MANUAL FOR NONCOMMISSIONED OFFICERS. 815 



you put in on a piece of ground a lot of stakes, each one of 

 wiiich is exactly the same height above sea level — that is, run 

 a line of levels — then make a map showing the location of the 

 stakes, a line drawn on the map through all the stake positions 

 is a cpntour and shows the position of all points of that par- 

 ticular height. 



On any given map all contours are equally spaced in a ver- 

 tical direction, and the map shows the location of a great 

 number of points at certain fixed levels. If you know the 

 vertical interval between any two adjacent contours, you 

 know the vertical interval for all the contours on that map, 

 for these intervals on a given map are all the same. 



With reference to a point through which no contour passes, 

 we can only say that the point in question is not higher than 

 the next contour up the hill, nor lower than the next one 

 down the hill. For the purposes of any problem, it is usual to 

 assume that the ground slopes evenly between the two adja- 

 cent contours and that the vertical height of the point above 

 the lower contour is proportional to its horizontal distance 

 from the contour, as compared to the whole distance between 

 the two contours. For instance, on the map, find the height 

 of point A. The horizontal measurements are as shown ou 

 the map. The vertical distance between the contours is 2v. 

 feet. A is about one-quarter of the distance between the SOO. 

 and the 820 contours, and we assume its height to be one 

 quarter of 20 feet (5 feet) higher than 800 feet. So tM 

 height of A is 805 feet. 



The vertical interval is usually indicated in the corner oi 

 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 usuaU!? 

 small — perhaps as small as 1 foot ; on maps of large areas o^ 

 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 ths. 

 map the horizontal distance between any two contours caa 

 be found. For example: On the map the horizontal dis- 

 tance between D and E is 90 yards, or 270 feet. The verti- 

 cal distance is 20 feet, the V. I. of the map. The slope then is 



