The top line represents differences of altitude in feet. The first column gives vertical angles in degrees and minutes. The body of the table gives distances in miles and hundredths of a mile, corresponding to the number of feet at the top of the column, and the angle at the left of the line.
Additional publication details
USGS Unnumbered Series
Table of differences of altitude to nearest foot for angles from 1 minute to 2 degrees and for distances under 1 mile