Civil Engineering Reference
In-Depth Information
Once we have collected all of the data, we would bring the receivers into the
office and upload the data to a personal computer where the network would be
analyzed and adjusted. The requisite software is usually supplied by the receiver
manufacturer. When the data is analyzed, we would instruct the software to hold
point NGS1 and enter the state plane coordinates for that point. Because GPS col-
lects 3-dimensional data, we would also need to enter the elevation of the point.
Assuming there were no flaws in the data, after adjusting the network the software
would calculate coordinates for points C1 and C2. The bearings and distances for
the three lines would also be calculated. The coordinates would then be uploaded
into our total station data collector. When we went back into the field to run the
traverse around our Locus, we would begin by setting the total station on point
C1. After setting a prism on C2 we would use the total station to measure the dis-
tance between C1 and C2. Because our GPS software has already calculated this
distance, we would be able to compare the results of the GPS measurement and
the total station measurements. This would provide us with a confirmation that this
portion of our network was accurate. Of course we would need to take into con-
sideration the differences between ground and state plane grid measurements that
we discussed earlier. While the total station occupied C1 we would place points T1
and T4 online so we could begin our traverse through the wooded parcel.
As our example demonstrates, GPS can be a valuable asset for a boundary
surveyor. For large boundary surveys a number of control points can be located
throughout the area where traverse lines will be run and where the total station
will be able to occupy these points as a part of the traverse. This will provide
redundant control and the traverse can be adjusted between the GPS points.
Static GPS allows even small boundary surveys to be economically and quickly
tied into the state plane coordinate system. Once a survey is tied into state plane,
it will always be possible to reproduce the survey, even if all of the permanent
monuments on the site are destroyed. Static GPS can also be useful when it is nec-
essary to locate evidence that is too far away to be located economically using
conventional traversing techniques. One example that comes to mind is when a
state, county or town line runs through the property being surveyed or the line
constitutes one of the parcel's boundaries, and the closest monuments on the line
are miles away. Traversing to the monuments may take days but, using GPS it
might be accomplished in a few hours.
Consider the survey shown in Fig.
12.10
. A state line runs through the locus.
The locus traverse consists of points 14 through 17. A state line boundary marker,
SB1, was found in a wooded area two miles away. A GPS receiver cannot be set
on the boundary marker because it is in a wooded area surrounded by tall trees.
One way to locate the boundary marker would be to establish a random short line
just outside the wooded area in the open. This line is labeled 1-2. GPS receivers
can then occupy points 1 and 2 and one or more of the points on the locus trav-
erse. The points on the locus traverse would serve as control. Once coordinates
for points 1 and 2 have been obtained, a total station could be set up on point 2, a
prism set up on point 1 and SB1 could be located using an angle and distance. In
this example it would not be necessary to traverse two miles and back in order to
