I 
rks of man), such as towns, cities, roads, railroads, and 
ndaries. 
twn and 
The symbols used to ent these features are 
yrrn «uu ^plained below. Variations appear on some earlier 
jps, and additional features are represented on some special 
Ips* * ' ’ - _ • , 
|V11 the water features are represented in blue, the smaller 
jams and canals by single blue lines and tB^larger streams, 
\ lakes, and the sea by blue water lining or blue tint. Jnter- 
[tent streams—those whose beds are dry for a large part of 
year'—are shown by lines of blue dots and d^ht ^. 
belief is shown by contour lines in brown, which on some 
is are supplemented by shading showing the effect of^Hght 
)wn from the nortliwest across the area, represented, for the 
.pose of giving the appearance of relief and thus aiding in 
interpretation of the contour lines. A contour line repre- 
s an imaginary line on the ground (a contourflevery part 
inch is at the same altitude above sea level. Such a line 
jld be drawn at a v ^e con “ 
at certain regular intervals of altitude are shown, lne 
of the seacoast itself is a contour, the datum or zero of alti- 
being mean sea level. The 20-foot contour would be the 
ire line if the sea should rise 20 feet. Contour lines show 
shape of the hills, mountains, and valleys, as well as their 
tude. Successive contour lines that are far apart on the 
> indicate entle slope; lines that are close together indi- 
C a steep slope; and lines that run together indicate a cliff. 
ler.in which contour lines express altitude, form, 
1 grade is shown in the figure below. 
] 
