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This application is a continuation of Ser. No. 08/975,801, filed Nov. 21, 1997, now U.S. Pat. No. 6,102,136, which is a continuation of Ser. No. 08/583,303, filed Jan. 16, 1996, now U.S. Pat. No. 5,711,381. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to underground bore location systems and, more particularly, to a novel underground bore location system and method for detecting and compiling bore location data so that a bore map may be generated. 
     Those of ordinary skill in the art should recognize that the term “bore” refers to the excavation of a hole, typically for utilities, through the ground and to the excavated hole itself. The present invention relates to systems and methods for locating such bores, but also to such systems and methods for locating existing buried utilities, whether such existing utilities were initially installed by boring or trenching techniques. Accordingly, unless otherwise indicated, the term “bore” as used herein refers to new bores and to existing buried utilities or similar lines. 
     Boring location systems are utilized in a variety of circumstances. For example, in horizontal boring systems as are typically used for installing utilities, it is desirable to maintain a directional boring head in a desired boring path and to avoid known obstacles such as existing utilities. Accordingly, systems are known to trace existing utilities from an aboveground position. Similarly, it is often desirable to map existing utilities. 
     While such known systems are capable of indicating the position and depth of a bore at a specific location, they are generally unable to produce a corresponding plot during a new bore or as an existing bore is located. Such plots may, for example, track the position of a new bore with respect to known underground obstacles. Thus, an operator may, by monitoring the plot, control the directional boring head to avoid the obstacles. Such mapping of a new or existing bore has in the past been accomplished manually. For example, an operator with an aboveground monitoring device that detects a signal radiated from a probe proximate a directional boring head may walk on the ground surface tracing the progress of the probe during the bore. The operator may manually relay information to a second operator by voice or other communication means so that a plot of the bore may be generated. 
     Manual plotting methods are slow, inefficient, and prone to error. Thus, it is desirable for a bore location system to automatically compile data relating to the depth of a bore so that a bore plot may be automatically generated. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes and addresses the disadvantages of the prior art. Accordingly, it is an object of the present invention to provide an improved bore location system. 
     It is a further object of the present invention to provide a system and method for mapping a horizontal bore which determines the position of the bore through detection of electromagnetic signals radiated from the bore. 
     It is a still further object of the present invention to provide a system and method for producing bore plot information in real time as a bore is located. 
     These and other objects are achieved by providing a system for mapping horizontal bores below a ground surface. The system includes a transmitting source configured to radiate a location signal from the bore. A receiver device is configured to receive the location signal and to indicate, responsively to the location signal, the lateral position of the horizontal bore with respect to the receiver. A measurement device is configured to measure the depth of the bore with respect to the ground surface at selected locations along the bore, and a monitor device is configured to receive depth data from the measurement device. The monitor device is also configured to compile the depth data associated with the selected locations with respect to at least one reference position and to output the compiled depth data so that the depth of the bore at the selected locations may be collectively identified. 
     The measurement device may be configured to measure the underground depth of the bore by a variety of methods. For example, depth measurements may be taken by determination of a field gradient of a received location signal or by detection of a radiated magnetic field indicating deviation from a predetermined desired path. Furthermore, the measurement device may be embodied by the receiver device. Accordingly, in one presently preferred embodiment, the receiver device and measurement device comprise a single portable assembly which the operator uses to locate the lateral position of the bore via the location signal, taking a depth measurement in a known fashion at the selected location. Thus, a visual display device at the receiver may display information indicative of both the bore&#39;s lateral position and depth. 
     Similarly, the transmitting source may generate the location signal in a number of ways. For example, the source may be a probe fed through an existing bore or housed within a boring head cutting a new bore. Alternatively, the source may be an aboveground unit that directly creates or indirectly induces an electric current along an existing utility which, in turn, generates a radiating magnetic field which is received by the receiver device. Furthermore, the source may be the utility itself where the utility conducts an electric current that generates its own magnetic field. 
