Frank, I have long used a medium-size pack on a rear rack to hold my measuring tape, bike cable lock, adhesive tape and chalk, and miscellaneous gear. After riding past too many splits, I mounted a front basket in which I place a clipboard case and my notebook. I use cable ties to hold the case with my notebook open so that I have the numbers for my splits right under my eyes. This has been a big help - despite how dorky I look while I'm measuring. I also keep my adhesive tape and a calculator in the basket for easy access while I am measuring. Sometimes I carry an extra can of paint in the basket. The clipboard case keeps my notebook and draft maps dry in the event of precipitation. I have a cell mount for my phone on the stem, which I keep open to my GPS program.
Measurers have lots of different ways of configuring their bikes, some of which are visible in the archives here. I tried panniers and I did not like them - too bulky and inconvenient.