Compute the ratio of land to water. When many values are available, students should average these values to obtain one that better approximates the true value. Search the Internet or use an encyclopedia to find the area of the continents and the radius of the Earth. As opportunities arise during the activity, discuss sample bias, representative samples, and the importance of simple random samples. Large-scale maps are available for free over the Internet, so a class might consider obtaining a copy to consult during the activity.
Atlases report land areas in square miles or square kilometers.
Students should check to insure that values are correctly converted. There is wide variability in the values of land areas printed by different sources. Students could participate in an ancillary activity of finding data from multiple sources over time and exploring trends in the figures that they provide. The problem of accurately measuring land mass is related to the problem of finding the length of a coast line. They both deal with measurement differences that occur when one uses yardsticks of different lengths, so both can lead to a study of fractal sets.