Surveying Indirect Measurements with GNSS

Video Transcript
Download Video
Right-click and save to download

Detailed Description

This video details how to survey indirect measurements with GNSS. Both RTN and RTK setups are discussed, along with tips and tricks and troubleshooting.


Date Taken:

Length: 00:08:43

Location Taken: Henderson, NV, US

Video Credits

Todd Geiger, Brandon Meier, Hampton Childres


Hi, this is Megan Poff and I’m the Field Office Chief at the USGS in Las Vegas, Nevada.  I’ll be talking briefly about surveying indirect measurements with GNSS.  First of all, what is GNSS?  GNSS stands for Global Navigation Satellite System.  A Global Positioning System, or GPS, is the exclusive U.S. space-based constellation component of GNSS.  Complete information on using GNSS can be found in the USGS Techniques and Methods book 11, chapter D1, which I’ll be referring to herein as the GNSS T&M.  In the Vegas area, we really love surveying our indirect measurements with GNSS.  We have found it to be faster than traditional surveying methods and it gives us the option of doing a one-person indirect measurement survey.  I have done a number of solo indirect measurement surveys with GNSS, and I can get the simple ones done in just a couple of hours.  Another advantage is that surveys can easily be accomplished with real-world coordinates.  We have also found that our GNSS systems can collect points in areas where traditional surveying methods would not work, like in extremely heavy cattails.  In heavy cattails, you often can’t shoot through to see a prism or barcode level rod unless you do a lot of chopping and trimming, but with a GNSS system, you can just force your way into the mess and get points! 

Let’s talk about traditional RTK versus RTN surveying methods along with the equipment we’ll need for each.  Traditional RTK, or Real-Time Kinematic, uses a stationary receiver known as a base station to provide real-time differential corrections to a mobile receiver known as a rover.  For an RTK survey, you’ll need two receivers, a tripod for the base station, a rod (with bipod recommended) for the rover along with cables, communication equipment like radios, power supplies, and some sort of handheld data collector.  For the tripod, the GNSS T&M recommends a center-pole fixed-height tripod, which is also my preference because you know exactly how high the tripod is at all times.  An RTN, or Real-Time Network, uses a network of fixed receivers that can provide differential corrections to the rover via wireless broadband technology from cell phone towers.  All you need to do an RTN survey is a rover – the mobile receiver I mentioned previously, a rod for the rover, a subscription to your local RTN, a cell phone, mifi, or SIM card that accompanies handheld data collectors, and a data collector. 

Wow – seems like we need less equipment for an RTN survey than we need for an RTK survey.  Why wouldn’t I just do RTN surveys all the time and save myself the expense of buying a second receiver?  That’s easy!  RTN isn’t available everywhere, like in rural areas or areas without wireless data access.  Also, not all RTN services are free and are often fee based.  Even if you plan to do an RTN survey at an urban site, it’s always a good idea to bring along a base station as a backup.  In general, if you have wireless data access and a subscription to your local RTN, you can go ahead and do an RTN survey.  If not, plan to do an RTK survey.  The rest of this video will focus on RTK surveys since many USGS gages are located in remote locations.

The general procedure for a GNSS indirect measurement survey is as follows: 1) before starting the survey, measure and record the height of the base on its tripod and rover on its rod to verify their heights.  2) Set up the base station over a known benchmark or some other marked feature you can accurately re-occupy and establish communications with satellites. 3) Turn on the rover and establish communication with the base station.  4) Make sure you have a “fixed” position.  5) Collect a data point.  Most GNSS systems will collect data at one-second intervals.  See the GNSS T&M to determine the amount of time you’ll need to occupy each point in your survey for the accuracy you desire. 

