Abstract:
Described herein are various embodiments of a novel gauge apparatus for rapidly and repeatably attaching a conventional survey target to a structural element in order to facilitate plumb and level alignment and erection. Relatively unskilled workers can use this gauge to set a survey target a predetermined distance from the centerline of the structural element and maintain a consistent registration, thus saving time. The plumb/level alignment gauge consists, in an exemplary embodiment, of a graduated shaft with a target head fitted to receive both a standard surveyor&#39;s target and a prism. Attached to the shaft are two sliding jaws that magnetically attach to the structural element. The gauge is positioned by setting the scale mark corresponding to the offset distance directly above the centerline of the column. Plumbing is achieved by aligning the target to the offset line through conventional means.

Description:
BACKGROUND 
   In erecting buildings, the surveyor is commonly charged with the task (among other tasks) of ensuring that all of the vertical structural elements are plumb and level, i.e., each vertical element (for example, a column, steel beam, or stud) must be installed so that it is precisely vertical with respect to gravity. In particular, each element must be plumb (vertical) in two directions. These two directions are sometimes referred to as “fore and aft” and “side-to-side” as viewed by a surveyor. In general, however, every structure has a perimeter, referred to herein as the structural envelope. In a simple rectangular plan structure, the envelope has four sides, but obviously there is no limit to the number of edges or even a prohibition on curved edges. Accordingly, the two directions may be generally understood to be “along the structural envelope” and “into the structural envelope,” respectively, in buildings of an arbitrary number of sides and curved vertical surfaces. 
   The typical surveying/erecting process for ensuring verticality or plumbing each structural element includes setting out an offset line parallel to each edge of the building perimeter at a known distance away from the actual building footprint. The offset distance is commonly from two to six feet from the centerline of the structural element, although surveyor&#39;s preference and site conditions usually dictate, as will be discussed below. A surveyor positioned at a known point on the offset line next takes a bearing (“shoots a line”) along the offset line and adjusts the vertical angle of this line to sight in a target attached to the structural element to be plumbed. The target may be any standard surveying target known to persons having ordinary skill in the art, such as a card target, retroreflector, or prism, and is typically attached (again by well-known means) to a target staff or builder&#39;s rod. The rod is typically held perpendicular to the vertical element&#39;s centerline and over the offset line by a helper known as the rod man. The target is positioned so that its center point or zero line is at the pre-determined offset distance from the building perimeter. 
   Since this process can take some time, and because it is unreasonable to expect the rod man to hold the rod perfectly still, the rod or staff is usually clamped to the column or other structural element. However, since the rod cannot be clamped to the column at its vertical center, the survey team must measure the distance from the column&#39;s centerline to the optical center of the target in order to ensure that all distance measurements are indexed relative to the vertical centerline of the structural element. These measurements must be made by hand to determine the compensation distances between the center of the column in both the fore-and-aft (along the structural envelope) and side-to-side (into the structural envelope) distances and the position of the target center. The compensation distances thus take into account the space between the target center and the true physical vertical centerline of the structural element. The rod must also be set perpendicular to the vertical center of the column, so that the compensation distance measurements are made on a straight line. This is time consuming and prone to both measurement and mathematical error. 
   Once the rod is set, the surveyor then directs the erectors to move the column into or out from the plane of the envelope (using conventional building steel erection methods) to plumb it in the lateral or side-to-side direction. Once plumbed in this first direction, the surveyor then measures the distance to the target along the offset line, using common surveying tools such as a theodolite equipped with electronic distance measurement (EDM) hardware and appropriate software. This process is well known in the art and will not be further described. Comparing this distance with the expected fore-and-aft position of the column, adjusted for the compensation distance between the rod&#39;s clamped position and the center of the column defined in the building plans, the surveyor then directs the erectors to adjust the column&#39;s position fore-and-aft, along the structural envelope. 
