Hi, I am not a teacher, but I am an engineer, so maybe I can help.
First, I am almost certain that your sentences repeat one phrase by mistake. I think it should be:
"The change in elevational difference between two consecutive measurements of elevational difference each measured over 300mm not exceeding 5mm in 95% of measurements and not exceeding 7.5mm in 100% of measurements."
The 300mm is how far apart in distance they take the measurements. They measure the elevation at one spot, then move over 300mm and measure the elevation at the new spot. The difference in elevations should be less than 5mm in 95% of the cases and less than 7.5mm in all of the cases.