photograph, and figure 11-3 shows the line map
`made by the use of overlays.
`
`are all used for measuring unknown angles and
`distances and/or for laying off known angles and
`distances.
`
`BASIC SURVEYING INSTRUMENTS
`
`Most fieldwork done by an Engineering Aid
`(especially at the third- and second-class levels)
`is likely to consist of field measurements and/or
`computations that involve plane surveying of
`ordinary precision. This section describes the basic
`instruments, tools, and other equipment used for
`this type of surveying. Other instruments used for
`more precise surveys will also be described briefly.
`Surveying instruments come in various forms,
`yet their basic functions are similar; that is, they
`
`MAGNETIC COMPASS
`
`A magnetic compass is a device consisting
`principally of a circular compass card, usually
`graduated in degrees, and a magnetic needle,
`mounted and free to rotate on a pivot located at
`the center of the card. The needle, when free from
`any local attraction (caused by metal), lines itself
`up with the local magnetic meridian as a result
`of the attraction of the earth’s magnetic North
`Pole.
`
`Figure 11-3.-Line map made by overlays from the aerial photograph in figure 11-2.
`
`11-12
`
`

`

`ENGINEER’S TRANSIT
`
`of
`consists
`fieldwork
`survey
`A primary
`measuring horizontal and vertical angles or
`directions and extending
`straight
`lines. The
`instruments that can perform these functions have
`additional refinements (built-in) that can be used for
`other survey operations, such as leveling. Two types
`of instruments that fall into this category are the
`engineer’s transit and the theodolite. In recent years,
`manufacturing improvements have permitted con-
`struction of direct-reading theodolites that are soon to
`replace the vernier-reading transits. However, in
`most SEABEE construction, the engineer’s transit is
`still the major surveying instrument.
`
`The magnetic compass is the most commonly
`used and simplest
`instrument
`for measuring
`directions and angles in the field. This instrument
`has a variety of both civilian and military
`applications. The LENSATIC COMPASS (available in
`your Table of Allowance) is most commonly used for
`SEABEE compass courses, for map orientation, and
`for angle direction during mortar and field artillery
`fires.
`
`In addition to this type of compass, there are
`several others used exclusively for field surveys. The
`ENGINEER’S TRANSIT COMPASS, located between
`the standards on the upper plate, is graduated from
`0° through 360° for measuring azimuths, and in
`quadrants of 90° for measuring bearings (fig. 11-4).
`Notice in figure 11-4 that the east and west markings
`are reversed. This permits direct reading of the
`magnetic direction.
`
`The compass shown in figure 11-5 is commonly
`called the BRUNTON POCKET TRANSIT. This
`instrument is a combination compass and clinometer.
`It can be mounted on a light tripod or staff, or it may
`be cradled in the palm of the hand.
`
`Other types of compasses can also be found in
`some surveying instruments, such as the theodolite
`and plane table.
`
`Figure 11-4.-Engineer’s transit compass.
`
`Figure 11-5.-A Brunton pocket transit.
`
`45.742
`
`11-13
`
`

`

`The transit (fig. 11-6) is often called the
`
`universal survey instrument because of its uses. It
`
`may be used for measuring horizontal angles and
`
`directions, vertical angles, and differences
`
`in
`
`elevations; for prolonging straight lines; and for
`
`measuring distances by stadia. Although transits of
`
`various manufacturers differ in appearance, they are
`
`alike in their essential parts and operations.
`
`The engineer’s transit contains several hundred
`parts. For-descriptive purposes, these parts may be
`grouped into three assemblies: the leveling head
`
`assembly, the lower plate assembly, and the upper
`many plate or alidade assembly (fig. 11-7).
`
`Leveling Head Assembly
`
`The leveling head of the transit normally is the
`four-screw type, constructed so the instrument can be
`shifted on the foot plate for centering over a marked
`point on the ground.
`
`Lower Plate Assembly
`
`The lower plate assembly of the transit consists
`of a hollow spindle that is perpendicular to the
`
`Figure 11-6.-An engineer’s transit.
`
`11-14
`
`29.242
`
`

