- - AGRICULTURAL CORE CURRICULUM - - (CLF2000) Advanced Core Cluster: AGRICULTURAL MECHANICS (CLF2700) Unit Title: SURVEYING _____________________________________________________________________________ (CLF2704) Topic: Differential Leveling Time Year(s) 4 Hours 1 / 2 / 3 / 4 _____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome # (O-4) - Determine the difference in elevation between two or more points. Special Materials and Equipment: Tripod level, level rod, field notebook. References: Bowers, W., Jones, B. A., Jr., & Olver, E. F. (1973). Engineering Applications in Agriculture.Champaign, IL: Stipes Publishing Co. Kissam, Phillip. (1978). Surveying Practice (3rd ed.). New York: McGraw-Hill. Evaluation: Test/quiz by instructor. Map the topography of a field, produce a base map, and identify the high and low spots. Designate several fixed spots around the schoolyard as benchmarks and perform a differential leveling exercise to determine the relative difference in elevation between the spots. TOPIC PRESENTATION: Differential Leveling A. Mapping the Topography of a Field 1. Purpose a. To determine the relative ground surface elevations of a tract of land and to plot those elevations on a base map. b. Completion of the above enables determination of cuts and fills to grade the land for an exact slope for surface irrigation. 2. Procedure a. Staking the Field 1) Stakes should be set at equal distances across the field to be surveyed. 2) 3/8" x 1-3/4" x 4' lathes pointed at one end are commonly used. 3) A survey reading will be taken at each stake location in order to enable calculation of relative topography of the field. 4) A field set up with stakes might look like: x-----------------------------------------------x | 0 0 0 0 0 0 0 | | | | 0 0 0 0 0 0 0 | | | | 0 0 0 x 0 0 0 | | | | 0 0 0 0 0 0 0 | | | | 0 0 0 0 0 0 0 | x-----------------------------------------------x 0 Stake location x Level location b. Setting and Reading the Level 1) The level should be set at the center of the field as indicated above. 2) The level should be set up and levelled as described previously or as described in the operator's manual for the level. 3) A level rod reading is taken at each stake location. a) If the ground is soft, a tractor can be driven along the line of stakes and rod readings can be taken in the wheel tracks. 4) Differences in rod readings for each stake represent differences in elevation at each stake. a) The rod reading itself represents the vertical distance from the ground at each stake to the level of the cross hair in the level instrument. b) The relative elevation at a particular stake location is higher when the rod reading is small. c) The relative elevation at a particular stake location is lower when the rod reading is large. c. Recording Information 1) A rod reading is recorded for each stake in the field. 2) Rod readings should be recorded for a permanent reference point location (called a bench mark or BM) as well as at each corner of the field. a) A benchmark is a marked point of known elevation. They should be marks that are easily recognized, easily found, and not likely to be moved. They can be cross marks or bronze tablets set in masonry in a good foundation or they can be a certain part of a fire hydrant, pump base, or nail in a tree. d. Producing a Base Map 1) If the exact elevation of the reference BM is not known, it is common to assume it is 100 feet in elevation. 2) Add the rod reading at the benchmark to the elevation of the BM. a) The resulting number is the elevation of the cross hair in the instrument. 3) At subsequent stakes, subtract the stake rod level reading from the number computed in paragraph 2). above. 4) The following map illustrates recording of the pertinent rod reading information: x-----------------------------------------------x | 0 0 0 0 | | 3.20 3.42 3.46 3.22 | | 0 0 0 0 | | 3.21 3.44 3.46 3.21 | | 0 0 x 0 0 | | 3.21 3.44 3.46 3.22 | | 0 0 0 0 | | 3.21 3.44 3.48 3.21 | | 0 0 0 0 | | 3.20 3.45 3.47 3.22 | BM-----------------------------------------------x 3.00 0 Stake location 3.22 Rod reading at each stake in feet x Level location BM Bench mark location = 100 feet (assumed) 5) The following base map represents the surveyed field with the elevations relative to the 100 foot BM. Elevation of the cross hair = 100.00' + 3.00' = 103.00' x----------------------------------------------------x | 0 0 0 0 | | 99.80' 99.58' 99.54' 99.78' | | 0 0 0 0 | | 99.79' 99.56' 99.54' 99.79' | | 0 0 x 0 0 | | 99.79' 99.56' 99.54' 99.78' | | 0 0 0 0 | | 99.79' 99.56' 99.52' 99.79' | | 0 0 0 0 | | 99.80' 99.55' 99.53' 99.78' | BM------------------------------------------------------x 100.00' 6) Interpretations from the mapping: a) All stakes within the field are lower than the BM location. b) The field is low through the middle and high at the left and right sides. B. Differential Leveling 1. Purpose a. Differential leveling is used to determine the difference in relative elevation of several points that are varying distances apart. b. It is commonly used to establish relative elevations of permanent points (called benchmarks) around the farm which are useful for making drainage and soil conservation surveys. c. Differential leveling is similar in form to mapping the topography of a field as described above. Typically involves resetting the level (i.e., moving it from location to location) and a more involved set of record keeping and calculations. 