   The whole process is then repeated for the next structural element, and so on, down each edge of the structure and again on each successive floor. The offset distance may be varied to take into account structural or architectural projections (such as cantilevers) extending outside the building envelope or ground conditions (such as traffic) that preclude setting up the theodolite. 
   Prior art devices include the aforementioned builder&#39;s rods with permanent scales or graduations on them, which allow direct positioning to the offset line in the lateral direction. These devices are generally not furnished with clamping devices or other means of attaching them to a structural element. Also known is a magnetic-base target, such as the Sokkia RT50M, which can be directly attached to a steel column or other element. The RT50M, however, measures about four to eight inches from the magnetic base to the center of its retroreflective target and is not adjustable in length beyond a nominal amount. As such, it is too short to reach far enough outside of the building envelope to lie over a typical offset line. The Sokkia RT50M is made by Sokkia Co., Ltd. of 260-63 Hase, Atsugi, Kanagawa, Japan. 
   Another prior art device once used in the trade consists of a flat plate magnet with a shaft screwed perpendicular to the plane of the plate. Mounted on the shaft is a sliding target that can be positioned along its length. This device is prone to breakage as it can be easily knocked off the face of the column by passing workers, material, or equipment. It has no scale for direct measurement and requires careful setup to measure and compute the compensation distances from column center to target position. 
   A third prior art device is a tool known as an “erection rod.” This device consists of the long arm of a common carpenter&#39;s framing square (with its inscribed scale) mounted into a magnet that can be attached to a flat face (i.e., the flange) of a wide web column. A surveyor&#39;s target card is positioned on the arm by sliding it along the scale. This device, too, has problems with staying affixed to the steel and it also requires careful setup to measure and compute the compensation distances from column center to target position. Additionally, the target card is prone to blowing off in a stiff wind and movement in general. Furthermore, the whole device has to be rotated to place the target in view of the surveyor. 
   The deficiencies in the above-described conventional approaches include several sources of error and inaccuracy, potentially contributing to out-of-tolerance measurement accuracy. For example, the rod man must position the rod correctly on each structural element relative to its center and the offset line and measure—again, with high accuracy—the compensation distances from the column center in both the fore-and-aft and side-to-side directions. These offsets must be communicated to the surveyor without error and properly entered into the calculations. The target must also be pointed at the survey instrument, a direction that varies from column to column and floor to floor, which requires careful initial positioning of the entire tool. 
   What is needed is an adjustable alignment gauge that can be attached to a structural element and positioned rapidly, precisely, and repeatably with respect to the structural element&#39;s centerline and an offset line a short distance away from the building envelope. 
   SUMMARY 
   Described herein are various embodiments of a novel gauge apparatus for rapidly and repeatably attaching a conventional survey target to a structural element in order to facilitate plumb and level alignment and erection. Relatively unskilled workers can use the disclosed gauge apparatus to set a conventional survey target a predetermined distance from the centerline of the structural element to maintain a consistent registration with the centerline, thus saving labor for the more highly trained rod men and other members of the survey team. 
   The plumb/level alignment gauge consists, in an exemplary embodiment, of a graduated or otherwise scaled shaft with a target mounting head fitted to receive, on one end, a standard surveyor&#39;s target or prism. Attached to the shaft are two sliding jaws that can be clamped or otherwise attached to a structural element. In the case of structural elements made of steel (such as the common steel wide web or I-beam), one exemplary embodiment utilizes magnets on the inside faces of the sliding jaws to attach to the structural element. In some embodiments, the shaft passes through a hole in each jaw that allows the whole shaft to rotate in order to orient the target to a distant surveyor. In an alternate embodiment using a square or rectangular shaft, only the target head rotates around the shaft&#39;s long axis. 