`

`Figure 11-7.-An engineer’s transit, exploded view.
`
`29.242
`
`center of a circular plate and accurately fitted the
`socket in the leveling head. The lower plate contains
`the graduated horizontal circle on which the values of
`horizontal angles are read with the aid of two
`verniers, A and B, set on the opposite sides of the
`circle. A clamp controls the rotation of the lower plate
`and provides a means for locking it in place. A slow-
`motion tangent screw is used to rotate the lower plate
`a small amount to relative to the leveling head. The
`
`rotation accomplished by the use of the lower clamp
`and tangent screw
`is known as the LOWER
`MOTION.
`
`Upper Plate or Alidade Assembly
`
`The upper plate, alidade, or vernier assembly
`consists of a
`spindle attached plate
`to a
`
`11-15
`
`

`

`circular plate carrying verniers, telescope
`standards, plate-level vials, and a magnetic
`compass. The spindle is accurately fitted to
`coincide with the socket in the lower plate spindle.
`A clamp is tightened to hold the two plates
`together or loosened to permit the upper plate to
`rotate relative to the lower plate. A tangent screw
`permits the upper plate to be moved a small
`amount and is known as the UPPER MOTION.
`The standards support two pivots with adjustable
`bearings that hold the horizontal axis and permit
`the telescope to move on a vertical plane. The
`vertical circle moves with the telescope. A clamp
`and tangent screw are provided to control this
`vertical movement. The vernier for the vertical
`
`circle is attached to the left standard. The
`telescope is an erecting type and magnifies the
`image about 18 to 25 times. The reticle contains
`stadia hairs in addition to the cross hairs. A
`magnetic compass is mounted on the upper plate
`between the two standards and consists of a
`magnetized needle pivoted on a jeweled bearing
`at the center of a graduated circle. A means is
`provided for lifting the needle off the pivot to
`protect the bearing when the compass is not in use.
`
`LEVEL VIALS.— Two plate level vials (fig.
`11-6) are placed at right angles to each other. On
`many transits, one plate level vial is mounted on
`the left side, attached to the standard, under the
`
`Figure 11-8.-Horizontal scales, 20 second transit.
`
`11-16
`
`

`

`vertical circle vernier. The other vial is then
`parallel to the axis of rotation for the vertical
`motion. The sensitivity of the plate level vial
`bubbles is about 70 sec of movement for 2 mm
`of tilt. Most engineer’s transits have a level vial
`mounted on the telescope to level it. The
`sensitivity of this bubble is about 30 sec per 2-mm
`t i l t .
`
`CIRCLES AND VERNIERS.— The hori-
`zontal and vertical circles and their verniers are
`the parts of the engineer’s transit by which the
`values of horizontal and vertical angles are
`determined. A stadia arc is also included with the
`vertical circle on some transits.
`
`The horizontal circle and verniers of the transit
`that are issued to SEABEE units are graduated
`to give least readings of either 1 min or 20 sec of
`arc. The horizontal circle is mounted on the lower
`plate. It is graduated to 15 min for the 20-sec
`transit (fig. 11-8) and 30 min for the 1-min transit
`(fig. 11-9). The plates are numbered from 0° to
`360°, starting with a common point and running
`both ways around the circle. Two double verniers,
`known as the A and B verniers, are mounted on
`the upper plate with their indexes at circle readings
`180° apart. A double vernier is one that can be
`read in both directions from the index line. The
`verniers reduce the circle graduations to the final
`reading of either 20 sec or 1 min.
`
`Figure 11-9.-Horizontal scales, 1-minute transit.
`
`11-17
`
`