     A method according to the present invention for mapping horizontal bores below a ground surface includes the steps of receiving a location signal radiated from the bore, determining depth of the bore responsively to the received location signal, compiling data corresponding to the depth of the bore at selected locations along the bore, and displaying the compiled data so that the depth of the bore at the selected locations may be collectively identified. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification including reference to the accompanying figures in which: 
     FIG. 1 is a perspective view of a wireless remote boring system in accordance with an embodiment of the present invention; 
     FIG. 2A is a perspective view of a receiver/transmitter in accordance with an embodiment of the present invention; 
     FIG. 2B is a perspective view of a signal generating probe; 
     FIG. 3 is a perspective view of a remote receiver/display in accordance with an embodiment of the present invention; 
     FIG. 4 is a perspective view of a directional boring head associated with a signal generating probe and drill rod; 
     FIG. 5 is a block diagram illustrating the operation of a receiver/transmitter unit in accordance with an embodiment of the present invention; 
     FIG. 6 is a block diagram illustrating the operation of a remote receiver unit in accordance with an embodiment of the present invention; 
     FIG. 7 is an exemplary visual display of a receiver and/or monitor device in accordance with an embodiment of the present invention; 
     FIG. 8 is an exemplary visual display of a receiver and/or monitor device in accordance with an embodiment of the present invention; 
     FIG. 9 is an exemplary visual display of a receiver and/or monitor device in accordance with an embodiment of the present invention; 
     FIG. 10 is an exemplary bore plot generated in accordance with an embodiment of the present invention; 
     FIG. 11A is a perspective view of a transmitting source in accordance with an embodiment of the present invention; 
     FIG. 11B is a perspective view of a transmitting source in accordance with an embodiment of the present invention; 
     FIG. 11C is a perspective view of a transmitting source in accordance with an embodiment of the present invention; 
     FIG. 11D is a perspective view of a transmitting source in accordance with an embodiment of the present invention; 
     FIG. 12 is a schematic illustration of a transmitting source in accordance with an embodiment of the present invention; 
     FIG. 13A is a partial graphical representation of a depth measurement procedure practiced in accordance with a preferred embodiment of the present invention; 
     FIG. 13B is a partial graphical representation of a depth measurement procedure practiced in accordance with a preferred embodiment of the present invention; and 
     FIG. 13C is a partial graphical representation of a depth measurement procedure practiced in accordance with a preferred embodiment of the present invention. 
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without parting from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalence. 
     Referring to FIG. 1, a directional boring device  10  in accordance with an embodiment of the present invention is illustrated. A boring machine  12  is located in an initial position and includes a boring rod  14  and a directional boring head  16 . The boring machine includes a control panel  18  with actuators  20  for controlling the operations of the boring device. In accordance with the present invention, means for wireless receipt of location signals from a transmitting source includes a receiver device  22 . Receiver  22  includes a display  24  and the means for wireless transmission from the receiver device of information received from the transmitting source to a remote monitor device. As embodied herein, the means for wireless transmission includes a wireless transmitter  26  with an antenna  28 . 
     A signal generating probe  30  is located generally adjacent boring head  16  for emitting location signals containing information about the boring device as will be discussed in more detail below. The improved guidance system further includes a remote monitoring device  32  located generally adjacent to boring machine  12  for receiving the transmitted information from transmitter  26  via wireless transmission. Remote monitor  32  includes a display  34  so that the operator  36  of the boring device can see and/or hear the information transmitted from transmitter  26 . 
     Accordingly, a workman  38  at a distant location from the boring machine  12  utilizes receiver  22  to receive a location signal from signal generating probe  30 , which signal contains information with respect to the boring head  16 . Such information may be, for example, its location, its depth below the ground, its pitch, its angular position or roll, its temperature, and/or the remaining battery life of the probe. This information is received by receiver  22  as will be described in more detail below and is processed on display  24  at this location. 
     Substantially simultaneously and in real time, transmitter  26  transmits signals carrying the information that is displayed on display  24  to the monitor  32  via wireless transmission. Remote monitor  32  processes these signals and displays them on display  34 . Both data and image signals may be transmitted between the wireless transmitter and remote monitor  32 . Thus, operator  36  at the boring device is able to obtain real time information with respect to the boring head just as the workman  38  is able to obtain this information at the location of the boring head. The particular mechanisms for accomplishing this with respect to a preferred embodiment will be described in more detail below. 
