The most accurate traditional method of measuring distances uses a steel tape. When proper procedures are followed the error will be less than 1.0 ft in 3,000 ft. The standard equipment for a taping party, usually at least three people, consists of a tape, two range poles, a set of 11 chaining pins, two plumb bobs, a hand level, and a field notebook. Individual items of taping equipment are described in the following paragraphs.
12.3.3.1. Surveyor's Chain
The traditional device for measuring distance accurately is the surveyor's steel chain. Today accurate surveyor's chains constructed of non metalic materials are available. The modern standard steel chain is 100 ft long and 3/8 inch wide, and weighs 2 to 3 lb per 100 ft. The older tapes are graduated with marks set in
Babbitt
Zero point 1 foot mark 2 foot mark
Babbitt-metal bosses with each foot marked, from 0 to 100 ft. Notice that the tape between the first and second foot mark does not have any graduations. This is the common design of surveyor's chains.
Newer surveyor's chains have the marks etched on the face of the tape. Highway chains are fully graduated, but the traditional surveyor's chain only subdivides one foot of the tape. The subdivisions are usually in tenths or hundredths of a foot. The graduated foot may be the first foot, as in Figure 12.3, or in an extended foot, Figure 12.4. A chain with the first foot graduated is called a cut chain and a chain with the extend foot is called an add chain.
Determining the style of chain that is being used is important otherwise it is easy to have an error of one foot when reading an add or cut tape incorrectly, Figure 12.5.
Metal surveyor's chains are manufactured from an alloy of steel. It is very important to wipe them down with a clean rag after each use and lightly oil them periodically to prevent them from rusting. They are also very easy to break by bending them sharply or pulling a loop tight.
FIGURE 12.3. Segment of Babbitt style tape.
Tapes are popular for making rough measurements or when the accuracy of a chain is not needed. Tapes are constructed of metal and different nonmetallic materials. Tapes are usually fully graduated and range in length from 25 to 200 ft.
FIGURE 12.4. Typical add tape.
FIGURE 12.4. Typical add tape.
FIGURE 12.5. Segment of cut tape.
FIGURE 12.5. Segment of cut tape.
Nonmetallic tapes are usually lighter and easier to use. They require less maintenance and are not as easily damaged by moisture. Inexpensive nonmetallic tapes will stretch under tension and therefore are not as accurate as steel chains or tapes.
Chaining pins are made of heavy gauge wire and are 12 to 15 inches long, are painted red and white, and sometimes have a bright cloth attached to help locate them in tall grass. They are used to mark the end of each tape length and come with 11 pins in a set; One pin is used to mark the start of the chaining (taping), and the remaining 10 pins are used to mark lengths of tape. When all 11 pins have been used, assuming a 100 ft tape, 1,000 ft has been measured.
Range poles are 1-inch-diameter tubular steel or nonmetallic shafts 6 to 10 ft long with one pointed end. They are alternately painted red and white and are used for "lining in" when one is taping or measuring angles, Figure 12.6.
Plumb bobs with 6 to 10 ft of cord attached are used when measuring horizontal distances on sloping or irregular ground to transfer the distance from the horizontally held tape to the point on the ground. They also are attached to a surveyor's level, to locate the level over a stake, when one is measuring distances by stadia or EDM, or laying out angles, Figure 12.7.
A hand level consists of a small sighting tube 5 to 6 inches long equipped with a spirit level, a glass tube filled with a liquid and a bubble. The image of the bubble
/f is reflected by a prism and can be observed by looking through the tube. The instrument is held to the operator's eye and is leveled by raising or lowering the end until the cross-hair intersects the image of the spirit-level bubble. This is a low-precision instrument used to make rough measurements of the slope and as an aid in keeping the surveyor's tape level. Newer models may also include stadia cross-hairs and direct-reading angle scales, Figure 12.8.
There are six basic steps involved in taping: (1) lining in, (2) applying tension, (3) plumbing, (4) marking tape lengths, (5) reading the tape, and (6) recording the distance.
Pre computer and pre calculator taping methods were measured by stations. A distance of 100 ft is called a full station and is written as 1 + 00 ft. A distance of 123 ft is written as 1 + 23 ft. The modern practice is to record all distances in decimal feet.
