Instructables user hpstoutharrow was sick of storing his hiking maps inside a plastic bag, so he decided to look into ways to print on plastic. He ended up finding a way to print simple maps onto garbage bags using an iron and an ink jet printer.
The process itself requires several sheets of printer paper, a large clear plastic kitchen bag, a white plastic garbage bag, an ink jet printer, and an iron. There are a lot of steps involved here to ensure you get a good print, but it works by using the heated iron to bond the paper and the plastic together. You print directly on the plastic and seal it with the iron again.
You're not going to be able to get a highly detailed topographic map using this method, but it's a cost effective way to print your own waterproof maps. Hit up hpstoutharrow's guide below for a full walkthrough on the process.
Print Waterproof Plastic Maps [Instructables]