     The present invention may also be utilized with systems for locating existing utilities. For example, referring to FIG. 11A, transmitting source  50  radiates a location signal from utility  52  located within the bore. Cables from transmitting source  50  are clipped directly to buried utility  52  and to ground at  54 . AC current will carry along the length of the conductor and will return through a grounded stake to transmitter  50 , providing a signal loop. Current strength displayed on both transmitter  50  and receiver  22  is at its maximum as receiver  22  moves directly over and traces the utility. Receiver  22  may indicate the maximum current by audible or visual means, thereby indicating the lateral position of the horizontal bore with respect to the receiver device. Thus, an operator carrying a portable receiver device can move to his left or right until the receiver device is approximately directly above the utility. 
     Faults can be detected by current fluctuation. A microprocessor within receiver  22  rejects the depth reading when receiver  22  strays over other utilities in the area by indicating “DETECTING ERROR” on the visual display. Current strength is adjustable to avoid bleeding onto other utilities in congested areas, and to “power-up” for a longer locate in areas where no other utilities are present. One transmitting source configured to operate in accordance with the present invention as described above and below is the SpotDTek® marketed by McLaughlin Manufacturing Company, Inc. of Greenville, S.C. 
     Transmitter  50  may also be configured to indirectly generate the location signal when a direct connection to the utility is impractical. For example, referring to FIG. 11B, transmitter  50  is placed on the ground surface in an upright position above the utility  52 . Transmitter  50  emits a varying magnetic field  54  to generate a current along utility  52  which, in turn, induces a magnetic field along the length of utility  52 . Accordingly, receiver  22  may detect the location signal. Using the McLaughlin SpotDTek®, this indirect mode is effective for utilities buried at depths of 6.5 feet or less and produces a location signal detectable up to 200 feet. As in the direct connection mode, “DETECTING ERROR” will be displayed if receiver  22  picks up other utilities in the area. The current induction strength is adjustable in this mode to tune out other utilities in congested areas. Current readout on the digital display will also detect faults as receiver  22  is moved along the surface. 
     Referring now to FIG. 11C, for depths between 6.5 and 16 feet transmitter  50  is placed over utility  52 . A strong signal  54  is generated by twin coils, and high AC power provides an effective detection range of over 1,000 feet. As in the short span indirect mode, current strength can be fine-tuned so that other utilities and faults may be readily detected. 
     Referring to FIG. 11D, a coil clamp can be used on metallic lines or to induce a signal through PVC lines. The coil clamp does not have to close around the conductor. It need only be placed on and parallel to the utility  52 . The SpotDTek® external coil mode has a detection range of over 1,000 feet. 
     The above-described methods for detecting an existing utility involve radiating a location signal from a metallic utility. Referring to FIG. 12, a transmitting source for use with nonmetallic pipe includes a battery operated transmitter probe  56  inserted in PVC or other nonmetallic pipe having a 1″ or larger internal diameter. Probe  56  emits a magnetic location signal  54  that is received by a receiver  22  which traces the progress of probe  56  as it is routed through the utility  52 . 
     Furthermore, the transmitting source may be simply the utility itself. For example, power and telecommunication lines emit their own electromagnetic radiation which may be used as location signals. Thus, receiver  22  may trace these utilities while detecting the self-emitted location signal. The SpotDTek® device may be programmed, for example, for three passive frequencies, 50-60 Hz for live power and 13-17 KHz and 18-22 KHz for two radio frequencies. Thus, such utilities may be located without the need of signal inducement as long as current is flowing on the lines. 
     It should also be understood that the receiver  22  may be stationary. For example, the present invention could be utilized in bore homing systems like those disclosed in Chau U.S. Pat. No. 4,881,083 and Bridges et al., U.S. Pat. No. 4,646,277. 
     Furthermore, as will be apparent to those of ordinary skill in the art, a variety of suitable apparatus and methods may be employed to radiate a location signal from a bore, to receive the location signal, to determine the depth of the bore responsively to the received location signal, to compile data corresponding to the depth of the bore at selected locations along the bore, and to display the compiled data so that the depth of the bore at the selected positions may be collectively identified. 
     Thus, for example, a receiver device may be a fixed device or a portable device carried by an operator to trace a new or existing bore. Similarly, the depth measurement device may measure bore depth in a variety of ways. For example, depth may be measured by determination of a field gradient of a received location signal or as a function of the pitch angle of a directional boring head. Furthermore, the measurement device may be an independent device or a device embodied by other system components, for example the receiver device. 