   In operation, the surveyor sets out an offset line parallel to the edge of the building envelope containing the structural elements to be plumbed. (This setting out and use of an offset line is well known in the art and will not be discussed in greater detail herein.) After first establishing a survey plane coincident with and plumb to the offset line, the surveyor acquires the target on the outside end of the plumb/level alignment gauge apparatus. With the target center in view, the surveyor directs the erectors to plumb the structural element by moving it in or out relative to the plane of building envelope, i.e., side-to-side, perpendicular to the survey plane. Once plumb in this “lateral” direction, and using the conventional EDM features of a modern theodolite well known in the art, the surveyor directs the erectors to plumb the column along the plane of the structural envelope, i.e., fore-and-aft, parallel to the survey plane. This latter step is accomplished by comparing the measured distance to the gauge&#39;s target against the expected (planned) distance obtained from the building construction plans through standard means well known in the art. 
   The plumb/level alignment gauge of the present invention is designed to be firmly affixed to a column or other structural element, in one exemplary embodiment by a conventional clamp device well known in the art. In a preferred embodiment, the jaw plates include strong magnets, eliminating the need for an external clamp or clamps. 
   The shaft portion of the gauge apparatus has a permanently inscribed or otherwise firmly attached graduated scale indexed to the center of the target. This configuration enables the gauge to be directly positioned on a column by setting the scale mark corresponding to the offset distance directly above the centerline of the column. For example, if the offset line is set three feet from the building envelope and the column measures 12×12 inches, the worker simply clamps the gauge to the column and slides the shaft over until the 3-foot mark on the scale is aligned with the column&#39;s vertical centerline. 
   Accordingly, the present plumb/level alignment gauge addresses the shortcomings of the prior art by providing a simple device to plumb structural elements during building erection. The plumb/level alignment gauge is capable of being positioned rapidly, accurately, and repeatably with respect to the centerline and an offset line a short distance away from the building envelope and remains firmly attached. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  illustrates, in a simplified plan view, the elements of a building site for reference in subsequent descriptions. 
       FIG. 2  illustrates, in a simplified elevation view, some of the elements of the building site of  FIG. 1  as seen from a surveyor&#39;s point of view. 
       FIG. 3  shows a plumb/level alignment gauge according to one embodiment of the present invention. 
       FIG. 4  shows a portion of a plumb/level alignment gauge according to an alternate embodiment of the present invention. 
       FIG. 5A  illustrates a simplified elevation view of the plumb/level alignment gauge in use, according to one embodiment of the present invention. 
       FIG. 5B  illustrates a magnified view of target  560  in  FIG. 5A , as seen through the theodolite reticule, according to one embodiment of the present invention. 
       FIG. 6  depicts a simplified elevation view of the plumb/level alignment gauge in use as seen from a point on a line perpendicular to survey plane  224  of  FIG. 5A . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a simplified plan view of a building  100  under construction. At this stage in the construction, several structural elements  110 - 1 ,  110 - 2 , and  110 -N have been erected and are undergoing the process of plumbing and leveling. In this exemplar, structural envelope  115  is depicted as simple rectangle; clearly, however, a building can have any shape consisting of straight or curved segments of any length or radius. A single, flat side of envelope  115  is discussed herein for simplicity and clarity of illustration. A person having ordinary skill in the art will readily understand that the present invention is not limited in its application to use on any particular building configuration. 
   Offset lines  122  and  124  are set out by conventional means well-known in the surveyor&#39;s art from offset (or coordinate) points  120 A and  120 B. Other conventional reference points commonly used in the building and surveying trades may also be used. 
   In order to plumb and level (as those terms are known and used in surveying) structural element  110 -N on the side of building  100  parallel to offset line  124 , the surveyor positions a theodolite with EDM capabilities (or similar survey instrument) at offset point  120 A or at a known distance from offset point  120 A on line  124 . (The theodolite is omitted from this drawing for clarity; see instead  FIG. 2 .) 
   Although a theodolite with EDM capabilities is described, those skilled in the art will realize that an entire class of survey instruments with distance measuring capabilities other than a theodolite can be used, including (for example, but not by way of limitation) a manual or fully-robotic total station. Accordingly, the invention is not limited to any particular type of survey instrument with EDM capabilities. Indeed, one of ordinary skill in the art will also see that a dumpy level or simple transit may also be used to confirm lateral (side-to-side) plumb. 
