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 and 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 and 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.
One, before starting the survey, measure
and record the height of the base on its tripod
and rover on its rod to verify their heights.
Two, set up the base station over a known benchmark
or some other marked feature you can accurately re-occupy
and establish communications with satellites.
Three, turn on the rover and establish
communication with the base station.
Four, make sure you have a fixed position.
Five, collect a data point.
Most GNSS systems will collect data at one-second intervals.
See the GNSS T and 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.
One, record 180 epochs of data on the object.
Two, walk at least 100 feet away from the object.
Three, 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.
Four, if you aren't using a fixed-height rover rod,
adjust the height of the rod by at least 0.8 feet
and enter the new height into the data collector.
Five, walk back to the object using
a different approach path, and six,
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 and 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 and 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.