Lubbock, TX

john.lipe@noaa.gov

806-745-3916

Glossary

Bench Mark (BM) - A permanent marking with an established elevation (msl). The best such mark is one established by a United States Geological Survey, USGS. It is usually a bronze tablet set in concrete with the elevation stamped into it. Bench Marks can also just be a "X" or "+" etched in concrete, rock, or some other permanent type structure. At many river locations, a Reference Mark is used in place of a bench mark.

Reference Mark - A permanent marking, typically an "X" or "+" sign etched in concrete, a bolt, or other "permanent" feature whose elevation is defined as the height above gage zero. The elevation of gage zero is often approximated from USGS section maps when no USGS bench mark is available. These markings are made by hydrologist so they can re-establish staff gages destroyed by flood or other reasons.

Temporary Bench Mark (TBM) - A marking, or location, with a known elevation. These markings are not usually permanent. They are used by a person doing leveling work as intermediary points when carrying a known elevation from one point to another. TBMs can as simple as the top of the ground surface where you drag your foot to mark the spot. They could also be the top of a rock or wooden stake, or even an "X" or "+" scratched into concrete or rock. It is desirable for TBMs to be something that can be found at a later time so that elevations can be rechecked.

Gage Datum or Gage Zero - This is the elevation above mean seal level (msl) of the established Zero gage height. Most often, this elevation was picked to represent the level of zero flow for a river or stream. However, because of shifting sand and other sediments in the river bottom, the point of zero flow can significantly deviate (by several feet) from the gage datum.

Bench Mark Above Datum (BM _{abv datum}) - This is the relative height of the Bench Mark or Reference Mark above gage zero (gage datum). When setting staff gages, it is desirable to deal in elevations above gage datum rather than true elevations.

Rod - A survey tool, typically a three section graduated wooden pole. When extended, they have a length of 12 feet. They are typically graduated in tenths and hundredths of a foot.

HI (Height of Instrument) - This is the elevation of the crosshairs, or viewing elevation of the level. Typically, this elevation is measured as a height above gage datum.

Backsight (BS) Reading - A reading off the Rod, generally to the nearest 1/100th foot, taken when looking through the instrument (level) back toward a point of known elevation. This point is typically a Bench Mark or Temporary Bench Mark. This reading when added to the elevation of the bench mark, yields the height of the instrument (level).

Foresight (FS) Reading - A reading off the Rod, generally to the nearest 1/100th foot, taken when looking through the instrument (level) forward toward a point of unknown elevation. This point is typically a temporary bench mark whose elevation you are establishing. When subtracted from the height of instrument (HI), it yields the elevation of the surface under the bottom of the rod.

Step-By-Step Instructions for Leveling a Staff Gage

Leveling a staff gage requires several steps.

1. Find a Bench Mark (or Reference mark) which will be your starting elevation.

2. Convert this elevation to elevation above Gage Datum if necessary.

3. Ready your equipment (level, tripod, and rod).

4. Pick an initial placement for the tripod and level.

5. Level the instrument on top of the tripod.

6. Look through the instrument and take Backsight reading.

7. Calculate the Height of Instrument (HI).

8. Have person with rod move to next location (Temporary Bench Mark) and take a Foresight reading.

9. Calculate the elevation of the ground at a second location known as a Temporary BenchMark (TBM₂).

10. Repeat steps 3 - 8 as necessary until you reach the desired location of the staff gage.

11. Set Staff Gage.

12. Recheck Elevations.

1. Find a Bench Mark, then convert the height to a height above gage datum.

By referring to the E-19 or other description of gaging station, locate a bench mark (BM) or reference mark (RM). The location of each BM or RM is explicitly defined. There are often several marks listed for each gaging location since any one might be destroyed in a flood.

If the site is a new gaging location, you will need to find an existing bench mark if available. If not available, or located some distance from the gaging site, you should establish a reference mark by etching a "x" into a concrete or rock surface using a hammer and chisel, or you could use some other readily available and easily defined point like the SE corner of a concrete porch of a nearby building. Reference marks are often defined as an elevation above gage zero, especially when no USGS bench mark is available. Even if defined simply as the height above gage datum (zero), the approximate elevation above mean sea level can be approximated from USGS section maps.

Bench Marks are usually located on top of some sort of semi-permanent concrete structure like a bridge abutment. The best type of bench mark, often referred to as "permanent bench mark" is the top of a USGS bronze tablet, set in concrete, which is stamped with the exact elevation above sea level.

2. Convert The Bench Mark Elevation to a Height Above Gage Datum - This is done by subtracting the gage datum elevation (msl) from the bench mark elevation.

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BM elevation( msl) - Gage Datum (msl) = BM _{above datum}

3. Set up your Tripod and Level

Once you have located the bench mark you will be using, the next step is to set your level.

