Civil Engineering Reference
In-Depth Information
For each state plane projection area, a central meridian corresponding to a line
of longitude is chosen that is near to the center of the area to be mapped. The cen-
tral meridian corresponds to true north. As the distance east and west of the central
meridian is increased the distortion becomes greater and grid north deviates from
true north to a greater extent. This happens because lines of longitude converge at
the earth's poles but the grid lines on the state plane coordinate system remain par-
allel to the central meridian.
Boundary surveys may be related to a state plane coordinate system by run-
ning survey lines to N.G.S. points or by using global positioning as described ear-
lier. Connecting a boundary survey into a state plane coordinate system will add a
level of redundancy because it has the potential to make the reestablishment of lost
monuments easier and more certain. We will discuss GPS methods and state plane
coordinates in detail in later sections.
4.10 Magnetic North
Magnetic north is determined by using a magnetic compass. A magnetic compass
needle aligns itself with the earth's magnetic field which, in most geographic loca-
tions, does not correspond to true north. Approximately 90 % of the earth's mag-
netic field is generated by the earth's outer core. The relationship between true
north and magnetic north varies with the geographic location of the person making
the measurement.
Declination
is the term used to describe the angular difference
between true north and magnetic north. In 1980 in New Hampshire magnetic north
was approximately 15° west of true north. In the Florida panhandle it was 0°. In
central Texas it was 8° east and in Oregon it was 20° east. The magnetic declina-
tions for North America in recent times are shown in Fig.
4.9
.
A magnetic compass needle aligns itself with the earth's magnetic field
which, in most geographic locations, does not correspond to true north.
The location of magnetic north also varies with time.
Variation
is the term used to
describe how declination varies with time. When interpreting old deeds and plans
it is necessary to make variation adjustments for historical differences in magnetic
north. The National Geophysical Data Center (NOAA) maintains a web site where
users can compute the historic declination for locations in the U.S. Having this
information available becomes very important when a surveyor needs to locate the
boundaries of a compass survey that was performed many years ago.
Although it is probably not an immediate concern for surveyors, the earth's
magnetic poles are subject to polar reversals. Every 200,000-500,000 years north
becomes south and vice versa. It has been about 750,000 years since the last rever-
sal. It is not known when the next reversal will happen but it is believed that it
