Civil Engineering Reference
In-Depth Information
scale factor corrections to coordinates. Combined factors are only applied to dis-
tances. Coordinates are not scaled. When we speak of converting coordinates from
one system to the other we are just saying that the values of the coordinates are
changed. We do not change the length of the foot or meter which is used for meas-
urements of or on the grid.
When working with conversions between ground coordinates and state plane
coordinates, it is important to realize that the coordinates themselves are not
distorted.
In boundary surveying, we are primarily interested in the distance measured on
the ground. In most cases the distance will be measured with a total station. As
an example, suppose we begin a survey by using GPS to set a number of con-
trol points on a 200 acre boundary survey. We will perform the remainder of the
survey with a total station and use the GPS control points as traverse points. We
decide that the GPS coordinates will be based on the state plane coordinate sys-
tem. Let us also assume we are in an area where the ground elevation and distance
from the central meridian causes a substantial amount of distortion. When we go
out into the field and measure between the GPS points with our total station, the
distances which we measure will not agree with the distances calculated from the
GPS measurements. As we have seen, for very short lines, the differences may not
be that large, but for long lines, the differences may be substantial.
When working on a survey which incorporates state plane values, a surveyor
must decide which coordinate system to use. He can use the state plane values
or he can use ground values. The calculations can proceed either way, by simply
converting coordinates from one system to the other. Because GPS is now being
used in many boundary surveys, it would seem to make sense to work on the state
plane coordinate system. Should a surveyor decide to convert the GPS control to
a ground based coordinate system, and to work on that system it should be kept
in mind that a single scale factor will only be accurate for a small survey. So, if
the survey covers a large area it will be more accurate and less confusing, to work
with state plane coordinates, otherwise more than one scale factor may be needed,
making the calculations more cumbersome.
If a surveyor decides to work on the state plane system, boundary dimensions
will have to be converted to ground dimensions so that that the distances between
lot corners are correct when they are measured on the ground. The decision to use
a state plane coordinate system does not convey a license to the surveyor to mod-
ify boundary dimensions from their true values. Boundary surveys often require
that plans be prepared. Whenever a survey plan shows boundary dimensions there
are legal implications. For example, when a surveyor prepares a plan to be used
as the basis of a parcel description, the distances on that plan must be actual dis-
tances as measured on the ground, not distances adjusted to fit the state plane
coordinate system. When a client purchases a parcel 1,000.00 feet square she does
