If you need good absolute locations, then you hire a company to put up markers for one or more reference points with high or even extremely high precision. Maybe down to mm depending on needs.

But when the goal is to just measure distances with the traveling GPS, then there is no such need. Distances are relative so it doesn't matter what the position of the fixed GPS is - it's enough that it sees lots of satellites.

And even when you need absolute positions, you can get quite good results by having that fixed GPS sample the location for a couple of days and take the averaged center. There will still be a residual system error, but way smaller than the normal expected error you have from a snapshot position.

Anyway - for really good results, you need a RTK (Real Time Kinemaic) GPS. But they are very expensive.
http://en.wikipedia.org/wiki/Real_Time_Kinematic