I'm really struggling with a 'resonable' technical implementation of the map system used by the popular MMO World of Warcraft. Not really sure on a data structure format and how to save/store it in a SQL relational DB. I've come up with an idea below, but wondered if anyone had any better ways?

A brief summary of the map system used by World of Warcraft:

• Loading up the map starts with a blank outline of a continent
• Each continent has multiple 'zones', which are outlined, but blank
• Selecting a 'zone' zooms into the zone, but it is initially blank
• As the player explores the zone, certain areas within the zone will 'be discovered' and will be coloured and detailed in
• Players need to explore the whole zone for it to be fully discovered and show a 'complete' map
• Players can discover any area of the map in any order
• Maps and areas are not rigid, they can be complex and different shapes and sizes

My current thinking:

• Use a 2D tool to create the following 2D assets
• The blank outline of a zone
• Multiple transparent layers with a certain area 'coloured', which could be overlayed onto the blank outline
• Assign an integer ID to each area layer
• When a player discovers a zone
• Attempt to find the correct corresponding layer and assigned 'integer'
• Add this integer to an array corresponding to a zone
• When a player logs off
• save this array to a string, seperating the integers with a underscore
• save this string to a seperate SQL table, using the character ID as a primary key
• When a player logs on
• load all rows using the character ID as a lookup
• Load all of the integers sperating them out with the underscore
• use the integers as a ID reference to the transparent layers to show as visible

Thoughts?