Measurement of Horizontal Distances
Pacing, taping, stadia, and electronic devices are used for measuring horizontal distances.
Pacing
Pacing may be used for approximate measurement when an error as large as 2 percent is permissible. Measurements by pacing generally are permissible for terrace and diversion layouts, preliminary profile work, and gridding for surface drainage surveys.
Measurement by pacing consists of counting the number of steps between two points and multiplying the number by a predetermined "pace factor," which is the average distance in meters or feet per step for each individual. It can be determined best if one walks, in a natural stride, a measured distance, usually 150 m (500 ft),several times. It should be paced enough times to make certain the number of paces for the distance does not vary more than two or three. The "pace factor" then would be the distance in meters or feet divided by the number of paces. The "pace factor" may vary with the roughness and slope of the ground. Adjustments should be made to take care of these variations.
Some people prefer to use a stride instead of a pace. It consists of two paces, so the "stride factor" would be two times the "pace factor."
Taping
Taping is the method of measuring horizontal distances with a tape. Survey distances are recorded by stations, which are usually 30 m (100 ft)apart.
The fractional part of a distance between full stations is called a plus station. Fractions of a meter (foot) are indicated by decimals, to the nearest 0.01 m (0.1 ft), depending upon the accuracy of the measurement required. For example, a point on a line 94.24 m (309.2 ft)beyond station 3+05 m (10+00 ft) is indicated as station 3+99.24 (13+09.2).
Stakes set along the line are marked with a waterproof lumber crayon, known asa "keel," or with an ink marker. Markings are placed on the face of stakes so that as a person walks along the line in the direction of progressive stationing, the station markings can be seen as each stake is approached.
Accurate taping with a steel tape or chain requires skill on the part of the surveyor in the useof plumb bobs, steel taping pins, range poles, hand levels, and tension indicator apparatus. This manual is written for execution of the less difficult conservation work. The following procedure should be observed:
1. Keep tape on line being measured.
2. Keep uniform tension on tape for each measurement.
3. "Break" chain on slopes as necessary to keep chain level (fig. 1-20).
4. Accurately mark each station.
5. Keep accurate count of the stations.
The following procedure is generally used for chaining a line:
1. If the line to be measured is a meandering line along a drainage ditch or gully channel, the measurements are taken parallel, or nearly so, to the meandering line. If the line is straight, a range pole is set ahead on the line asfar as can be seen, or the direction is marked by a tree, fence post, or other convenient point. This mark is used in sighting in a straight line from the point of beginning.
2. For purposes of this explanation, it will be assumed a straight line is to be measured, and a stake has been set at the point of beginning marked 0+00. (Seesection on "Profiles and CrossSections" for other methods of stationing the beginning station.)
3. The lead surveyor takes thezero end of the tapeand advances in the general direction of the line to be measured. When the end of the tape is near, the rearsurveyor calls out "chain." This signals the lead surveyor to stop.
4. The rear surveyor then sights-in the lead surveyor on the line to be measured and holds the required distance mark of the tape exactly on the beginning stake. The lead surveyor pulls the tape straight and reasonably tight and setsa stake or pin on line exactly at the "zero" end of the tape.
5. Each time the rearsurveyor calls out the station number, the lead surveyor should answer with the number at that stake, indicating the front station. In so doing, the
rearsurveyor can mentally check and verify the addition to the forward station.
On slopes, the uphill end of the tapeshould be held on the ground and the surveyor at the other end should hold the tape so that it is level or at least as high as the surveyor can reach and "plumb" down by means of a plumb bob. On grades too steep for level taping, the tape should be "broken" in such convenient lengths that it can be held approximately level, plumbing down to the ground. Figure 1-20illustrates the process of breaking
chain and indicates the errors that can occur if this is not done on steep slopes.
Lesson 16
Stadia
The stadia method is a much faster way to measure distances than chaining, and is sufficiently accurate under some conditions.
The equipment required for stadia measurements consists of a telescope with two extra hozizontal hairs, called stadia hairs, and a graduated rod. Most transits and telescopic alidades, and some engineering levels, have stadia hairs. One of the stadia hairs is above the center horizontal crosshair and the other is an equal distance below it.
To take a stadia measurement, observe through the telescope the interval in meters or feet on the rod between the two stadia hairs when the rod is held vertically on some point. The stadia rod must be help plumb because considerable error in distance can result if it is not. this interval, called the stadia interval, is a direct function of the distance from the instrument to the rod. On most instruments the ratio of this distance to the stadia
interval may be taken as 100to 1 with no appreciable error. The exact ratio for instruments
with stadia hairs is usually indicated on the card placed in the instrument box. To determine the distance from the instrument to any given point, observe the stadia interval on the rod help on the point and multiply this interval by 100.
In reading stadia intervals, it is usually convenient to set the lower stadia hair on some even meter or foot mark and read the interval to the upper stadia hair. When the distance is such that one of the stadia hairs falls off the rod, one-half the interval may be read between one stadia hair and the horizontal crosshair. When this is done, the distance will be twice the interval that is read on the rod times 100.
The distances obtained by the stadia are as measured along the line of sight from the instrument to the rod. If the line of sight is on an appreciable grade, you will need to make a correction to obtain the true horizontal distance. The correction can be made by the use of tables or a stadia slide rule, either of which give both horizontal and vertical distances from stadia readings on various a grades. For slopes less than 5 percent, the horizontal
distance will be within 0.3percent of the measured distance and may be used without need of correction.
Aerial Photographs
Horizontal distances may be obtained between pointson an aerial photograph by direct scaling if the scale of the photograph is known. If the scale is not known or if you want to check it, the scale may be determined as follows:
1.Select two well-defined points on the photograph located so that measurement with the chain can be made conveniently between them on the ground and, also, so that the measurement between them on the photograph will cross a good portion of the center section of the photograph.
2. Measure, in centimeters (inches), the distance (A) between the points on the photograph.
3. Usea chain to measure, in meters (feet), the distance (B)between the points on the ground.
4. Scale of the photograph =B/A centimeters per meter (feet per inch). The scale is determined may be applied to other measurements of the same photograph, although the measurements near the edge may be affected some by the distortion of the photograph.
Electronic Equipment
To take a measurement with electronic measuring equipment, simply sight the unit onto a reflector target. Then, press the range button and numerals will appear on the display screen showing the distance in meters (feet). This procedure may vary slightly depending upon the type of equipment used.
Lesson 17
Differential Leveling
Planning and establishment of all permanent practices used in soil and water conservation work require information regarding the relative elevation of points on the earth’s surface.
Three principal methods are used to determine differences in elevation: barometric, trigonometric, and differential (or spirit) leveling. Differential leveling, the method most commonly used, is the only one explained here. It utilizes the phenomenon that a spirit level can be used to fix a line of sight perpendicular to the action of gravity. This line of
sight can then be used to determine differences in elevation between nearby points on the earth’s surface.