`

`The A vernier is used when the telescope is in its
`normal position, and the B vernier is used when the
`telescope is plunged.
`The VERTICAL CIRCLE of the transit (fig. 11-
`10) is fixed to the horizontal axis so it will rotate with
`the telescope. The vertical circle normally
`is
`graduated to 30´ with 10° numbering. Each quadrant
`is numbered from 0° to 90°; the 00 graduations define
`a horizontal plane, and the 90° graduations lie in the
`vertical plane of the instrument. The double vernier
`used with the circle is attached to the left standard of
`the transit, and its least reading is 1´. The left half of
`the double vernier is used for reading angles of
`depression, and the right half of this vernier is used
`for reading angles of elevation. Care must be taken to
`read the vernier in the direction that applies to the
`angle observed.
`In addition to the vernier, the vertical circle may
`have an H and V (or HOR and VERT) series of
`graduations, called the STADIA ARC (fig. 11-10). The
`H scale is adjusted to read 100 when the line of sight
`is level, and the graduations decrease in both
`directions from the level line. The other scale, V, is
`graduated with 50 at level, to 10 as the telescope is
`depressed, and to 90 as it is elevated.
`
`29.266
`Figure 11-10.-Vertical circle with verniers,
`scales, and stadia arc.
`
`11-18
`
`The VERNIER, or vernier scale, is an auxiliary
`device by which a uniformly graduated main scale
`can be accurately read to a fractional part of a
`division. Both scales may be straight as on a leveling
`rod or curved as on the circles of a transit. The
`vernier is uniformly divided, but each division is
`either slightly smaller (direct vernier) or slightly
`larger (retrograde vernier) than a division of the
`main scale (fig. 11-11). The amount a vernier division
`differs from a division of the main scale determines
`the smallest reading of the scale that can be made
`with the particular vernier. This smallest reading is
`called the LEAST COUNT of the vernier. It is
`determined by dividing the value of the smallest
`division on the scale by the number of divisions on
`the vernier.
`
`in
`Direct Vernier.— A scale graduated
`hundredths of a unit is shown in figure 11-11, view A,
`and a direct vernier for reading it to thousandths of a
`unit. The length of 10 divisions on the vernier is
`equal to the length of 9 divisions on the main scale.
`The index, or zero of the vernier, is set at 0.340 unit.
`If the vernier were moved 0.001 unit toward the
`0.400 reading, the Number 1 graduation of the
`vernier shown in figure 11-11, view A, would coincide
`with 0.35 on the scale, and the index would be at
`0.341 unit. The vernier, moved to where graduation
`Number 7 coincides with 0.41 on the scale, is shown
`in figure 11-11, view B. In this position, the correct
`scale reading is 0.347 unit (0.340 + 0.007). The index
`with the zero can be seen to point to this reading.
`
`Retrograde Vernier.— A retrograde vernier on
`which each division is 0.001 unit longer than the 0.01
`unit divisions on the main scale is shown in figure 11-
`11, view C. The length of the 10 divisions on the
`vernier equals the length of the 11 divisions of the
`scale. The retrograde vernier extends from the index,
`backward along the scale. Figure 11-11, view D,
`shows a scale reading of 0.347 unit, as read with the
`retrograde vernier.
`
`Vernier for Circles. — Views E and F of figure
`11-11 represent part of the horizontal circle of a
`transit and the direct vernier for reading the circle.
`The main circle graduations are numbered both
`clockwise and counterclockwise. A double vernier
`that extends to the right and to the left of the index
`makes it possible to read the main circle in either
`direction. The vernier to the left of the index is used
`for reading clockwise angles, and the vernier to the
`right of the index is used for reading
`
`

