30 
A convenient grid interval on the topographic maps for project plane 
coordinates is the ground distance represented by a spacing of 5 inches or 
10 centimeters when compilation is based on the foot or metric system of 
measurement, respectively. 
Before mapping is ündertaken, the horizontal control points (permanent 
station markers) in the basic control surveys for the project are plotted accu- 
rately on each applicable map sheet. Plotting is done using the plane coordi- 
nates of each of such control points as adjusted to apply at level of the sur- 
vey project, and these coordinates are recorded, numerically, near the margin 
of the maps outside mapping details and directly opposite the position of the 
station marker to which they apply. The position of each bench mark is sym=- 
bolized on the applicable map and its exact elevation is noted. 
Principal details required on the topographic maps are topographic fea- 
tures and spot elevations. Topographic features, compiled to scale and speci- 
fied accuracy in position, include contours, and streams, rivers, lakes, swamps, 
and so forth. Spot elevations, in true position, are needed at saddles and 
crests; at principal intersections of roads, streets, and canals; at centerline 
on ends of bridges and other structures; at no less than a 2-inch interval in 
all areas where contours are more than 3 inches apart; and at shore lines of 
streams and lakes. Other details required on the maps and compiled to scale 
are land use features, property boundaries, and political subdivisions.  Numer- 
ically noted or named on the maps are the plane coordinates (X and Y or East 
and North) of the coordinate grid lines, specific station markers where there 
is a change in datum from one segment of the survey project to another, prin- 
cipal land use and topographic features and their condition, property owners, 
and political subdivisions. Another detail shown near the center of each map 
sheet, at the intersection of an east-west grid line with a north-south grid 
line, is the deviation from true north of a specific line of the project plane 
coordinate system. 
The accuracy of plotted details, such as horizontal control points, is 
based on an equivalency of displacement with respect to true position. On the 
ground, this accuracy, in feet, is represented on the map by 1/100, or smaller 
fraction, of the map scale in feet to one inch; and in meters, by 1/3,931, or 
smaller fraction, of the denominator of the representative fraction expressing 
map scale. The accuracy of contours with respect to true elevation is such 
that the elevations determined from the contours on the maps have an accuracy 
of 3/2, or smaller fraction, of the contour interval for 90 percent of the 
contours tested and the elevations, so determined for the remaining 10 percent 
have an accuracy of one contour interval, or fraction thereof. 
Spot elevations meet accuracy standards when, with respect to true ele- 
vation, they are at an accuracy of 1/4, or smaller fraction, of the contour 
interval for 90 percent, and at an accuracy of 1/2, or smaller fraction, of the 
contour interval for the remaining 10 percent of these elevations. 
The accuracy of planimetric features is based on the map position of a 
feature with respect to its true position. For 90 percent of the features 
tested, this accuracy, in feet, is required to be 1/40 or smaller fraction of 
map scale in feet to one inch and, in meters, 1/1,575 or smaller fraction of 
the denominator of the representative fraction expressing map scale. For the 
remaining 10 percent of features tested, this accuracy, in feet, is required to 
be 1/20 or smaller fraction of map scale in feet to one inch, and, in meters, 
1/787 or smaller fraction of the denominator of the representative fraction 
expressing map scale. 
The title block for each map sheet contains essential identifying infor- 
mation and specific numerical data needed by the map user. These include the 
project name, type and purpose of the map; graphic scale and numerical expres- 
sion of the compilation scale; if the map is planimetric, notation regarding 
the minuteness in which the profile and cross sections were measured; the State 
plane coordinate system by State and zone, the numerical value of the combined 
adjustment factor and datum elevation to which it applies; and the method of 
map compilation.