     Accordingly, all suitable apparatus and methods for accomplishing the present invention should be understood to be in the scope and spirit of the present invention. For ease of explanation, however, the remainder of the specification will address an exemplary preferred embodiment for use with a directional boring system as shown in FIG.  1 . It should be understood that such an example is provided by way of illustration only and not in limitation of the invention. For example, the location signal may be radiated and received by any of the methods or systems described above. 
     Referring to FIGS. 2A and 2B, receiver  22  and signal generating probe  30  are illustrated. Receiver  22  includes a longitudinally extended plastic casing  22   a  which houses the receiving mechanism. Integrated with housing  22   a  is a display  24  and a handle  22   b  for positioning the receiver. Attached to the receiver is a wireless transmitter  26  whose operation will be described in more detail with respect to FIG.  5 . Of course, transmitter  26  may be incorporated within the receiver unit. Housing  22   a  includes a plurality of horizontally spaced apart coils  23  (shown in Phantom in FIG. 2 a ) for receiving signals from the signal generating probe  30 . Coils  23  form a crossed antenna configuration. Signal generating probe  30  generates a magnetic field that contains information with respect to the probe that is indicative of the boring head  16 . The multiple coils  23  in housing  22   a  utilize the field gradient of the magnetic field from the signal generator to generate information as to the location and depth of the boring head. The particular mechanism for generating the signals representative of information concerning the boring head, and the particular mechanism of receiving this information as is done by receiver  22 , does not form an essential part of the present invention in and of itself. One preferred method of measuring the signal generated by signal generating probe  30  is to measure the field gradient rather than the magnetic field strength in the manner as disclosed in U.S. Pat. No. 3,617,865 dated Nov. 2, 1971, the disclosure of which is incorporated herein by reference in its entirety. 
     In a preferred embodiment, the frequency of the signal output by the signal generator is approximately 38 KHz. Of course, any suitable frequency may be utilized, such as, for example, 1.2 KHz, 9.5 KHz, 114 KHz, etc. 
     Probe  30  in a preferred embodiment consists of a ferromagnetic core with copper windings on which an electrical current is placed to generate a magnetic field that is received by receiver  22  as set forth in U.S. Pat. No. 3,617,865. Probe  30  may be of varying types depending on the application desired, and may be capable of providing a variety of types of information. For example, location and depth of the probe (and, consequently, the boring head) may be measured by determining the field gradient of the magnetic field generated by probe  30 . Mercury switches may be provided in a probe  30  around its inside perimeter so as to indicate the angular position or roll of the boring head. When the boring head is rotated to a particular position, the appropriate mercury switches will close and, therefore, angular position information is generated. As is indicated in FIG. 4, a directional boring head  16  has a sloped portion  16   a  for controlling the direction of the boring head in conjunction with the propulsion of the boring machine. With information as to the angular location of the sloped portion  16   a , the boring head can be oriented to proceed in a desired direction. This is referred to herein as the roll of the directional boring head. 
     In addition, probe  30  may contain a cradle-type switch for indicating the pitch above or below a horizontal plane or a plane parallel to the surface of the ground at which the directional boring head is located. Finally, indicators may be contained in the boring head and probe to indicate the battery life remaining in the probe or signal generator  30  as well as the temperature of the boring head. All of this information may be conveyed to the receiver through the magnetic field generated by the signal generator. Thus, for example, FIGS. 7,  8  and  9  illustrate possible visual displays of the receiver  22  and/or monitor  32 . The display as in FIG. 7 illustrates the direction of the tapered surface  16   a  and pitch angle of boring head  16 . The display in FIG. 8 illustrates the depth of the boring head at a particular instance. 
     It should be appreciated by one skilled in the art that, although receiving a magnetic field is one preferred embodiment, any suitable type system for determining the desired information about the boring head through a wireless radiated signal would be within the scope of the present invention. In addition, while the signal generator is referred to herein as a probe, it should be appreciated that other types of signal generators would also be within the scope of the present invention. 