`

`Figure 11-11.-Types of verniers.
`
`11-19
`
`

`

`counterclockwise angles. The slope of the
`numerals in the vernier to be used corresponds
`to the slope of the numerals in the circle being
`read. Care must be taken to use the correct
`vernier. In figure 11-11, view E, the circle is
`graduated to half degrees, or 30 min. On this
`vernier, 30 divisions are equal in length to 29
`divisions on the circle. The least reading of this
`vernier is 30 min divided by 30 divisions, or 1 min.
`The index (fig. 11-11, view E) is seen to lie between
`342°30´ and 343°. In the left vernier, graduation
`Number 5 is seen to coincide with a circle
`graduation. Then, the clockwise reading of this
`circle is 342°30´ plus 05´, or 342°35´. When the
`right vernier is used in the same way, the
`counterclockwise reading of the circle is 17°00´
`plus 25´, or 17°25´. In figure 11-11, view F, the
`circle is graduated in 15-min divisions and each
`half of the double vernier contains 45 divisions.
`The least reading on this vernier is 20 sec. The
`clockwise reading of the circle and vernier is
`3 5 1 ° 3 0 ´ p l u s 0 5 ´ 4 0 " o r 3 5 1 ° 3 5 ´ 4 0 " . T h e
`counterclockwise reading is 8°15´ plus 9´20", or
`8°24´20".
`
`THEODOLITE
`
`A theodolite is essentially a transit of high
`precision. Theodolites come in different sizes
`and weights and from different manufacturers.
`Although theodolites may differ in appearance,
`they are basically alike in their essential parts and
`operation. Some of the models currently available
`for use in the military are WILD (Herrbrugg),
`BRUNSON, K&E, (Keuffel & Esser), and PATH
`theodolites.
`
`To give you an idea of how a theodolite differs
`from a transit, we will discuss some of the most
`commonly used theodolites in the U.S. Armed
`Forces.
`
`One-Minute Theodolite
`
`The 1-min directional theodolite is essentially
`a directional type of instrument. This type of
`instrument can be used, however, to observe
`horizontal and vertical angles, as a transit
`does.
`
`The theodolite shown in figure 11-12 is a
`compact, lightweight, dustproof, optical reading
`
`instrument. The scales read directly to the nearest
`minute or 0.2 mil and are illuminated by either
`natural or artificial light. The main or essential
`parts of this type of theodolite are discussed in
`the next several paragraphs.
`
`HORIZONTAL MOTION.— Located on the
`lower portion of the alidade, and adjacent to each
`other, are the horizontal motion clamp and
`tangent screw used for moving the theodolite in
`azimuth. Located on the horizontal circle casting
`is a horizontal circle clamp that fastens the circle
`to the alidade. When this horizontal (repeating)
`circle clamp is in the lever-down position, the
`horizontal circle turns with the telescope. With
`the circle clamp in the lever-up position, the
`circle is unclamped and the telescope turns
`independently. This combination permits use
`of the theodolite as a REPEATING INSTRU-
`MENT. To use the theodolite as a DIREC-
`TIONAL TYPE OF INSTRUMENT, you should
`use the circle clamp only to set the initial reading.
`You should set an initial reading of 0°30´ on the
`plates when a direct and reverse (D/R) pointing
`is required. This will minimize the possibility
`of ending the D/R pointing with a negative
`value.
`
`VERTICAL MOTION.— Located on the
`standard opposite the vertical circle are the vertical
`motion clamp and tangent screw. The tangent
`screw is located on the lower left and at right
`angles to the clamp. The telescope can be rotated
`in the vertical plane completely around the axis
`(360°).
`
`LEVELS.— The level vials on a theodolite are
`the circular, the plate, the vertical circle, and the
`telescope level. The CIRCULAR LEVEL is
`located on the tribrach of the instrument and is
`used to roughly level the instrument. The PLATE
`LEVEL, located between the two standards, is
`used for leveling the instrument in the horizontal
`plane. The VERTICAL CIRCLE LEVEL (verti-
`cal collimation) vial is often referred to as
`a split bubble. This level vial is completely built
`in, adjacent to the vertical circle, and viewed
`through a prism and 450 mirror system from the
`eyepiece end of the telescope. This results in the
`viewing of one-half of each end of the bubble at
`the same time. Leveling consists of bringing the
`two halves together into exact coincidence, as
`
`11-20
`
`

