Spatial Analysis: Allow user to draw multiple reference areas

[danangmassandy]

Allow user to draw multiple reference areas and select the polygon.

TBD: Do we allow selection of mutiple polygons?

[danangmassandy]

Current implementation is that user can draw multiple polygons for the reference area. But, the polygons will be combined for calculating the relative difference layer. We could enable user to select single polygon and use it as relative difference layer.

Also, note that when clicking draw, the previous polygons are cleared.