

{
	"query": {
		"bool": {
			"must": [
				{ "term": { "person": "004" } },
				{ "term": { "trajectory": "20090131002206.plt" } }
			]
		}
	},
	"aggs": {
		"make_line": {
			"geo_line": {
				"geo_point": {"field": "location"},
				"sort": { "field": "timestamp" }
			}
		}
	}
}

sample response

{
    "took": 21,
    "timed_out": false,
    "_shards": {...},
    "hits": {...},
    "aggregations": {
        "make_line": {
            "type": "LineString",
            "coordinates": [
                [
                    121.52926194481552,
                    38.92878997139633
                ],
                [
                    121.52922699227929,
                    38.92876998055726
                ],
             ]
        }
    }
}

visual response

limitations

Due to the cardinality of points, an initial max of 10k points
will be used. This should support many use-cases.

One solution to overcome this limitation is to keep a PriorityQueue of
points, and simplifying the line once it hits this max. If simplifying
makes sense, it may be a nice option, in general. The ability to use a parameter
to specify how aggressive one wants to simplify. This parameter could be
the number of points. Example algorithm one could use with a PriorityQueue:
https://bost.ocks.org/mike/simplify/. This would still require O(m) space, where m
is the number of points returned. And would also require heapifying triangles
sorted by their areas, which would be O(log(m)) operations. Since sorting is done,
anyways, simplifying would still be a O(n log(m)) operation, where n is the total number
of points to filter........... something to explore