`

`Figure 11-12.—One-minute theodolite.
`
`29.267
`
`11-21
`
`

`

`The telescope of the theodolite is an inverted
`image type. Its cross wires can be illuminated by
`either sunlight reflected by mirrors or by battery
`source. The amount of illumination for the
`telescope can be adjusted by changing the position
`of the illumination mirror.
`
`TRIBRACH.— The tribrach assembly (fig.
`11-15), found on most makes and models, is a
`detachable part of the theodolite that contains the
`leveling screw, the circular level, and the optical
`plumbing device. A locking device holds the
`alidade and the tribrach together and permits
`interchanging of instruments without moving the
`tripod. In a “leapfrog” method, the instrument
`(alidade) is detached after observations are
`completed. It is then moved to the next station
`and another tribrach. This procedure reduces the
`amount of instrument setup time by half.
`
`CIRCLES.— The theodolite circles are read
`through an optical microscope. The eyepiece is
`located to the right of the telescope in the direct
`position, and to the left, in the reverse. The
`microscope consists of a series of lenses and
`prisms that bring both the horizontal and the
`
`Figure 11-13.-Coincidence- type level.
`
`11-13. The TELESCOPE
`shown in figure
`LEVEL, mounted below the telescope, uses a
`prism system and a 450 mirror for leveling
`operations. When the telescope is plunged to the
`reverse position, the level assembly is brought to
`the top.
`
`TELESCOPE.— The telescope of a theodolite
`can be rotated around the horizontal axis for
`direct and reverse readings. It is a 28-power
`instrument with the shortest focusing distance of
`about 1.4 meters. The cross wires are focused by
`turning the eyepiece; the image, by turning
`the focusing ring. The reticle (fig. 11-14) has
`horizontal and vertical cross wires, a set of
`vertical and horizontal ticks (at a stadia ratio
`of 1:100), and a solar circle on the reticle
`for making solar observations. This circle covers
`31 min of arc and can be imposed on the
`sun’s image (32 min of arc) to make the pointing
`refer to the sun’s center. One-half of the vertical
`line is split for finer centering on small distant
`objects.
`
`Figure 11-14.-Theodolite reticle.
`
`Figure 11-15.-Three-screw leveling head.
`
`11-22
`
`

`

`vertical circle images into a single field of view.
`In the DEGREE-GRADUATED SCALES (fig.
`11-16), the images of both circles are shown as
`they would appear through the microscope of the
`1-min theodolite. Both circles are graduated from
`0° to 360° with an index graduation for each
`degree on the main scales. This scale’s graduation
`appears to be superimposed over an auxiliary that
`is graduated in minutes to cover a span of 60 min
`(1°). The position of the degree mark on the
`auxiliary scale is used as an index to get a direct
`reading in degrees and minutes. If necessary, these
`scales can be interpolated to the nearest 0.2 min
`of arc.
`
`The vertical circle reads 0° when the
`theodolite’s telescope is pointed at the zenith, and
`180° when it is pointed straight down. A level line
`reads 90° in the direct position and 2700 in the
`reverse. The values read from the vertical circle
`are referred to as ZENITH DISTANCES and not
`vertical angles. Figure 11-17 shows how these
`zenith distances can be converted into vertical
`angles.
`
`Figure 11-17.-Converting zenith distances into vertical
`angles (degrees).
`
`In the MIL-GRADUATED SCALES (fig.
`11-18), the images of both circles are shown as
`they would appear through the reading micro-
`scope of the 0.2-mil theodolite. Both circles are
`graduated from 0 to 6,400 mils. The main scales
`are marked and numbered every 10 mils, with the
`
`Figure 11-16.-Degree-graduated scales.
`
`Figure 11-18.-Mil-graduated scales.
`
`11-23
`
`

`

`last zero dropped. The auxiliary scales are graduated
`
`from 0 to 10 roils in 0.2-mil increments. Readings on
`
`the auxiliary scale can be interpolated to 0.1 mil.
`
`The vertical circle reads 0 mil when the telescope
`is pointed at the zenith, and 3,200 mils when it is pointed
`
`straight down. A level line reads 1,600 roils in the direct
`position and 4,800 roils in the reverse. The values read
`are zenith distances. These zenith distances can be
`converted into vertical angles as shown in figure 11-19.
`
`One-Second Theodolite
`
`The 1-sec theodolite is a precision direction
`type of instrument for observing horizontal and
`vertical directions. This instrument is similar to,
`
`Figure 11-19.-Vertical angles from zenith distances (mils).
`
`Figure 11-20.-A 1-second theodolite.
`
`11-24
`
`45.632
`
`

