Civil Engineering Reference
In-Depth Information
Fig. 7.2
Parallels and meridians
equator. As the meridians progress north or south from the equator, they converge
and come to a point at the poles. This is called
Convergence
. Figure
7.2
shows
parallels and meridians. As shown in Fig.
7.1
, principal meridians were established
which pass through initial points. Principal meridians extend both north and south
from these points. All principal meridians run true north and south.
Base Lines
are parallels of latitude. Parallels are so named because they are
parallel to the equator. Unlike meridians, Parallels do not converge. As shown in
Fig.
7.1
, base lines are extended east and west from the initial points. The intersec-
tion of a base line and a meridian forms a 90° angle. Base lines run true east and
true west.
Notice in Fig.
7.3
near R4E that south of the base line, where the first guide
meridian east intersects with the base line, there are two intersections. The inter-
section to the south of the base line is called a
Closing Corner
. The closing corner
is the intersection of the meridional line with the parallel. The intersection to the
north of the base line is called a
Standard Corner
. The offset in the two cor-
ners is caused by convergence of the guide meridians and occurs on each standard
parallel.
7.3 Quadrangles
Once the principal meridians and base lines were established, a series of
Standard
Parallels
were established 24 miles north and south of the base lines (in some
cases 30 miles was used). The labeling was sequential, such as first standard paral-
lel north, second standard parallel north and first standard parallel south, second
standard parallel south. An example of these parallels is illustrated in Fig.
7.3
.