I want to go over some quality assurance of a GNSS survey.  Just like with any other survey, the first point you survey should be a benchmark preferably representing a leveled height, or at a minimum a published height disseminated from a height modernization campaign using GNSS.  When you start your survey, take 180 epochs of data on that first point.  A bipod will help you keep the rover rod steady.  You will then perform a blunder check on this point.  Blunder checks are required for all shots that identify control or those that need to maintain specific uncertainties. The procedure for doing a blunder check is as follows: 1) record 180 epochs of data on the object.  2) Walk at least 100 ft away from the object.  3) Invert the rover rod so the receiver is upside down long enough to lose initialization on the satellites.  Keep in mind that some newer handhelds will allow you to perform the re-initialization without inverting the rod, so make sure you read your owner’s manual. 4) If you aren’t using a fixed-height rover rod, adjust the height of the rod by at least 0.8 ft and enter the new height into the data collector.  5) Walk back to the object using a different approach path, and 6) set up on the point and complete another 180-epoch shot.

When you stop to take a break to, say, plot your high-water marks in the field, it would be a good idea to return to that point and take another 180 epochs of data.  When you finish your survey, you should go back to that point a final time and “close” on it with another 180 epochs of data.  You’ll be able to determine if your data are usable or not depending on how consistent your shots are.  

I have a few tips and tricks for getting the most out of your GNSS system when you survey an indirect measurement.  GNSS equipment can be finicky in regards to the communications between base to rover, or the rover or base to the satellites, or Bluetooth communications between the controller to either the base or the rover.  These are complex systems and little issues can happen!  Practice with your GNSS system in your parking lot before you take it out to survey for an indirect measurement.  It’s much easier to troubleshoot when you are calm and can go inside and ask a colleague some questions if you get stuck.  Consider the alternative – you’re out at a remote field site staring down at the aftermath of an enormous flood.  There’s mud and debris everywhere, you’re hours from the office, there is no cell phone coverage, the rover won’t communicate with the base, and you don’t know what to do.  If only you had set up and practiced in the parking lot at work beforehand! 

Another tip – and this might seem like a no-brainer – but always charge the batteries before you go out, and make sure those batteries can hold a charge for however long you’ll be surveying!  If your field vehicle is equipped with an inverter, take the battery chargers with you as well.  If a survey ends up taking longer than you anticipated, you’ll be covered.  If the survey is done before you anticipated, you’ll be able to survey at another site. 

One more tip – know how to download the data from the controller and be able to do just enough processing to be able to plot the high-water marks in the field.  It can be tempting to skip this step, but it is a requirement for most indirect methods used by the USGS.  Many GNSS controllers have some data visualization capability, but I have never seen one that can plot high-water marks with elevation on the y-axis and station on the x-axis.  Take the time and do it right in the field using a spreadsheet or the SAC-GUI and reap the rewards of having a reliable survey for your computation. 

I also want to mention a couple of other items to keep in mind during GNSS surveys.  Don’t forget your good tenets of surveying an indirect measurement just because you’re using a different instrument.  Make your site sketch.  Document what you see with pictures.  Make sure you know how to make remarks in your handheld GNSS controller so you can record the quality and type of high-water marks.  If you’re out surveying different sites over multiple days, download the data in the evening so you don’t lose data.  Furthermore, if you are doing an RTN survey, you will need to evaluate it daily with adjustments using a spreadsheet as discussed in the GNSS T&M.

Finally, let me present a few caveats.  GNSS won’t work everywhere.  If it can’t see the sky, you’re out of luck.  If it can only see a limited sky view, you might be out of luck.  That means that areas in dense tree cover or canyons with steep and high walls probably won’t work for a GNSS survey.  I encourage you to take the time to evaluate your possible indirect measurement survey locations for GNSS usage before you are staring down the flood of the century.  I also recommend that you read through the entire GNSS T&M before attempting GNSS surveys for an indirect measurement, but I would like to point you specifically to the section on mission planning, which has some great information on things like space weather, position dilution of precision, and satellite outages.  Keep in mind that GNSS is a great tool to have in your toolbox, but it isn’t the only tool.  You wouldn’t use a screwdriver to hammer in a nail, right?  Same thing with GNSS. 

If you need help in the field, call your supervisor, surface-water specialist, GNSS specialist, or indirect measurement specialist.