Observation Tasks are sent to users when there is a new image to review.
Tasks contain the image id, but no overlays (visualisations). If there are no overlays, then BFW contacts EE to create an overlay.
Overlays can be for either of the following roles: latest; prior or adhoc.
The latest overlay relates to the current observation.
The prior overlay relates to observations older than the curent observation. This may be a cloud-masked amalgam of several older images.
The latest overlay is displayed on the left map, the prior overlays on the right.
Once calculated, BFW stores the overlay as a tiled map object so it does not need to be recalculated when the page is refreshed until earth engine deletes the tiles ( after a day or so).
BFW automatically gets Earth Engine to regenerates the overlay when the tiles return 404. AdHoc overlays are generated by the user clicking the View button.
They are not stored in the database of overlays.All overlays have a slider and checkbox. They can be hidden or made transparent.