Horizontal and Vertical Control
In this type of survey, the elevations of points, as well as their locations in a horizontal plane, are determined. This may be done after the original traverse for horizontal control has been established or may be carried on along with the establishment of the transit stations.
To obtain elevations of the transit points and other details after a traverse has been completed and checked, use the following procedure (using same survey as figure 1-34).
1. Set the instrument on E. Set an angle of 175'13' on the horizontal circle and sight the instrument on BM . This is the azimuth of E to (fig. 1-34).
2. Instrument is now oriented. Proceed with establishing elevations as follows:
Find elevation of point E and HI by taking rod reading 1.4 m (4.63 ft)on BM [elevation 432.953 m (1,421.45 ft)]. Next, measure4 3I with tape 1.33 m(4.37 ft). Then 432.953 + 1.41 - 1.33 m (1,421.45 +4.63 - 4.37 ft) = elevation of point E = 433.033 m(1,421.71 ft). A BM with an assumed elevation may be used if sea level datum is not required. Last,
turn azimuths, and take stadia readings and elevations of points accessible to point A.
When it is necessary to take vertical angles for the determination of elevations:
1. Tie a narrow band of red cloth around rod to mark HI, 1.33 m (4.37 ft).
2. Sight the middle horizontal crosshair at a point on the rod equal to the HI. This will give the vertical angle between the ground at the instrument and the ground at the foot of the rod for any location the rod may be placed.
3. Clamp the vertical circle and read the angle. To locate points, take readings in the following order:
1. Set the vertical hair on the rod and clamp the upper plate.
2. Read the distance by setting one stadia hair on an even number and noting the position of the other stadia hair on the rod.
3. Set the middle horizontal hair on the point (HI) on the rod to which the vertical angle is to be taken.
4. Then signal the rodholder to go to the next point and record the stadia interval, horizontal and vertical angles, bearing, and remarks.
Contour maps can be prepared from the surveyed points and their elevations, with the contour lines located by interpolation.
Lesson 29
Grid Surveys
Grid surveys are particularly applicable to surveys of small areas and where substantial
topographic data are needed. The system is simple in that a level, rod, and tape are all that are necessary, but it may require more time than planetable and alidade or transit and stadia surveys.
Obtaining topography by using the grid system consists of selecting and laying out a series of lines on the ground that can be reproduced to scale on paper. All topography, including ground elevation, is then obtained in the field with reference to these lines and is later plotted. Contour lines can be drawn in by interpolating between plotted ground
elevations. In any survey for a topographic sketch, a survey plan must be determined in advance. The following questions should be considered:
1. What ground features are conveniently located for use as base lines?
2. Can the base lines be reproduced on a drawing in their true relationship to each other?
3. How far apart shall gridlines be set?
4. How close together will ground elevations need to be taken?
5. What is the most efficient procedure to use?
In planning the survey procedure, remember that rod readings for elevations cannot be read with accuracy over 90 m (300 ft) with the ordinary level and level rod. If a stadia rod is used, it can be read at a distance of 150 to 180 m (500 to 600 ft). Ordinarily,
the distance between any two adjacent shots for ground elevations should not exceed 60 m
(200 ft). On very flat, uniform terrain, this distance might be increased to 90 m (300 ft).
In some instances it may be necessary to establish right angles for making grid layouts. If the surveying instrument does not have a horizontal circle for turning off right angles, they can be established by the 3-4-5 method (fig. 1-35). An alternative procedure is to use a right-angle prism. The prism saves time and is accurate enough for grid layouts and cross sections.
The following example explains the procedure for gridding a field. See figures 1-36 and 1-36(a) with accompanying field notes in figures 1-37 and 1-37(a). It is assumed that a level with a horizontal circle is available for use in laying out right angles from base lines and gridlines. This gridding procedure has the advantage over other systems in that fewer instrument setups are required for taking the levels. Also the entire job can be done at a
reasonable rate with a survey party of two people. In the example, a particular area is to be gridded for preparation of a topographic map. Inspection of this area on an aerial photo showed that the north line of the area was the farm boundary which was well defined, clear of brush, and would serve best as the base line. Further inspection showed that the
south field boundary was parallel to the north L boundary and that the east and west sides diverged from the north to the south.
1. First, an onsite check was made to determine if this gridding plan was workable. No trees, hills, or other obstacles were noted that would prevent useof the plan. The field and grid plan (figures 1-36 and 1-36a) were sketched in the field notebook. A range pole was set in the northwest corner of the field at point A, 6 (A, 2). This point was called A, 6
(A, 2) so that no minus coordinates would have to be used. Had it been called A, 0, or A, 3 (A, I), the point at the southwest corner of the field might be G, 0 (G, -1) or some other minus designation.
Distances of 120 m (400 ft) and 180 m (600 ft) from A, 6 (A, 2) were chained off eastward along the base line, and a range pole was set at A, 18 (A, 6) and a stake was set at A, 24 (A, 8).
2. The level was then set up over stake A, 24 (A, 8) and sighted on range pole at A, 6 (A, 2). A 90-degree angle was then turned off. A range pole was sighted in and set at point G, 24 (G, 8). This established direction for gridline 24 (8). Line 24 (8) was then chained off, beginning at point A, 24 (A, 8) and working toward the range pole at point G, 24(G, 8). Long stakes were set at 60-m (200-ft) intervals along the line. The rear surveyor lined-in each stake by eye between the succeeding stake and range pole at G, 24 (G;8) .The distance between points F, 24 (F,8) and G, 24 (G,8) was found to be 58 m (190 ft)and was so noted on field book sketch (fig. 1-37).
3. Gridlines D and E were staked next. The level was set up over points D, 24 (D, 8) and E, 24 (E,8), and 90-degree angles were turned off from line 24 (8). Long stakes were set at 60-m (200-ft) intervals on each line, starting measurement from the level and staking to east field boundary, then returning and staking to west field boundary.
4. Line 18 (6) was then staked, starting at point G, 18 (G, 6) on the south property line; 58 m (190 ft) was chained off to correspond to the stake previously set at F, 24 (F, 8). Staking of line 18 (6)was continued at 60-m (200-ft) intervals to the range pole at
A, 18 (A, 6). This completed lines 18 (6) and 24 (8).
5. With the four gridlines staked, the elevation shots for the entire field can be completed without further use of tape and with a minimum of pacing measurements, simply by locating any other points by lining-in by eye from the N and S and E and W stakes. Obviously, the gridline stakes must be distinguishable for a considerable distance. A field
up to 24 ha (60 acres) in area can be gridded with a four-line layout such as this. A survey party of three people can lay out and stake these lines much quicker than two people. A motor vehicle can be used to advantage in setting range poles and distributing stakes when crops and other field conditions will permit. Survey notes (figs. 1-37 and 1-37(a)) illustrate a commonly used method of keeping notes for this type of survey. Note the intermediate shots, which are not on regular grid points. Nearly all gridding will require some intermediate shots to locate and get elevations in low spots, high spots, existing ditches, etc. A sheet of cross-section paper on a clipboard or on a planetable is often used to record the survey notes directly on a sketch drawn to scale in the field.
Lesson 30