     FIG. 2  depicts the surveyor&#39;s view of an elevation of building  100 . In particular, this drawing highlights a single structural element  110 -N in need of alignment with respect to plumb and level as seen from the surveyor&#39;s position. This exemplar structural element, here a column or similar building component, is shown on the third floor  220  of a building under construction; one of ordinary skill in the art will recognize that such a column could be on any floor or ground level  210  and still be plumbed with the aid of the present invention. Survey plane  224  is a plumb plane coincident with offset line  124  (which extends into the plane of the drawing in this view) and offset point  120 A. Survey plane  224  is established with a survey instrument  260 , such as a theodolite or total station, located at offset point  120 A. 
     FIG. 3  depicts a plumb/level alignment gauge  300  according to one embodiment of the present invention. The gauge comprises a shaft  302  permanently marked with a scale  305 . In this figure, scale  305  is depicted as a series of permanently marked lines on shaft  302 . Alternately, the markings could be graduations as typically seen on a builder&#39;s or surveyor&#39;s rod or a level staff commonly used in the art. The scale preferably includes numerical markings that measure the distance from the center point  327  of target  325  in (typically) feet, inches, and tenths (omitted here for illustrative simplicity and clarity). 
   Shaft  302  may have a round, square, rectangular, or any other cross-section, as long as it is straight along its long axis. Shaft  302  may be made of any material, although steel or another sturdy material is preferred in order to avoid breakage or bending in the harsh environment of the construction jobsite. For example, but not as a limitation, in one embodiment shaft  302  is a standard steel builder&#39;s rod, such as a Sokkia Level Rod Model CR-08-T, which has a rectangular cross section and feet and tenths graduations. In another embodiment, shaft  302  is a six-foot length of ¾ inch steel tubing, such as thin-wall conduit. 
   Furthermore, although an inches and tenths scale is described, those skilled in the art will realize that scales in units other than inches and tenths or feet can be used. Additionally, the scale graduations and numbering may be etched, scribed, cut, painted, or marked onto shaft  302  by any means now known or to be discovered without limitation. Removable scales of any kind may also be used, as long as they can be reliably indexed to an origin or zero point at target center point  327  and read from “0” at center point  327  in ascending order (e.g., 0, 1 meter, 2 meters, etc.) Accordingly, the invention is not limited to any particular scale or method of marking or affixing it to shaft  302 . 
   Jaws  310  are attached to shaft  302  so that they can slide from side-to-side (along the long axis of shaft  302 ) independently of one another. Jaws  310  may thus be spread apart or slid together to engage the sides of a structural element of any size up to the length of shaft  310 . Jaws  310  each have a hole in them to accept shaft  302 . The size of the holes in the jaws relative to the outside dimensions of shaft  302  is selected such that the shaft can slide through the jaws (once attached to structural element  110 -N) with some slight friction laterally (i.e., along the long axis of shaft  302 ), but not move appreciably in any other direction, in order to reduce position uncertainty error at target center  327 . 
   In one embodiment, the friction fit of the jaw-shaft interface holds shaft  302  in place. In an alternate embodiment, one or two thumbscrews  370  or similar locking devices commonly employed on a sliding mechanical apparatus may be used to ensure that shaft  302  cannot move relative to jaws  310 . 
   Jaws  310  may contact structural element  110 -N directly and be held in place with a convention clamp (such as a common C-clamp or bar clamp, not shown). Preferably, however, jaws  310  are configured to act as backing plates to magnets  315 , as shown in  FIG. 3  (and in greater detail in the embodiment depicted in  FIG. 4 ). Such magnet-equipped jaws are thus self-attaching, eliminating the need for external clamps. 