`

`but slightly larger than, the 1-min theodolite. The
`WILD theodolite shown in figure 11-20 is
`compact, lightweight, dustproof, optical reading,
`and tripod-mounted. It is one spindle, one plate
`level, a circular level, horizontal and vertical
`circles read by an optical microscope directly to
`1 sec (0.002 roil), clamping and tangent screws for
`controlling the motion, and a leveling head with
`three foot screws. The circles are read using the
`coincidence method rather than the direct method.
`There is an inverter knob for reading the
`horizontal and vertical circles independently. The
`
`essential parts of a l-see theodolite are very
`similar to that of the 1-min theodolite, includ-
`ing the horizontal and vertical motions, the
`levels, the telescope,
`the tribrach, and the
`optical system shown in figure 11-21. The
`main difference between the two types, besides
`precision, is the manner in which the circles are
`read.
`The CIRCLE to be viewed in the 1-see
`theodolite is selected by turning the inverter
`knob on the right standard. The field of the
`circle-reading microscope shows the image of the
`
`Figure 11-21.-Circle-reading optical system.
`
`11-25
`
`

`

`circle (fig. 11-22) with lines spaced at 20-min
`intervals, every third line numbered to indicate
`a degree, and the image of the micrometer scale
`on which the unit minutes and seconds are read.
`The numbers increase in value (0 0 to 360 0,
`clockwise around the circle. The coincidence knob
`on the side of, and near the top of, the right
`standard is used in reading either of the circles.
`The collimation level and its tangent screw are
`used when the vertical circle is read.
`The circles of the theodolite are read by the
`COINCIDENCE METHOD in which optical
`coincidence is obtained between diametrically
`opposite graduations of the circle by turning the
`MICROMETER or COINCIDENCE KNOB.
`When this knob is turned, the images of the
`opposite sides of the circle appear to move in
`opposite directions across the field of the
`CIRCLE-READING MICROSCOPE. The grad-
`uations can be brought into optical coincidence
`and appear to form continuous lines crossing the
`dividing line. An index mark indicates the circle
`graduations that are to be used in making the
`coincidence. The index mark will be either in line
`with a circle graduation or midway between two
`graduations. The final coincidence adjustment
`should be made between the graduations in line
`with the index mark or when this index mark is
`halfway between the two closest graduations.
`
`HORIZONTAL CIRCLE.— To read the
`HORIZONTAL CIRCLE, turn the INVERTER
`or CIRCLE-SELECTOR KNOB until its black
`line is horizontal. Adjust the illuminating mirror
`to give uniform lighting to both sections of the
`horizontal circle; the micrometer scale is viewed
`
`through the circle-reading microscope. Focus the
`microscope eyepiece so that the graduations are
`sharply defined. The view through the microscope
`should then be similar to figure 11-22, view A.
`From this point, continue in the following way:
`
`1. Turn the coincidence knob until the images
`of the opposite sides of the circle are moved into
`coincidence. Turning this knob also moves the mi-
`crometer scale. The view through the microscope
`now appears as shown in figure 11-22, view B.
`2. Read the degrees and tens of minutes from
`the image of the circle. The nearest upright
`number to the left of the index mark is the number
`of degrees (105). The diametrically opposite
`number (the number ± 1800) is 285. The number
`of divisions of the circle between the upright 105
`and inverted 285 gives the number of tens of
`minutes. In figure 11-22, view B, there are five
`divisions between 105 and 285; and the reading,
`therefore, is 105050´. The index may also be used
`for direct reading of the tens of minutes. Each
`graduation is treated as 20 min. Thus, the number
`of graduations from the degree value to the index
`mark multiplied by 20 min is the value. If the
`index falls between graduations, another 10 min
`is added when the tens of minutes is read directly.
`3. Read the unit minutes and seconds below
`from the image of the micrometer scale. This scale
`has two rows of numbers below the graduations;
`the bottom row is the unit minutes and the top
`row, seconds. In figure 11-22, view B, the unit
`minutes and seconds are read as 7'23.5''
`4. Add the values determined in Steps 2 and 3
`above. This gives 105057'23.5''as
`the
`final
`reading.
`
`Figure 11-22.-View of a 1-second theodolite circle.
`
`11-26
`
`

