Making your own lake map or improving an existing one has never been easier. Navionics allows users to record sonar logs and then upload them to navionics.com. Within a week your map will be updated. Then when you update your Navionics+ card, you will have the latest map data.
I have been doing this all summer and here is an example of a before and after for Long Lake, WA.
You can find detailed instructions for all of the major electronics brands here.
Video How-To for Lowrance HDS Gen 1