GEOG 2043
GEOG 2043BACKGROUND: Differential leveling is the process of determining relative elevations at various points of interest. The elevations can be tied into a global control system (meters above mean sea level), or referenced to a local benchmark with an assumed elevation (i.e. Elev.=100.0 m). In most cases, since the difference in elevation between two points is what we are interested in, the absolute elevations usually have little bearing on local decision-making.
What does a level look like?
Differential leveling is the process by which differences between relative elevations (vertical differences) are measured. All measurement differences are made independent of horizontal distance. Since this method of measuring elevation differences is based on relative differences from one elevation, it is imparative to get the instrument level before starting. If the instrument comes out of level while you are working, it may be necessary to return to a point of known elevation before proceeding. If the instrument is not level, you will not get accurate values from your work. Differential leveling may require changing locations (set-ups), depending on two things:
1. The vertical distance between two points. The maximum height change that can be measured is dependant on the length of the rod used and the height of the instrument (HI). If the HI is 1.8 meters and the next point you want to record is 2.00 meters above your elevation, you will not be able to see it because you will be looking .2 meters below the bottom of the rod.
2. The distance between sights should not exceed 70 meters to maintain reading accuracy.
There are three basic terms used in differential leveling that help
keep things straight when recording notes in your field book. The terms
of importance are backsight, foresight, and turning point.
A backsight is a point with a known elevation, e.g. a benchmark. A backsight
may also be a point in which an elevation is assumed so that relative vertical
differences can be established. For example, a surveyor can assume an elevation
on a control point in the absence of a known elevation. A foresight is
a point with no known coordinates. This means that the foresight elevation
can be established only after a vertical difference is calculated between
the backsight (known elevation) and the foresight (unknown elevation).
A turning point is a point that initially begins as a foresight with an
unknown elevation. After an elevation has been established by determining
the vertical difference, the foresight has a known elevation and can therefore
be used as a backsight as you move along a transect. In review, after the
foresight has an established elevation, it can then be used as a backsight
point whereby a known elevation can be transferred to other foresight points
with unknown elevations.
METHODS: The level instrument is set up on a tripod. A screw on the tripod inserts into the bottom of the level so that it can be tightly fixed to the tripod. Next, extend the legs of the tripod so that the instrument is just above eye level. Find a point where the line of sight will allow to see your backsight, as well as your foresights and a potential turning point. The tripod should be set up with the legs firmly set in the ground.
Figure 1 Differential leveling.
It is that simple. In the event that you must traverse long distances,
turning points must be employed. Turning points are arbitrary points on
the ground that are not within the scope of your survey. You need to calculate
the elevation accurately through the point but will not use this point
in your analysis. Turning points are used to link elevations for points
that are not within the line of sight. The maximum distance you should
sight is about 70 meters and the legs of your traverse should be approximately
the same. You can verify the traverse distances by pacing them off.
NOTES AND CALCULATIONS: The note taking process is very important for field measurements in any discipline. The notes that you take must allow you to determine, after the fact, exactly what steps were taken. This is especially for calculations of spot elevations where confusion may be possible. Set up your field book with five category headings for the columns at the top of the page.
This should include (in order from left to right) the following (Figure 2):
Date: 2-11- 01
Party: JP and DM
Weather: 6 degrees C and cloudy
Location: Boulder Creek junction with Foothills HWY
| STA | BS | HI | FS | ELEV | REMARKS |
| A | 2.45 | 100.00 | USGS BM | ||
| 102.45 | |||||
| TP1 | 0.60 | 101.85 | moved inst. | ||
| 1.70 | |||||
| B | 103.55 | top of hill |
The field notes should include pertinent information such as date, persons working, site name and description, and weather conditions. This information will allow you to better keep track of the work that has been done at your site. It is important to set up the field book in the same order each time to reduce human error.
CHECKING YOUR CALCULATIONS: You can quickly check your calculations by summing the entries in the BS and FS columns. The difference between these two values should be equal to the calculated difference in elevation between the starting point and endpoint of the survey. Therefore,
Sum (BS) + sum (FS) = Final elevation - Initial elevationCOMPLETING THE SURVEY: Whenever possible the survey should loop back to the start point or "tie in" to another point of known elevation. This will ensure the accuracy of your measurements. If the traverse loop is complete, then your calculations should show that the point used to begin and end the survey has the same elevation (i.e.- a benchmark with an elevation of 100.00). In this case
Sum(BS) = Sum(FS)Under ideal conditions, the values calculated for the same point should have identical elevations. For example, if you run a leveling traverse with three turning points, and you return to the original benchmark (backsight), you should calculate the same elevation when you return to the point. In practice, this is rarely the case. The flexibility to your accuracy is dependant on the reason for your data collection. For example, land surveyors will not accept error greater than .03 hundredths per 100 meters. In contrast, field workers studying the geomorphology of a steep alpine stream may be happy with measurements accurate to a few centimeters. Once you have set an error allowance for your data collection, you can calculate your readings to see if your level loop "worked". If not, you can always redo it to ensure accurate transfer of elevation from your benchmark to your foresighted point.
Objectives:
Assuming you have the data entered in a spreadsheet, and you have calculated the elevation of each point,
1. Plot the profile of the streambed (distance vs. elevation)
2. Fit a trendline through the data. The command for trendline can be found under the "Chart" menu (if you don't see this option, it's probably because you don't have the chart selected). Select the option for a linear trendline, and then select the tab labeled "options"; check the box that says "show equation on chart". Print the profile and attach it to your write-up.
3. On a separate sheet, calculate the discharge required to flood the higher terrace:
a) The formula for discharge is
Q = A * Vwhere A is the area in sq. meters and V is the velocity in meters/sec. On the basis of the cross section survey, we determined that A = 23.5 m^2. To determine the velocity we use the Manning equation:
V = (H^2/3 S^1/2)/nwhere H is the average depth, S is the average slope and n is a roughness coefficient.
b) Calculate V using H = 1.49 m (from the surveyed cross section), S = the value determined from the plot above, and n chosen from information given at the following USGS web site:
http://wwwrcamnl.wr.usgs.gov/sws/fieldmethods/Indirects/nvalues/This web page provides photos and brief descriptions of locations where n values have been verified. Select an n value from the possible choices; this is clearly a subjective process, but there appear to be several sites that resemble ours. If your calculated velocity exceeds 4 m/s, then your n value is probably too low; if your calculated V < 2 m/s, your n is probably too high.
4. The flood that occurred on May 4-8, 1969 was estimated to have a discharge of 2,500 cfs near Broadway. Convert that value to cubic meters per second (Q in cfs x 0.02832 = Q in cms), and compare that result with your calculated value. Show your work neatly.
5. Provide a few sentences commenting on the results.