`

`VERTICAL CIRCLE.— When reading the
`VERTICAL CIRCLE, turn the circle-selector
`knob until its black line is vertical. Adjust the
`mirror on the left standard and focus the
`microscope eyepiece. You then go on in the
`following way:
`
`1. Use the vertical circle tangent screw to
`move the collimation level until the ends of
`its bubble appear in coincidence (fig. 11-23)
`in the collimation level viewer on the left
`standard.
`2. Read the vertical circle and micrometer
`scale as described before. Be sure to have proper
`coincidence before you take the reading.
`3. The vertical circle graduations are num-
`bered to give a 00 reading with the telescope
`pointing to the zenith. Consequently, the vertical
`circle reading will be 900 for a horizontal sight
`with the telescope direct and 2700 for a horizontal
`sight with the telescope reversed. Figure 11-23
`shows the view in the circle-reading microscope
`for direct and reversed pointings on a target.
`These readings are converted to vertical angles as
`follows:
`
`There are two separate occasions for setting
`the horizontal circle of the theodolite. In the first
`case, the circle is set to read a given value with
`the telescope pointed at a target. With the
`theodolite pointed at the target and with the
`azimuth clamp tightened, the circle is set as
`follows: Set the micrometer scale to read the unit
`minutes and the seconds of the given values. Then,
`with the circle-setting knob, you turn the circle
`until coincidence is obtained at the degree and tens
`of minutes value of the given reading. This setting
`normally can be made accurately to plus or minus
`5 sec. After the circle is set in this manner, the
`actual reading should be determined.
`In the second case, the circle is set to a value
`of a given angle. When a predetermined angle is
`measured, you first point the instrument along the
`initial line from which the angle is to be measured
`and read the circle. Add the value of the angle
`to the circle reading to determine the circle reading
`for the second pointing. Set the micrometer scale
`to read the unit minutes and the seconds of the
`value to be set on the circle. Then, you turn the
`instrument in azimuth and make coincidence at
`the degrees and tens of minutes value that is to
`be set. The predetermined value can usually be
`set on the circle in this way to plus or minus 2 sec.
`
`ENGINEER’S LEVEL
`
`The engineer’s level is a widely used instrument
`for leveling operations. Its sighting device is a
`30 ± 3 variable power telescope, with a
`maximum length of 18 in. and with an erecting
`eyepiece. Some models use internal focusing,
`while others use external focusing objective
`
`Figure 11-23.-View of a vertical circle for direct and reversal pointings.
`
`11-27
`
`

