GNSS / GPS
You can finish your surveys sooner and with greater precision by leveraging the power of GPS. Understanding what your instrument can and can’t do before you get started is crucial. On the plus side, you don’t need intervisibility between points. GPS works 24/7 in almost any weather and gives extremely geodetically accurate results. Best of all, you can get more done in less time with fewer people. At the same time, be aware of GPS’s limitations.
Several Measuring Techniques
Your GPS needs to be able to “see” at least four satellites. Trees or tall buildings can block your GPS’s line of sight, and GPS isn’t suited for indoor surveying. In such situations, an optical total station may be more cost-effective. Depending on your application, you can choose from several measuring techniques:
For long lines, geodetic networks, or tectonic plate studies, use static measuring. It’s extremely accurate over long distances, but slower than other methods.
To establish local control networks, network densification, or related jobs, use rapid static measuring. Also highly accurate for baselines up to 20 kilometers, rapid static is much quicker than static measuring.
If you’re doing detail surveys or measuring many points close to each other, kinematic measuring may be your best bet. This depends on whether your instrument can see four satellites in the clear. Otherwise, you’ll need to reinitialize, which can take between 5 and 10 minutes – unless your GPS has on-the-fly processing capabilities.
Another technique is real-time kinematic (RTK), which uses a radio data link from the satellites to the reference receiver and on to the rover. This allows for real-time measurements, as long as there’s no radio interference or line-of-sight blockage. RTK is useful for detail surveying, stakeout, and COGO applications.
Static measuring is the original GPS surveying technique. To measure a long baseline (20 km/16 mi and up), place the reference receiver at a known point. Next, place a second receiver (called the “rover”) at the other end of the baseline. Then, set identical data recording times – usually 15, 30, or 60 seconds – and measure for at least one hour. Depending on how long the line is, how many satellites are in sight, and their relative geometry, you may need to measure for a longer time. Remember: measure twice to avoid mistakes. Once you collect enough data, turn off the rover, move it to the next baseline, and repeat the process. For even greater speed, add another rover and alternate the two rovers’ placement to measure each line.
Rapid Static Measuring
If you’re working on a site that’s never had any GPS surveys done, you’ll first need to establish several points with known coordinates in order to calculate transformations. Choose a point for the reference receiver, then move one or more rovers to each of the other known points. As in static measurement, the time that each rover must measure depends on the baseline length and the GDOP. After you send the data to your office for processing, check for errors by measuring the same points again at a different time of day.
Set up the reference receiver, then place the rover at the end of the baseline. Making sure not to move the instruments from their stationary positions, turn on both receivers and wait 5 to 20 minutes (depending on the number of visible satellites and the baseline length). After the data is acquired, you can walk with the rover. It’s possible to record positions at a predefined rate, at predetermined positions, or both. However, try to avoid objects that might block the receiver signal. Should the rover lose sight of receivers so that there are fewer than four, move it to where it can see four or more satellites, then re-initialize it before you continue measuring.
Real-Time Kinematic (RTK) Measuring
RTK is replacing kinematic measuring. The rover gets signals from the reference, but because it also has its own GPS antenna, it receives satellite signals directly instead of through the reference receiver. The rover then processes both signals to resolve the ambiguity. Start by setting up the reference receiver. Once it is picking up the satellite signal, you can turn on the rover. Wait until the rover starts tracking both the satellites and the reference. When this happens, the rover will initialize, resolve ambiguities, and be ready to record both points and coordinates. Baseline measurements are accurate between 1 cm and 3 cm. Be sure not to lose contact with the reference, which causes the rover to lose the ambiguity calculations and thus accuracy. Check your transmitting radio for interference. Make sure the antennas on both the transmitting and receiving radios aren’t blocked by tall buildings, and that your cable to the antenna isn’t so long that the signal degrades.