Leica 320SG Laser Question, Having issues while laying pipe in Texas Heat

I've been trying out a Leica Rugby 320SG high end laser for laying 18" C905 waterline. The unit seems to get off an inch or more after 100' or more. The salesman said may be UV rays from the white bedding rock or the 100 degree heat. Anyone ever hear of this? Very frustrating! Posted Pics

Many thanks for any input

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Leica 320SG Laser Question, Having issues while laying pipe in Texas Heat