Unfortunately, I can sympathize with you and other measurers who have to measure in the rain. I am not saying that it rains a lot here in Vancouver, but I have been caught more than once in the cold and wet.
I exclusively use waterproof notepads when I am measuring (rain or shine). They come under the brand name: "Rite in the Rain" All Weather Writing Paper and are available from survey equipment suppliers. The ones I use are 4.5 by 7 inches and come either spiral bound or in booklet form for about $5.95 CA. They work best with pencil rather than pen, but don't fall apart or become illegible in the rain.