Civil Engineering Reference
In-Depth Information
Fig. 12.9
Hybrid GPS and conventional survey
have one person in the field, it will be necessary to set up the receivers on points
C1 and NGS1, turn on receiver C1, drive to NGS1 and turn on that receiver. If it
took 15 min to drive from C1 to NGS1, that portion of the data would not be used
because both receivers would not have been on for that period. The same would be
true for turning off the receivers. The only data that would be useable would be the
data collected while both receivers were simultaneously collecting data.
Once we have collected data for the required amount of time, we would have
enough data to calculate survey line L1. We would then move the C1 receiver to
C2 and collect data for another hour. This will give us data for survey line L2.
Finally, we would move the receiver at NGS1 to C1 and repeat the process for
survey line L3. In our example, it would take at least 3 hours to collect the data for
our network. If we only had a single person working it would take longer, depend-
ing on the time required to travel between the points. Notice that we have to col-
lect data for each line in our network. It can be seen that if we had three receivers
instead of only two, data for all points would be collected simultaneously and it
would only take us an hour to collect all of the data, instead of 3 hours.
A word of caution is in order when leaving GPS receivers in the field. There is
an understandable tendency to leave the receivers unattended when they are collect-
ing data. Doing so reduces the size of the field crew. It also seems like a waste of
time to have a crew member stand there for an hour or more watching the receiver.
However, leaving the receivers unattended may be too great a temptation for some
and having the receivers stolen or even mischievously moved or tipped over is not
unheard of. Because of the necessity of simultaneous data collection, loss or move-
ment of a receiver means starting over again for that phase of the work.
