Go to a large graveyard that has gravestones that range widely in age. Finding appropriate stones for each decade of the 20th century would work well.
Select the particular stones you will use in the research. Choose only stones that have no pedestal or other protruding features. You want standard rectangular stones that were originally made to be the same thickness at the top as at the bottom.
Determine the location of the first gravestone with the GPS and record it in the notebook, as well as the information on the stone, in particular the year of death.
Measure the thickness of the bottom of the gravestone with the calipers. Then measure the thickness of the top. A smaller number at the top is evidence of erosion. Enter these two numbers in your notebook.
Repeat steps 3-4 for all of the gravestones in your study. Convert the numbers you have to percentages to determine the change in each gravestone since it was put in place. Let's assume your stone measured 6 inches at the bottom and 5 at the top. First calculate the absolute difference: (6 - 5) = 1. Then get the average value: (6+5) / 2 = 5.5. Then get the percentage by dividing the first result by the second and multiplying by 100: (1 / 5.5) x 100 = 18.18 percent.
Create a line graph based on your percentage numbers. In general, the older stones will have more weathering, but you should be able to see an increase in the weathering rate over time if your area is having acid rain problems. For example, if stones placed one hundred years ago have an average erosion percentage of 20 percent, you would expect (if there were a constant rate of erosion) stones placed fifty years ago to show an average of 10 percent. A number higher than 10 for the fifty year old stones would show possible evidence of increased acid rain.