We (read I) are currently surveying all the stormwater pits and pipes in our shire, and have run into a few issue with handling unknown network connections. The problem is that a lot of our network is not pit-to-pit like any good network should be (I can thank lack of standards for that) so we have a lot of pits that have a pipe running out to another pipe somewhere underground and it’s just busted in with no junction pit.
We are able to get a invert on the outgoing pipe from the pit but not where it joins the main pipe, because well, we don’t know where that main pipe is or even how deep it is. I think a picture will help with what I am describing:

So the green dot is the new surveyed pit and the X is the invert position. We have a rough direction for the outgoing pipe but no idea how far back into the footpath the main pipe is, so creating a connected network is bloody hard.
Now imagine over half your network is like this, and this is an easy case. How do you handle it?
I’m interested to know how other people would/have handled doing a full survey of their stormwater (or any other underground) network. Our ultimate goal is to have a complete connected network with inverts at all joins in order to be able to run models.
Any help or people to contact would be of great help.
I'm very interested to know how you would get inverts for the pipes that have no junction that comes to the surface. Is digging each junction up the only option?