2. Procedure a. The procedures for differential leveling will be detailed with the aid of the example below. The surveying problem is to determine the elevation of points TP-1, TP-2, and TP-3 as diagrammed below. The BM elevation is known and is 100.00 feet. Because of obstructions and distances, it will be necssary to reset the level at four locations (designated by X's) below. 0 TP-1 X#2 0 TP-2 X#3 X#1 0 TP-3 0 BM X#4 X#1 FIRST LEVEL SET-UP LOCATION b. Set up and level the instrument at the approximately halfway between the BM and TP-1. 1) It is generally recommended that the distance from the instrument to the benchmark not exceed 400 feet; approximately 200 feet is the usually recommended distance. 2) It is also recommended that the instrument be set up close to halfway between two benchmarks in order to cancel out leveling errors. c. Send the rodman to the BM and record the rod reading (3.02') at this point. This is what is known as a backsight (BS) reading and is recorded under BS in the field notebook. 1) A backsight (BS) reading is sometimes designated with a + sign and always results from a reading at a point of known or calculated elevation. 2) Add the BS reading to the elevation of the BM to give the height of the instrument (HI), i.e., HI = 3.02' + 100.00' = 103.02'. d. Move the rodman to station TP-1, take a rod reading (3.68') and record this as a foresight (FS) reading in your field notebook. 1) Subtract this FS reading from the HI reading to obtain the elevation at point TP-1, i.e., 103.02' - 3.68' = 99.34'. Note that the process is repeated in steps e through g below e. Move the level instrument to the second level set-up location (X#2) and set up and level the instrument. f. Take another rod reading from X#2 back to TP-1. Record this backsight reading (3.34') in the field notebook. 1) Add the BS reading to the elevation of TP-1 to give the height of the instrument (HI), i.e., HI = 3.34' + 99.34' = 102.68'. g. Move the rodman to station TP-2, take a rod reading (4.17'), and record this as a foresight (FS) reading in the field notebook. 1) Subtract this FS reading from the HI reading to obtain the elevation at point TP-2, i.e., 102.68' - 4.17' = 98.51'. Note that the process is again repeated in steps h through j below. h. Move the level instrument to the third level set-up location (X#3) and set up and level the instrument. i. Take another rod reading from X#3 back to TP-2. Record this backsight reading (4.00') in the field notebook. 1) Add the BS reading to the elevation of TP-2 to give the height of the instrument (HI), i.e., HI = 4.00' + 98.51' = 102.51'. j. Move the rodman to station TP-3, take a rod reading (3.95'), and record this as a foresight (FS) reading in the field notebook. 1) Subtract this FS reading from the HI reading to obtain the elevation at point TP-3, i.e., 102.51' - 3.95' = 98.56'. Note that the process is again repeated in steps k through m below. k. Move the level instrument to the fourth level set-up location (X#4) and set up and level the instrument. l. Take another rod reading from X#4 back to TP-3. Record this backsight reading (3.56') in the field notebook. 1) Add the BS reading to the elevation of TP-3 to give the height of the instrument (HI), i.e., HI = 3.56' + 98.56' = 102.12'. m. Move the rodman to the original BM, take a rod reading (2.11'), and record this as a foresight (FS) reading in the field notebook. 1) Subtract this FS reading from the HI reading to obtain the elevation at point BM, i.e., 102.12' - 2.11' = 100.01'. FIELD NOTEBOOK FOR SURVEY STATION BS HI FS ELEVATION BM 3.02' 103.02' 100.00' TP-1 3.34' 102.68' 3.68' 99.34' TP-2 4.00' 102.51' 4.17' 98.51' TP-3 3.56' 102.12' 3.95' 98.56' BM 2.11' 100.01' SUMS +13.92' -13.91' BS - FS CHECK = +13.92' - 13.91' = +0.01' BM CHECK = 100.01' - 100.00' = +0.01' 3. Checking for Survey Arithmetic Errors a. The backsight and foresight columns should be added separately and compared with the difference between the original and final benchmark elevations. The differences should be the same in numerical value and algebraic sign. If either error is present, there is a mistake in the addition or subtraction in the field notebook and the calculations should be refigured. 4. Rules of Differential Leveling a. All rod readings are recorded by both surveyors (rodman and level person) as soon as they are checked. All rod readings are recorded in the line that refers to the point sighted. b. All arithmetic is carried out as far as is possible with the readings available. The computations are made by each person while the other is moving. c. All work must stop until each person agrees on the elevation of each TP. d. The backsight and foresight columns should be added separately and compared with the difference between the original and final benchmark elevations. The differences should be the same in numerical value and algebraic sign. e. Every benchmark must be used as a turning point. 7/12/91 JR/tf #%&C