   Shaft  302  is attached to target head  320  in such a way that the target head can be removed for safekeeping and avoidance of damage. In one embodiment, target head  320  has a threaded stud configured to mate with a like-threaded receptacle on the end of shaft  302 . In another embodiment, an attachment fitting  318  provides a tight slip fit between shaft  302  and target head  320 . The means and method of attachment between target head  320  and shaft  302  are not important to the functioning of the present invention, except that the target head (at least) must be rotable, i.e., able to be rotated to face the surveyor. Whether this is accomplished by means of a rotating attachment fitting  318  or by rotating the entire shaft  302 -target head  320  assembly is immaterial. One of ordinary skill in the art will readily see that a wide variety of attachment means are adaptable for use with the present invention and will satisfy the above criteria. 
   Target head  320  consists, in one exemplary embodiment, of a tubular section, preferably of a durable metal, approximately 6 inches in length with a narrow slot running its length. This slot is sized so that a standard target card  325  (such as but not limited to a Sokkia Model 8126-20 target) can slip tightly into the slot. The slot length is sized so that the zero point  327  of target  325  can be aligned over a zero line  328  inscribed (or otherwise permanently marked) on target holder  320 . Alignment of target  325  with zero line  328  is an important step in assembling plumb/level alignment gauge  300  because the horizontal registration of target head  320  with scale  305  is critical to establishing the position of target  325  with respect to the centerline of the structural element  110 -N. In one exemplary embodiment, the integrity of scale  305  is ensured by configuring (through means well-known in the manufacturing arts) attachment fitting  318  and target head  320  so that a tight attachment will reliably and repeatably position scale zero line  328  at the true origin of scale  305 . 
   Although a slot running the length of target head  320  and a standard card target  325  are described, those skilled in the art will realize that a variety of slot lengths and common survey targets other than rigid or semi-rigid cards can be used. Stick-on, plastic, and metal targets of various shapes and sizes are well known in the art. The slot need not run the full length of the target head either; it need only be long enough to hold the target (of whatever material) firmly. The size of target head  320  and the card-holding slot therein can easily be adapted by a person having ordinary skill in the art to function with the plumb/level alignment gauge described herein. Indeed, one need not install the target in a slot at all; any target fitted to stand above the centerline of rod  302  will work. Accordingly, the invention is not limited to use with any particular type of target. 
   Target head  320  also includes a prism attachment fitting  330  mounted on scale zero line  328 . Fitting  330  is, in one exemplary embodiment, a simple brass disc 7/16 of an inch in diameter and 5/16 of an inch thick, drilled through its center and tapped to receive a 6 mm×1.0 thread per mm (also known as M6 x1.0) stud. This type of mounting thread is commonly used on surveyor&#39;s mini-prisms and other EDM retro-reflective devices. 
   Prism attachment fitting  330  must be attached to target head  320  so that its center is set on scale zero line  328 . This alignment registers the center of prism  350  with the origin of scale  305 , just as the center  327  of target  325  was aligned in the previous embodiment. This alignment represents only one direction, however; it provides a lateral zero reference only. In order to ensure that prism  350  is at a precisely known position in the direction parallel to the survey plane  224  (referencing  FIG. 2 ), prism attachment fitting  330  is mounted so that prism  350 &#39;s “back” (as the effective reflection surface/point of the prism is referred to in the art) is co-planar with the surface of shaft  302  that contacts the side of structural element  110 -N. 
   Prism  350  (shown in dashed outline for clarity) is, in one exemplary embodiment, a typical surveyor&#39;s mini-prism used with EDM devices, such as (for example but not by way of limitation) the OMNI JR. 25.4 mm prism. This prism is equipped with an M6 x1.0 stud that mounts into fitting  330 . OMNI JR. Prisms are available from Omni Optical Products, Inc., 17282 Eastman Ave., Irvine, Calif. 92614. 
   Shaft  302  may also include, in some embodiments, a pin  380  that prevents jaws  310  from sliding off. Other devices, such as a cotter pin, screw-on cap, or flared end on shaft  302  may also be used. Accordingly, the invention is not limited to any particular type of pin  380  or similar part placed to prevent either or both jaws  310  from becoming separated from shaft  302 . 
