Heat map using areas

MagnatAgri

Hi all, I'm not very experienced in using Global Mapper but hoping someone can help with an issue I'm trying to resolve.  I've created areas using the "create area features from equals values" by selecting a colour range from satellite imagery using the eyedropper tool.  I've had to tile the imagery to get high enough detail to make it worthwhile, so I have multiple layers (75) containing multiple area features.  It's a large area (several thousand hectares) and I'd like to summarize it if possible using a heat map.  It's this possible for areas or points only?  When I select all of the 75 layers and select 'Create Density Grid (Heat Map)' it comes up with the notification 'You must first enter vector data (i.e. line, point, and/or area feature data) before you can perform this operation'.  Any guidance would be appreciated.