From my own experiences, posting like this is a good first step. Secondly, I'd suggested providing X/Y coords of start to finish of the canal as well as well as a screenshot from the online livemap of it's location. Things you'll need to consider: what is the highest point of elevation along the route? Slopes can only be dug or surface mined a max of a 300 slope - so if the highest point is 500, you'll need to factor that in.
I'd also ask the GM's if they could put up signposts along the length of the canal project informing players to not deed in that area.
Also, depending on the length of the canal, plan to have bridges to cross the canal every 20 tiles so players to provide ease of crossing.