   Although jaws  310  having an asymmetric cross section (with respect to shaft  302 ) are illustrated in  FIG. 3 , those skilled in the art will realize that jaws of any shape can be used. In one embodiment, shown in  FIG. 4 , jaws  410  consist of circular backing plates  412  for flat toroidal plate magnets  415 , where both the jaws and the magnets are in the shape of a flat torus mounted concentric with the long axis of shaft  302 . Accordingly, the invention is not limited to any particular jaw and (if employed) magnet shape. 
   The embodiment depicted in  FIG. 4  also includes pinch protection sleeves  420  placed between jaws  410  to prevent magnets  415  from meeting and potentially pinching someone&#39;s finger. Besides being painful, strong plate magnets of the type employed here may be extremely difficult to separate once joined by mutual attraction; pinch protection sleeves  420  prevent this occurrence by maintaining a separation of about one inch or more. 
   Note that the function of pinch protection sleeves  420  may also be accomplished by a single sleeve or by more than two sleeves. Indeed, protection may be equally afforded by, for example, a block or blocks attached to shaft  302  or one or both magnets  415 . The only limitation on such pinch protection means is that they do not interfere with the repeatable and precise placement of the plumb/level alignment gauge on the structural element so that the center point of the target (or the effective back of the prism) is at a known and constant position relative to the centerline of the structural element. Accordingly, one of ordinary skill in the art will therefore recognize that the present invention is not limited as to the type or number of pinch protection sleeves. 
   In the circular-jaw exemplary embodiment of  FIG. 4  above, the diameter of jaw backing plate  412  is approximately five inches and employs a five-inch diameter toroidal plate magnet  415  approximately ⅜ of an inch thick. The center hole of each jaw  410  is 15/16 of an inch in diameter and slips over a ¾ inch outside diameter (O.D.) length of standard galvanized electrical conduit (EMT, or Electrical Metallic Tubing) material; the conduit forms shaft  302 . The function of pinch protection sleeves  420  is furnished by two half-inch long segments of one inch O.D. conduit slipped over shaft  302  and located between magnets  415 . Fitted on the target end of shaft  302  is a standard ¾-to-¾ inch EMT conduit compression coupler (functioning as attachment fitting  318 ) that allows for rotation and removal of target head  320  and target  325 . (The entire shaft  302  is also rotable within the center holes of jaws  410 .) Target holder  320  is constructed, in this exemplary embodiment, of a 6-inch length of ¾ inch EMT material with a 4⅛ inch slot cut along its length to receive a Sokkia Model 8126-20 target card. Target holder  320  also includes the simple brass disc prism attachment fitting  330  discussed above and is configured to accept the OMNI JR. prism. In this configuration, the effective back of the prism is coincident with the far end of its mounting stud, which extends 5/16 of an inch from its body. Accordingly, when this prism is screwed tightly into attachment fitting  330 , the end of the stud, and thus the effective back of the prism, is very nearly in contact with the outer diameter of target head  320 , offset only by an amount equal to the wall thickness of pinch protection sleeves  420 . 
   Because of this design, the surveyor can be confident that the compensation distance between the back of prism  350  and the centerline of structural element  110 -N depends solely on the size of the structural element. When the plumb/level alignment gauge is attached to the far side of structural element  110 -N, the compensation distance is subtracted from the line-of-sight distance to the prism; when attached to the near side, the compensation distance (plus the ¾ inch diameter of shaft  302 ) is added to the line-of-sight distance. The computation of such distances including compensation distances determined by the position of tools is well known and well within the experience of a person having ordinary skill in the art. 
   Although a particular size of prism attachment fitting  330  is described, those skilled in the art will realize that various attachment fittings sized to mate with industry-standard EDM retro-reflectors of various types can also be used. Accordingly, the invention is not limited to any particular type of fitting (or EDM retro-reflector), so long as fitting can positively register hold an EDM reflector with respect to the scale zero line and structural element contact surface alignment criteria discussed above. 