This procedure assumes you will be using a typical surveying level (shown below) which mounts on the top of a tripod with extendable legs. You should also have an extendable 3 section "rod," shown to the right of the tripod. If you are not familiar with reading a "rod," study the detailed view to the right closely. Reading one looks simple and obvious, but looks are often deceiving, especially when viewing one through the telescope. The rod is graduated in feet and tenths of a foot, not feet and inches. The top and bottom edges of each black tick-mark designate the actual reading. For example, 4.00' is the top edge of the longer black mark running through the large 4 near the top of the section shown. The bottom edge of that same black tick-mark is 3.99 feet. The small "4" in the center of the section is 3.40 feet.

Attach the Instrument (Level) to the Tripod with the leveling knobs on the instrument placed directly over the tripod legs. (Make sure Level is attached securely with the large screw which screws into the base of the level.

4. Pick a Location for the Tripod and Level, so that when looking through the level back toward your Bench Mark (BM), you would intersect the rod near it's base. (This assumes you will be working downhill as shown in the diagram below. If not working downhill, this is not as important.)

5. Leveling the Instrument (Level). - This can be difficult to the newcomer, but if you know and use the proper procedure, it becomes fast and easy.

5a. Set the tripod on the ground and extend the legs so as to raise the instrument to a comfortable viewing height and also level the instrument as much as possible.

5b. Step on the end of each leg to set them firmly into the ground.

Remember, at this point the height of the instrument should be a short distance above your bench mark, or temporary bench mark. This is done so you don't have to move the instrument as often.

Steps 5c through 5h can be a little confusing, so I suggest you read through the instructions once and then return to 5c and start the process.

5c. Rotate the top of the Level so as to place the round bubble level (see picture step 2) halfway between two of the leveling screws on the base of the instrument. This should also be directly between two of the legs of your tripod if you attached the instrument on top of the tripod as suggested in section 2 , step a.

5d. Place your thumbs and forefingers on two of the leveling screws either side of the round bubbling level. When adjusting the screws, always turn both at the same time and in opposite directions. The trick to leveling the instrument quickly is always move your thumbs toward or away from each other in equal amounts. This technique is the ***KEY*** to completing this process easily.

5e. Turn the leveling screws so as to move the air bubble to a position centered between the two leveling screws. Most likely this will not be in the center, but along either the back edge (12 o'clock position) or along the front edge (6 o'clock position) of the level after the first adjustment. If any of the adjustment screws run out of adjustment range, you will need to move it back toward the center of it's adjustment range, adjust the legs of the tripod, and start over.

5f. Reach to the third leveling screw on the opposite side of the Level. Adust it up or down until the bubble comes close to the center.

5g. Readjust the first two leveling screws to bring the bubble into the center of the circle.

5h. Grab the top of the Level and turn the instrument 120 degrees so that the round bubble level is directly between the next two leveling screws. This should also be directly between the next set of legs of the tripod. If the bubble moved out of the circle, make necessary adjustments.

5i. Once centered, the bubble should remain centered as you rotate the Level to any direction. However, some fluctuation is common on these instruments because they need calibration. Unless the error is large, it typically will not have a significant negative impact.

Download a blank table and example table to use while surveying or reading instructions 6 through the end.

6. Take a Backsight Measurement. A backsight measurement is taken by looking through the instrument back toward a bench mark or temporary bench mark. A foresight reading is taken when looking in another direction, typically toward the direction of the new elevation that you are trying to establish.

Have one person hold the rod on top of the bench mark (or temporary bench mark with a known elevation). Look through the instrument, focus and take a "backsight" reading. There are two focus knobs on the instrument. The first is the black ring around the eye piece. It focuses the cross-hairs and should only have to be focused once. The other knob on the side of the instrument helps you focus on the target. Once focused, take your backsight reading and record it in a table like the one shown in the following diagram. Read the rod to the nearest hundredth of a foot. Be careful to not lean on the tripod while taking measurements. Also, before you start, review how to read the rod. Readings like 3.01' are often confused with 3.10 feet.

Tip 1. When possible, ask the person holding the rod to slide their finger tip or a pencil up and down the edge of the rod until you see it next to the cross-hair in the telescope of the Level. This works as a good double check scheme so that a reading like 3.05 doesn't get mistaken for 3.50.

Tip 2. When working downhill, your backsight reading will normally be a low value because it's from the bottom of the rod. Assuming so, keeping the rod vertical is not nearly as critical of an issue as when you are reading numbers toward the top of the rod. There are two ways typically used to get accurate readings when the rod is extended and your are getting a value from the top. One is by holding a carpenter's level against the rod to keep it vertical. The second is for the person holding the rod to rock it toward and away from the instrument. Use the vertical cross-hair to tell them if they have it leaning to one side of the other. With it rocking, the lowest value you observe on the rod should be the proper reading.

7. Calculate the Height of the Instrument (HI) and record. Simply add the backsight reading to the elevation of the bench mark to get the height of the instrument above gage datum. Record this value in your table. HI = BM elev + BS

8. Move Rod to TBM₁ (Temporary bench mark) and take a Foresight reading. Have the person with the rod move to a new location (down the hill in the example shown in step 3) and set it on an object which will be called TBM₂. TBM₂ can be just the surface of the ground, but if possible, it should be the top of something you can find again to check your measurements. Try to make your TBM the top of a rock, a wooden stake, or some other object you can go back to so you can double check measurements later. Make sure the rod is vertical to get the best possible reading (See Tip 2 of step 5 above.).

