Hi, this is Megan Poff and I'm the Field Office Chief at the USGS in Las Vegas, Nevada.
On the first slope-conveyance video, we talked about theory, high-water marks, and cross-section locations.
This video will focus on surveying slope-conveyance indirect measurements.
More information on the slope-conveyance method can be found on page 334 of USGS Water Supply Paper 2175.
We've flagged our high-water marks and determined our cross-section location already.
Now we need to survey our slope-conveyance. You can complete a slope-conveyance using either a
For this video, I will demonstrate how to do a simple survey with a conventional digital level.
Just like with any other survey, follow the methods outlined in Levels at Gaging Stations-
USGS Techniques and Methods Report 3-A19. You can use either a digital or an optical level.
However, because we have to measure distance in addition to elevations, you're going to make use of the
horizontal circle or azimuth on the gun as well as the gun's distance-measuring capabilities.
Before you start, make sure you have a current good collimation test for your instrument.
You will want all of the accoutrements you typically take with you for any levels survey - that is, a tripod,
level rod, rod level, site-specific information such as a Station Description, and your field computer.
Don't forget your rod person!
Let's go to the site. Just like with any other levels circuit,
you will want to start your survey by doing a backsight on a reference mark.
However, remember that we are also measuring angles and distances with this type of survey.
We have to tell the instrument where 0 degrees, 0 minutes is located so our angles and distances will be correct.
What's another name for 0 degrees, 0 minutes? That's north! However, we have options.
It doesn't truly matter which direction you set to 0 degrees, 0 minutes, because we are surveying in an arbitrary datum.
You have three main options: 1) set the gun's zero to true north using a compass, 2) set the gun's 0 to "upstream" in the channel,
or 3) set the gun's 0 to the direction of the backsight shot. I actually prefer option 3 because I can check my angle at the end
of the survey very easily, but all options are valid. Whatever method you choose, point the gun in that direction,
and then rotate the bezel on the horizontal circle on the gun so you can set your 0 to that direction.
Once you've set the bezel, dont touch it for the duration of the survey.
Take the shot on your backsight, and also record the distance from the screen of the gun and the angle
that you'll read from the horizontal circle. When you read the horizontal circle, read down to the half of a degree,
which you will record in increments of 30 minutes. You're now ready to shoot all of the high-water marks.
You don't have to repeat high-water mark shots, and we also won't be doing a turning point for this simple, single-setup circuit.
Shoot each high-water mark first, making sure to record not only the foresight, but also the distance, angle, and the quality of the mark.
Remember that we are surveying high-water marks to a hundredth of a foot.
After you've surveyed your high-water marks, you can survey the cross section perpendicular to the flow.
We survey cross section points to the tenth of a foot, and you won't be repeating shots on these points either.
I try to place the cross section in an area that is representative of the entire reach if possible.
How many points should you choose for your cross section survey? Well, it depends.
Keep in mind though that your entire measurement will be based on the area that you define from this single cross section survey,
so it will be in your best interests to keep your survey relatively detailed.
Make sure you define the major changes in the shape of the channel as your rod person moves across the cross section.
For each cross-section point, record the foresight, distance and angle. You can find more information
on surveying both high-water marks and cross sections in the high-water marks and cross sections video in this series.
At the end of your survey, shoot back to your starting point again and record the foresight, distance, and angle to help with quality assurance.
You're done! You can now process the data through a spreadsheet to convert your angles and distances into northings and eastings.
I've been showing you how to measure distances using the electronic function your instrument, but what if the battery dies?
It's good to know how to measure distances manually. Take a look through the scope of the gun.
As you probably know, if you read the rod using the middle line, you can compute the elevation.
Have you ever looked at the top and bottom lines though? Those are stadia lines that are used for determining a distance.
It's just a little math. Read and record the value from the top cross hair, then read and record the value from the bottom cross hair.
Subtract the bottom from the top, multiply the result by 100, and that is your distance. However, if you use this method,
make sure you check the user's manual for your particular gun, because some optics have a different multiplier.
If you have a more complex survey that will require you to move the gun because you can't see all of the points you want to shoot in a single setup,
I recommend using a Total Station instead of the conventional level, or using a different indirect measurement technique altogether.
Remember that the slope-conveyance method often produces less accurate results and the computations do not provide any means for assessing
the results when compared to other techniques. If you're already out at the site with all of your surveying equipment, it may be better
just to add in another cross section or two and compute a slope-area measurement.
Slope-area measurements are discussed in detail in the slope-area videos.
For information on the theory behind a slope-conveyance measurement, high-water marks, or cross section selection,
surface-water specialist, or indirect measurement specialist.