     Referring to FIG. 3, a more detailed view of remote monitor  32  as illustrated. Remote monitor  32  may be held around the neck of operator  36  by strap  40  or mounted to boring machine  12  in any suitable fashion. Monitor  32  contains a display  34  for displaying the information received from wireless transmitter  26 . Display  34  is capable of displaying information identical to the information displayed on display  24  so that the operator  36  of the boring machine will have the same information as the operator  38  located at the boring head. In a preferred embodiment, display  34 , as well as display  24 , includes a clock face readout (FIG. 7) for indicating the angular position or roll of the boring head in quadrants, as well as indicators for the remaining information as discussed above. It should be understood that a graphic or visual display is one preferred form of display, but within the meaning of “display” or “indicate” as used herein, a voice or audio synthesizer could be substituted or other appropriate audible tones sufficient to convey the appropriate information to the operator. In addition, remote receiver  32  includes a touch pad control panel  42  for selecting the desired information to be displayed, adjusting the volume of the audible signal, or for other purposes as would be apparent to one skilled in the art. Display  24  has similar controls. 
     Referring to FIG. 4, directional boring head  16  includes a sloped or bent surface  16 a for assisting in the directional propulsion of the boring head as described above. Boring head  16  is connected through boring rod  14  to boring machine  12 . A component of the boring rod  14  contains a compartment into which the signal generating probe  30  may be inserted for generating the appropriate signals to convey the information with respect to the boring head as described above. As will be understood by those of ordinary skill in the art, as the boring head  16  advances through the bore, additional boring rods are added by operator  36 . Thus, the progression of the boring head  16 , and therefore the length of the bore, may be determined in terms of the number of boring rods expended. 
     Referring to FIG. 5, a block diagram is illustrated providing the operational characteristics of receiver  22  and wireless transmitter  26  to one skilled in the art. As illustrated, receiver  22  receives a signal generated by signal generating probe  30  via magnetic field as described above with respect to U.S. Pat. No. 3,617,865, or otherwise, and as would be readily apparent to one skilled in the art. The dual coil mechanism described above is illustrated at  42  in FIG.  5 . The signal received by coil  42  is filtered and converted from an analog signal to a digital signal at  44 . The digital signal is then processed in a central processing unit  46  to generate the appropriate audible signal as illustrated at speaker  47  and the appropriate visual signal through display  24 . The conversion of the received signals from the probe to a visual display and audible output as illustrated in FIG. 5 is done in a conventional manner as would be apparent to one skilled in the art. An example of a known commercial product suitable for this function is the Micro Computerized Pipe Locator marketed by McLaughlin Manufacturing Co., Inc., 2006 Perimeter Road, Greenville, S.C. 29605, under the product number MPL-H5. 
     In accordance with the present invention, central processing unit  46  simultaneously and in real time conveys a signal representative of the information displayed on display  24  and sent to audible means  47  to wireless transmitter  26 . Wireless transmitter  26  includes a frequency shift keyed modem  48  for receiving the signal from a central processing unit  46  and a transmitter chip  49  for transmitting the signal via wireless means to remote monitor  32 . In a preferred embodiment, the digital signal is transmitted between receiver  22  and transmitter  26  at 1200 bits per second. Also, in a preferred embodiment, between modem  48  and transmitter  49 , the “1” component of the digital signal is transmitted on a frequency of 1500 Hz and the “0” component of the digital signal is transmitted at approximately 2100 Hz. Of course, these are by way of example only. 
     Wireless transmitter  26  is capable of transmitting data and image signals and may be of any conventional type wireless transmitter with such capabilities. In a preferred embodiment, wireless transmitter  26  has selectable bands and transmits on a frequency of 469.50 MHz or 469.550 MHz with an output power of 18 milliwatts. Of course, these are by way of example also. In a preferred embodiment, the transmitter circuit corresponds to the Federal Communications Commission Standard no. ID-APV0290. The wireless transmitter is capable of transmitting both data and image signals and transmits the signals to the remote monitor  32  substantially simultaneously with the display on display  24 , thereby providing real time information to the operator  36  of the boring machine  12 . 
