Hello Elle / Xander -
Your options for adding and updating a track line as a Polyline feature record have not changed. In the thread Xander Bakker references, Generating Tracks from Events (April 2014), I mentioned a processor we were considering that would do this for you. The "reconstruct tracks" processor ended up being implemented as part of GeoAnalytics Server -- not GeoEvent Server. You can read more about that capability here: Reconstruct Tracks
In that same thread I suggested using an Incident Detector to help create your track line. That is still an option:
Please see my update to theRe: Generating Tracks from Events thread.
Using an Incident Detector this way is a very bad idea.
You would select Polyline for the processor's Geometry Type to configure the processor to build and update a polyline rather than point features. You will also need to make sure that the Incident Type is Cumulative (not PointInTime) ... and set the processor's Expiry Time to a sufficiently high value that the incident will not be automatically closed when no updates are received for a period of time; the default is 300 seconds (5 minutes).
GeoEvent Server Inbound connectors, such as Receive JSON on a REST Endpoint, offer the ability to Construct Geometry From Fields, but this is necessarily limited to Point feature geometry with a single X / Y coordinate value pair. The GeoEvent Server input cannot iterate over an array of coordinate values to build out a polyline or a polygon.
Your only other option, using GeoEvent Server, would be to incorporate a series of Field Calculator processors into a GeoEvent Service to perform specific string manipulations and transform coordinate information from a received event record into a JSON string representation of an Esri Polyline or Polygon geometry. The GeoNet thread How to switch positions on coordinates provides an example of how you might approach this. The specification for the Esri Feature JSON can be found beneath the ArcGIS REST API Geometry objects topic.
Please also see comments from Tanu Hoque and Thomas Colson in the thread Tracking Analyst in Pro. Thomas offers an approach using SQL to create a time ordered set of GPS points which can be used as vertices to create a polyline. Tanu includes animated illustrations of how he got this to work.
Hope this information was helpful --
RJ