     FIG. 5A  shows an expanded view of the building elevation depicted in  FIG. 2  with a plumb/level alignment gauge  500  in use. Here, a single vertical structural element  505  is shown for clarity. The vertical centerline  510  of structural element  505  is shown at a slight deviation to plumb line  520  (here exaggerated for illustration purposes). Plumb/level alignment gauge  500  is attached to structural element  505  with target head  555  and target  560  extending outside of the building envelope and nearly (but not quite yet) aligned with survey plane  224 . (Survey plane  224 , as described above with reference to  FIGS. 1 and 2 , is a plane containing offset line  124  and parallel to building envelope  115 .) 
   The surveyor&#39;s view of target  560  through theodolite reticule  575  prior to plumbing is shown in  FIG. 5B . Crosshairs  577   a  and  577   b  and stadia  578  are shown for illustrative purposes only, merely to point out the deviation of target center  527  from survey plane  224 , which is aligned with the surveyor&#39;s vertical crosshair  577   a . In this example, the surveyor would first instruct the erectors to level the plumb/level alignment gauge, thus bringing target  560  level. The surveyor would then instruct them to tilt structural element  500  to the surveyor&#39;s left to bring it into side-to-side level (i.e., plumb in the direction perpendicular to the survey plane), in accordance with standard methods of plumbing a structural element. 
   A similar process is followed when a prism and EDM device, such as a total station are used: side-to-side (lateral) plumb is determined by simple alignment with the prism&#39;s center, as discussed above with respect to target  560  and target center  527  of  FIGS. 5A and 5B . Fore-and-aft plumb (i.e., plumb in the direction parallel to the survey plane and perpendicular to lateral plumb) is determined by comparing the measured distance to the effective back of the prism  610  with the expected (computed distance) determined by the building plans. This geometry is illustrated in  FIG. 6 . Structural element  600  represents the column to be plumbed in the direction parallel to the survey plane. Here, structural element  600  is shown in a portion of building  100  on a floor  220  substantially elevated above foundation  210  in order to accentuate the initial deviation of column centerline  605  from plumb line  620  for illustrative purposes. Surveyor  640  uses a total station  645 , for example, to shoot a line  650  in the survey plane defined by offset line  124  to obtain the actual distance to prism target  610 . In this example, since structural element  600  is leaning to the right, towards surveyor  640 , the distance along line  650  will be shorter than expected for a plumb column. Accordingly, surveyor  640  will instruct the erectors to tilt structural element  600  away from total station  645  until the distance along line  650  matches the expected distance within a standard tolerance appropriate to the conditions. 
   Distance-based plumb/level computations, such as those necessary to determine the expected distance from the building plans, are well-known in the art and are not further discussed herein. 
   It must be noted that the present invention can also be used to plumb a vertical structural element in any location where direct viewing is difficult, e.g., the middle column in a long row of columns. One of ordinary skill in the surveying and erection arts will appreciate that many situations can arise beyond the offset line/exterior envelope scenario described for illustrative purposes above. Accordingly, the present invention is not limited in its application to structural elements on a building&#39;s perimeter. 
   Furthermore, the plumb/level alignment gauge presently disclosed may also be used to hold a target at the centerline of a structural element in order to rapidly enable an alignment check from a point perpendicular to the structural envelope and directly in front of the structural element. In one exemplary embodiment, this alignment check may be accomplished by inserting a target into a slot located on shaft  302  between jaws  310  centered, on e.g., the 3-foot mark (referring to  FIG. 3 ). By attaching this plumb/level alignment gauge embodiment and aligning the center of the target so mounted with the centerline of the structural element, the surveyor obtains a quickly and reliably mounted sighting target without the need to transport, calibrate, and mount a separate target device. 
   While particular embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims. In particular, the order in which the steps by which the present invention is used is purely illustrative in nature. In fact, the steps can be performed in any order or in parallel, unless otherwise indicated by the present disclosure. Accordingly, the appended claims encompass within their scope all such changes and modifications.