For many surveys the intent is to discover the true horizontal distance between two points. Three methods are commonly used: (1) tape and plumb bob, (2) tape and calculation, or (3) Electronic distance measuring (EDM). When method two is used, either the percent slope, change in elevation or vertical angle must be measured so that the horizontal distance can be calculated. When an EDM is used
the distance is horizontal if the instrument is horizontal at the time of measurement.
The need for horizontal measurements is illustrated in Figure 12.9. When measured along the surface of the ground, the distance would be 26.1 ft, whereas the true horizontal distance is 25.00 ft.
When slope distance is measured and the desired result is horizontal distance, either the percent slope, difference in elevation or the vertical angle must be measured and the true horizontal distance calculated or determined from tables. The percent slope can be measured by a hand level or a surveying level. When the percent slope is known the horizontal distance can be determined through the use of the slope equation and the Pythagorean theorem.
Problem: Determine the horizontal distance when the slope distance is 234.5 ft and the percent slope is 3.4%.
Solution: The first step is to determine the amount of change in elevation that occurs. This can be done using the slope equation:
rise
%Slope x run 3.4 x 234.5 ft rise =--=-—-= 7.973 ft
Where the rise is the change in elevation and the run is the horizontal distance, the horizontal distance can be determine by rearranging Pythagorean's theorem:
b = a — c b = Va2 - c2=V234.52 - 7.9732 = 234.36... or 234.4 ft
Table 12.1 can also be used to determine the horizontal distance for various slopes up to 30%. Note that for slopes up to 5% the correction factor is less than 0.125
True horizontal | ||
Correction factor |
distance | |
Slope |
(ft/100 ft) |
(100 ft) |
1 |
0.005 |
99.995 |
2 |
0.020 |
99.980 |
3 |
0.045 |
99.955 |
4 |
0.080 |
99.920 |
5 |
0.125 |
99.875 |
6 |
0.180 |
99.820 |
7 |
0.245 |
99.755 |
8 |
0.321 |
99.679 |
9 |
0.406 |
99.594 |
10 |
0.501 |
99.499 |
11 |
0.607 |
99.393 |
12 |
0.723 |
99.277 |
15 |
1.131 |
98.869 |
18 |
1.633 |
98.367 |
20 |
2.020 |
97.980 |
25 |
3.175 |
96.825 |
30 |
4.606 |
95.394 |
ft/100 ft (1.5 in/100 ft), but the correction factor increases more dramatically after 5%.
When Table 12.1 is used, the correction factor per 100 ft is multiplied by the slope distance and divided by 100, and the product is subtracted from the slope distance. It is important to remember that the correction factors are for 100 ft of distance. You must divide the measured slope distance by 100 first:
where HD = Horizontal distance; SD = Slope distance; CF = Correction factor.
Problem: Use Table 12.1 to determine the horizontal distance (HD) when the slope distance (SD) is 623.82 ft and the slope is 12.0%.
Solution:
£08.7 ft measured along the ground
200 ft measured by breaking chain FIGURE 12.10. Measuring sloping ground by breaking chain.
£08.7 ft measured along the ground
200 ft measured by breaking chain FIGURE 12.10. Measuring sloping ground by breaking chain.
When the ground slope is not uniform, the slope and the slope distance for each segment of the line must be determined and a separate correction applied to each segment.
When the horizontal tape method is used, one end of the tape is held on the ground, while the other end is raised until the tape is horizontal. The true distance is transferred to the ground from the elevated end of the tape by a plumb bob. When the slope is more that 5%, it is necessary to use aprocess known as "breaking chain." In this method the head chainman lays out the full length of the tape. The 100-ft length then is divided into convenient increments, usually 25 or 50 ft, with the chainman holding the tape horizontal and plumbing down to the ground at each increment. This process is illustrated in Figure 12.10. In this example, the chain was "broken" into 25-ft sections. Every 25 ft a plumb bob and line was used to set a pin.
For accurate results, a taping activity must be very carefully thought out and well organized. The following procedure for taping is recommended to ensure accurate results. A taping party consists of at least three people: the head chainman, the rear chainman, and a note keeper. An axe man also may be necessary in brushy areas. The 11 chaining pins serve as temporary markers for each station, and also help to count the number of full stations measured.