     Referring to FIG. 6, the signal transmitted by wireless transmitter  26  is received by remote monitor  32  at receiver unit  50 . Receiver unit  50  receives on the same frequency that transmitter  49  transmits on. In a preferred embodiment, such frequency is 469.50 MHz or 469.550 MHz. The circuitry utilized in remote monitor  32  also corresponds to FCC Standard ID-APV0290. The signal received at  50  is transmitted via frequency shift keyed modem  52  to central processing unit  53 . In a preferred embodiment, this is an eight-bit signal and represents the display and audio components of the signal transmitted to monitor  32 . A band pass filter  54  and carrier detector  56  may be utilized to filter and enhance the signal provided to the central processing unit  53 . The filter  54  may filter signals, for example, outside of a range of 1100 to 2300 Hz. In this embodiment, carrier detector  56  provides a one-bit signal to central processing unit  53  as to whether a radio wave is sending or not, and this controls the receipt by the central processing unit  53 . The signal between receiver unit  50  and band pass filter  54  is conveyed as described above with respect to the signal between modem  48  and transmitter  49  with respect to the frequencies. The central processing unit  53  processes the signal to produce an image on display  34  as well as an audible component if desired via speaker  58 . It should be appreciated that both transmitter  26  and monitor  32  may be of conventional design for the wireless transmission of data and the image signals, the particulars of which are not essential to the present invention. 
     As discussed above, receiver  22  is also a measurement device capable of measuring the depth of the probe below the ground surface. This information is transmitted to, and received by, monitor  32  as discussed above. Thus, referring to FIG. 9, display  34  indicates the depth of the boring head  16  at a particular selected location. In this embodiment, a depth of 4 feet 5 inches is indicated at a distance of 1 rod length, where 1 rod is equal to 10 feet. Operator  36  may record this information by depressing an appropriate key on keyboard  60 , causing CPU  53  to store the depth data associated with the appropriate rod length in EEPROM  61 . As each additional rod is expended, operator  36  may cause CPU  53  to record the depth data received by receiver  22 . CPU  53  has been preprogrammed by operator  36  via keyboard  60  prior to the boring operation to receive depth data in intervals of expended rods where each rod length is equal to 10 feet. Accordingly, when operator  36  depresses a “SET” key on keyboard  60 , the current depth measurement at CPU  53  is automatically stored in EEPROM  61  and associated with the current cumulative rod number. 
     Thus, as rods are expended and depth data is recorded, depth data associated with selected locations along the bore is compiled. Accordingly, a bore map may be generated at display  34  or, for example, at a personal computer included with monitor  32 , as illustrated in FIG.  10 . The vertical axis of the plot of FIG. 10 indicates feet below ground surface. The horizontal axis provides the length of the bore in the number of rods and rod feet. Thus, at the first extended rod, the bore illustrated was 2 feet deep while at the 10 th rod the bore was nearly 8 feet. Of course, the cumulative data may be presented in a variety of fashions, for example in tabular form. Accordingly, any and all suitable methods of identifying the compiled data should be understood to be within the scope of the present invention. 
     A system including the above described mapping capabilities is the MOLE MAP™, marketed by McLaughlin Manufacturing Company, Inc., 2006 Perimeter Road, Greenville, S.C. 29605. This system includes the capability to change the units at which depth measurements are taken. For example, in programming CPU  53 , keys on keyboard  60  may be used to adjust the length of the rods in a boring system. Thus, by adjusting the rod length utilized by CPU  53 , an operator may configure the system to record depth measurements at a partial rod length or at multiple rods. As the predetermined number of rods are expended, the operator would then press the “set” key on keyboard  60  to record the depth data at that point. Of course, those of ordinary skill in the art should understand that it is possible to create a control system that would automatically record the depth data received from receiver  22  as the rods are expended. As noted above, a map may be generated as in FIG. 10 at display  34  or at a PC included with monitor  32  as indicated in FIG.  6 . The plot data is provided to the PC via driver  62  and RS-232C connector  63  as indicated. Alternatively, monitor  32  may be embodied by a PC device. The information may be provided to the PC in real time as the depth data is recorded by operator  36  via keyboard  60 . Furthermore, a cumulative plot stored in EEPROM  61  may be downloaded to a PC and printer via connector  63 . It should be understood, however, that monitor  32  may or may not include a PC. 
