The vertical interval is usually indicated in the corner of the map by the letters "V. I." For instance: V. I.=20 feet.
On maps of very small pieces of ground, the V. I. is usually small--perhaps as small as 1 foot; on maps of large areas on a small scale it may be very great--even 1,000 feet.
Contours also show slopes. It has already been explained that from any contour to the next one above it the ground rises a fixed number of feet, according to the vertical interval of that map. From the scale of distances on the map the horizontal distance between any two contours can be found. For example: On the map the horizontal distance between D and E is 90 yards, or 270 feet. The vertical distance is 20 feet the V. I. of the map. The slope then is 20/270 = 1/13.5 = 7-1/2% = 4-1/2°, in all of which different ways the slope can be expressed,
On a good many contoured maps a figure like this will be found in one of the corners:
On that particular map contours separated by the distance
on the vertical scale show a slope of 1°: if separated by the distance