The head chainman begins by setting a pin for the starting point and then leads off with the 100 foot end of the tape. After one full station has been measured, the head chainman will have placed one pin in the ground at the beginning and one to mark the end of the first station. At this point he or she will have 9 pins left on the ring. As the chain is moved to the next station, the rear chainman does not pull the pin used to start the chaining. He only pulls the pins used at the 100 ft marks. After two full stations, the head chainman will have placed three pins in the ground and have 8 pins on the ring, and the rear chainman will have one in hand and one in the ground. If this system is carefully followed, the number of full stations measured will always be the same as the number of pins held by the rear chainman.
As described above, most surveyor's tapes are graduated in feet throughout their full length, with the first and/or last foot of the tape graduated in tenths or tenths and hundredths of a foot. A different procedure is required for measuring a distance shorter than a full tape length. When the party nears the end of the line and the remaining distance is less than 100 ft, the rear chainman holds the zero mark on the last pin and the head chainman pulls the chain taunt. The head chainman then moves the chain forward to the last foot mark if a add chain is used or backward to the next foot mark if a cut chain is being used. They both read the tape and the rear chainman's reading is subtracted from the head chainman's reading when a cut tape is used. The rear chainman's reading is added to the head chainman's reading when an add tape is used. For example, suppose that an add chain is being used and the head chainman reads 53 ft, and the rear chainman reads 0.21 ft. If the rear chainman has 6 pins, the total length of the line is 653.21 ft [(6 x 100) + (53 ft + 0.21 ft)]. If a cut chain was being used the distance would be 652.79 ft [(6 x 100 ft) + (53 ft - 0.21 ft)]. The following rules, if carefully followed, will help guarantee accuracy in taping.
1. Align the tape carefully, and keep the tape on the line being measured.
2. Keep a uniform tension of 15 lb of pull on the tape for each measurement.
3. Keep in mind the style of tape being used to avoid an error of 1 or 2 ft at each end of the tape.
4. "Break chain" on slopes as necessary to keep the tape level, or calculate the percent slope if measuring with the tape on the ground.
5. Carefully mark each station and keep an accurate count of the stations.
Measuring distance by stadia relies upon a fixed angle being designed into the instrument. "Stadia" comes from an early Greek word for a unit of length. Surveying instruments equipped for stadia measurement have two additional horizontal cross-hairs, called stadia hairs. They are placed equidistant above and below the horizontal leveling cross-hair, Figure 12.11.
The distance between the stadia hairs and the horizontal cross-hair is fixed by the manufacturer to provide a constant stadia interval factor (SIF) for the instrument. The most common stadia interval factor is 100. Instruments that have a SIF of 100 will have a one foot stadia interval, that is the difference between the top stadia reading (TSR) and the bottom stadia reading (BSR), when the rod is held 100 ft away from the instrument, Figure 12.12.
Surveying rod
Top stadia reading (TSR) Elevation
Bottom stadia reading {BSR}
/77777777T7777777777 -►
FIGURE 12.12. Measuring distance by stadia.
The angle formed by reading the top and bottom cross-hairs is constant, therefore the unit of the measure is determined by the unit of measure of the rod. When a Philadelphia style rod is used without the target the distance will be in units of feet. When a metric rod is used the distance will be in units of meters. When the stadia method is used to measure a distance, the instrument person reads the TSR and the BSR, and then multiplies the difference by the stadia interval factor. In the form of an equation:
Refer to Figure 12.13. The top stadia hair reading (TSR) is 6.29, and the bottom stadia hair reading (BSR) is 3.71. Thus the stadia interval is equal to 6.29 minus 3.71, or 2.58 ft. The distance from the instrument to the rod is equal to 2.58 multiplied by 100, or 258 ft.
Problem: What is the distance to the rod if the stadia readings are TSR = 6.07 and the BSR = 3.02?
D (ft) = (TSR - BSR) x 100 = (6.07 ft - 3.02 ft) x 100 = 305 ft
Solution:
TSR 6.29 ft
BSR 3.71 ft
/77777777777777777T7
FIGURE 12.13. Example using stadia.
Was this article helpful?