`

`assemblies. The reticle has two cross hairs at right
`angles to each other, and some models have stadia
`hairs. The telescope and level bar assembly is
`mounted on a spindle that permits the unit to be
`revolved only in a horizontal plane. It cannot be
`elevated or depressed. A clamp and tangent screw
`acts on this spindle for small motions to permit
`accurate centering. The spindle mounts in a
`four-screw leveling head that rests on a foot plate.
`The foot plate screws onto the threads on the
`tripod. When the instrument is properly leveled
`and adjusted, the line of sight, defined by the
`horizontal cross hair, will describe a horizontal
`plane.
`The two distinct types of engineer’s levels,
`classified according to their support, are the wye
`level and the dumpy level. The WYE LEVEL
`(fig. 11-24) is so called because its telescope is
`supported by a pair of wye rings. These rings can
`be opened for the purpose of turning the telescope
`or rotating it around its horizontal axis. The
`
`bubble tube (vial) can be adjusted, either vertically
`or laterally, by means of adjusting nuts at the ends
`of the bubble tube. All these features are provided
`for the purpose of making fine adjustments. The
`DUMPY LEVEL (fig. 11-25) has its telescope
`rigidly attached to the level bar, which supports
`an adjustable, highly sensitive level vial. During
`visual leveling operations and observations, both
`types handle similar basic operations. Their cross
`hairs are brought into focus by rotation of the
`eyepiece, and their target, into clear focus by
`rotation of the focusing knob. Their telescope can
`be exactly trained on targets by lightly tightening
`the azimuth clamp and manipulating the azimuth
`tangent screw.
`
`PRECISION LEVEL
`
`Other types of leveling instruments have been
`incorporated into the SEABEE units. In fact, the
`self-leveling level has now become standard
`
`Figure 11-24.-A wye level.
`
`11-28
`
`

`

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`11-29
`11-29
`
`
`
`

`

`equipment in the Naval Construction Force Table
`of Allowance (TOA). These precision instruments
`are essentially like the conventional levels except
`for added features.
`
`A precision level is one that is equipped with
`an extra-sensitive level vial. The sensitivity of a
`level vial is usually expressed in terms of the size
`of the vertical angle the telescope must be moved
`to cause the bubble in the level vial to move from
`one graduation to the next.
`
`The sensitivity of the level vial on an ordinary
`level is about 20 sec. On a precise level, it is about
`2 sec. The telescope level vial on an ordinary
`transit has a sensitivity of about 30 sec.
`
`The more sensitive the level vial is, the more
`difficult it is to center the bubble. If the level vial
`on an ordinary level had a sensitivity as high as
`2 sec, the smallest possible movement of the level
`screw would cause a large motion of the bubble.
`
`For this reason, a precise level is usually also
`a tilting level. On a tilting level, the telescope is
`hinged at the objective end so the eyepiece end
`can be raised or lowered. The eyepiece end rests
`on a finely threaded micrometer screw that can
`
`be turned to raise or lower the eyepiece end in
`small increments. The instrument is first leveled,
`as nearly as possible, in the usual manner. The
`bubble is then brought to exact center by the use
`of the micrometer screw.
`
`Military Level
`
`The military level (fig. 11-26) is a semi-precise
`level designed for a more precise work than the
`engineer’s level. The telescope is a 30-power,
`10-in.-long, interior-focusing type with an
`inverting eyepiece and an enclosed fixed reticle.
`The reticle is mounted internally and cannot be
`adjusted as in other instruments. It contains cross
`wires and a set of stadia hairs. The objective is
`focused by an internal field lens through a rack
`and pinion, controlled by a knob on the upper
`right-hand side of the telescope. The telescope and
`level vial can be tilted through a small angle in
`the vertical plane to make the line of sight exactly
`horizontal just before the rod reading is made.
`The tilting is done by a screw with a graduated
`drum located below the telescope eyepiece. A cam
`is provided to raise the telescope off of the tilting
`device and to hold it firmly when the instrument
`is being moved and during the preliminary
`
`Figure 11-26.-A military level.
`
`11-30
`
`45.749
`
`

`

`leveling. An eyepiece, located to the left of the
`telescope, is used for viewing the bubble through
`the prism system that brings both ends of the
`bubble (fig. 11-13) into coincidence.
`
`The level vial is located directly under the
`telescope, but to the left and below, directly in
`line with the capstan screws under the bubble-
`viewing eyepiece. The level vial’s sensitivity is
`given as 30 sec per 2-mm spacing. A circular
`bubble that is viewed through a 450 mirror is
`provided for the first approximate leveling before
`the long level vial is used. For night work,
`battery-powered electric illumination lights the
`long bubble, the reticle, and the circular level. The
`clamping screw and the horizontal motion tangent
`screw are located on the right-hand side; the
`former near the sp