Thank you for the clarification. If you are sending the data as MobileDeviceId,latitude,longitude then I would recommend creating a GeoEvent Definition with those fields in that order. Keeping in mind that every GeoEvent being processed must have a GeoEvent Definition which matches the event's schema. Comma-separated text being received via a UDP/Text Input must still have an Event Definition which specifies the event field names, data types, and field order.
You will want to configure your Input to create a Geometry from the received latitude and longitude fields, and include a field of type Geometry in the GeoEvent Definition so that the Input has somewhere to put the Geometry it creates.
- RJ