     As discussed above, the present system may be used to map existing utilities. In such a configuration, CPU  53  would be programmed to receive depth data in intervals of actual ground distance. Thus, an operator  38  as in FIG. 1 would trace the existing utility with receiver  22 . As the operator moves away from a starting point, the operator  36  would record depth data on a monitor at predetermined intervals from the starting point. Thus, a map of an existing utility similar to the map shown in FIG. 10 may be generated. However, the horizontal axis would be structured in terms of actual distance rather than rod lengths. Monitor  32 , via CPU  53  or a personal computer, may be configured to merge existing utility plots with a boring system plot. Of course, the horizontal axis of either the boring system plot or the utility plot must be converted so that the maps are compatible. 
     Referring again to FIG. 10, existing utilities running perpendicular to the new bore are indicated. Such utilities are known utility positions which the new bore must avoid. Accordingly, the ability of operator  36  (FIG. 1) to view a bore plot as the bore is being made enables the operator to control the directional boring head by controls  20  to avoid such existing utilities. 
     As noted above, stationary receiver devices may be used in preferred embodiments of the present invention to generate a boring system plot. One method of measuring bore depth with respect to the ground surface in such a system utilizes the pitch angle of the directional boring head. Referring to FIG. 13B, a bore is graphically illustrated beginning at ground level at  70 . The first ten foot rod section is expended at a 45° angle, and, thus, the depth of the bore at the first rod is 7.1 feet as shown. The pitch angle at the directional boring head may change as new rods are expended. In FIG. 13B, the pitch angles at the second, third, fourth, and fifth rods were 30°, 10°, 0°, and 0°, respectively. The depths at these points are 12.1 feet, 13.8 feet, 13.8 feet, and 13.8 feet, respectively. 
     This depth information may be transmitted from a stationary receiver device to a monitor device for use in generating a bore plot as described above. Again, the horizontal axis may be presented either in terms of expended rods (FIG. 10) or in actual ground distance. For example, if a plot were generated from the depth information of FIG. 13B, a depth of 7.1 feet would be marked at 1 rod (or 10 feet if a rod is 10 feet long) while a depth of 12.1 would be marked at 2 rods. If the plot is presented in terms of ground distance, a depth of 7.1 feet would be marked at 7.1 feet from starting point  70  while a depth of 12.1 feet would be marked at 15.76 feet from point  70 . 
     An accurate plot may be generated from the information as in FIG. 13B if the ground surface is substantially level. If the bore is made below a ground surface that is not level, the depth information of FIG. 13B must be modified if an accurate plot is to be obtained. For example, FIG. 13A graphically represents exemplary depth measurements made at each of the rod positions along the bore represented in FIG. 13B by, for example, a portable measurement device as described above. The depth data from FIG. 13B may then be modified to determine the position of the bore  52  with respect to an actual ground surface line  72  as illustrated in FIG.  13 C. The adjustment may be made by a central processing unit such as CPU  53  as in FIG.  6 . 
     It should also be understood by those of ordinary skill in the art that receiver  22  may also be configured to compile the data associated with the selected locations as described above with respect to monitor  32 . That is, monitor  32  may be at least partially embodied by a receiver  22 . This may be particularly advantageous in systems where only a utility plot is desired. In such case, a transmitting source radiates the location signal from the bore as described above, while a single portable unit may be used to receive the location signal, measure the depth, and compile the depth data associated with the selected location. Thus, a single apparatus would encompass the receiver device, measurement device and monitor device. Of course, in a directional boring system as shown in FIG. 1, receiver device  22  may be configured to simultaneously provide the same display as presented to the operator  36  at monitor  32 . 
     These and other modifications and variations of the present invention may be practiced by those of ordinary skill in the art without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims, and that the aspects of varying embodiments may be interchanged in whole or in part.

Summary:
A method for mapping a bore below a ground surface is provided. A boring rod head configured to transmit a signal indicating the pitch angle of the boring rod head with respect to a horizontal plane is disposed within a bore. The boring rod head is advanced within the bore in predetermined linear increments. The pitch angle is measured at each increment. The change in depth is determined at each increment from a prior increment based on the length of the increment and the measured pitch angle at the increment. For each increment, the changes of depth determined in the prior step are added, including the change of depth for the most recent increment. Thus, the depth of the bore at each increment with respect to a predetermined horizontal plane is determined. The depths determined in the prior step are then adjusted by an amount equal to the difference between the predetermined horizontal plane and a ground surface above the bore at each increment so that the adjusted depth at each increment describes the depth of the bore with respect to the ground surface at that increment. The adjusted depth data is accumulated.