9. Calculate the Elevation of TBM₂ , and Record in Table. Subtract the Foresight reading from the height of instrument (HI) to get the elevation of your temporary bench mark (TBM).

TBM elev = HI - FS

10. Move the Tripod and Level (down the hill if following example) and repeat steps 3 - 8.

Have the person holding the rod, remain at the temporary bench mark while you move the tripod and Level to a new position. Repeat steps 3-8 as many times as needed to reach the desired location of the staff gage to be installed.

11. Set the Staff Gage - Once you have the Level down to an elevation matching that of the staff gage, calculate the Height of the Instrument (HI) above gage datum. Adjust the staff gage up or down as needed so the height of the instrument matches the same elevation on the staff gage when looking through the telescope.

12. Recheck Elevations - Once the staff gage is set, it is a good idea to work back up the hill to the bench mark as a double check on your observations and calculations.

Practice in shop exercise:

Try an exercise in your office before going out in the field if you are unfamiliar with leveling. It is a lot easier to learn something new while in a comfortable environment. Here is an example of something you might do.

Assume the top of a short bookcase or table in your office is your Bench Mark

Top of bookcase elevation = 1080.00 ft.

Assume a gage datum 20 feet lower at 1060.00 ft.

Objective: Calculate the height of two different objects (temporary bench marks) within your office above this arbitrary gage datum. One might be the floor. You pick the other.

Step 1 Set up and level the Level.

Step 2 Fill out the example table through the first line. Your BM abv datum

should be BM = 20.00' abv datum

Step 3 Place the rod on top of bookcase.

Step 4 Take a backsight reading off the top of your TBM1 (top of bookcase). Fill in table.

Step 5 Calculate the height of your instrument (HI) abv datum. Record in table.

Step 6 Move rod to the floor. Extend if necessary and take a foresight reading. Record data.

Step 7 Calculate the elevation of the floor (TBM1) abv datum.

Step 8 Move the instrument to a new position (preferably lower). You should still be able to see TBM1. If you can't find a lower position for the Level, you may just need to lower the legs. Re-level the instrument.

Step 9 With the rod at TBM1, take a backsight reading and calculate the new height of instrument (HI). Record in book.

Step 10 Move the rod to a new object and calculate it's elevation. The new object could be another table, a book placed on top of the floor, etc. Record all data.

Step 11 Reset the instrument if necessary to see the original bench mark. Calculate the height of instrument, record, place the rod on top of the original bench mark and recalculate it's elevation. It should read 20.00 feet.

Alternate Leveling Technique Using Handheld Level

This technique is not recommend for reestablishing missing staff gages as errors are sure to occur. However, it is an excellent procedure for determining the elevation of objects and terrain within the flood plain.

This alternate technique assumes you are familiar with the primary technique of using the Level and tripod. In short, you replace the tripod with your body and replace the instrument with a handheld level. A typical handheld level is shown below beside it's case and a ball point pin. This type of level is easy and quick to use. Simply orient the small rectangular light hole to the top and look through the eye piece. The instrument has a small bubble that centers vertically in the view when held level.

Step 1. Take a blank worktable. Calculate the height of your bench mark above gage datum or use a reference mark. Fill out the worktable.

Step 2. Have an assistant take the rod and place it on top of the bench mark. Find a view point location slightly down the hill where your eye is on a level slightly above the elevation of the bench mark. Hold the level to your eye, level it while viewing the rod (your backsight reading). Without moving, record the reading to the nearest 1/10th foot in the log. Then calculate the Height of your eye (abv datum) and place that calculation in the box for HI. Again, you should not move since the elevation of your eye is replacing the height of the instrument.

Step 3. Have your assistant move to a new position down the hill (picked to be TBM2), extend the rod, and use a carpenter's level to hold it vertical. Without moving from your original position, turn to face the rod, hold the handheld level up, take a foresight reading, and record it in the table. Now you can move!....But tell the assistant with the rod to stay in place.

Step 4. Calculate the elevation of the ground under the rod (TBM1) and record in the table.

Step 5. Move to a new position down the hill as needed, find another comfortable viewing spot, hold up the level, take a new backsight reading, and record in the table. Don't move!

Step 6. Calculate the new height of your eye (above gage datum).

Step 7. Have the person with the rod move to a new TBM. Repeat the process until your eye is at the desired elevation of the object (gage, terrain, etc). Once your eye is at the desired elevation above gage datum, you simply set a staff gage so that it's height is the same as your eye. Ex. If your eye is at 6 feet, then the 6 foot mark on a staff gage should be at the same level as your eye.

Remember: This procedure is not recommended for reestablishment of missing staff gages as errors are sure to occur. However, it is an excellent procedure for deriving the elevation of terrain features which might flood so you can add detail to an E-19 of what will flood at various stages.

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