Pipe and cable locators are the most common instruments for both detecting and tracing underground utilities. There are many manufacturers of this type of equipment, and hundreds of separate pieces of equipment. Variations in equipment include antenna size, antenna shape, number of antennas, frequencies of antenna output, thresholds of signal output (automatic or controlled gain), types of signal displays, types of various attachments, indirect depth measurement indications, current flow direction indications, signal strength displays, and shape, size, and weight of transmitters and/or receivers. All pipe and cable locators have one or more receiving antennas, and, if they are an active device, they will also have one or more transmitting antennas. The size, shape, and type of antenna is directly related to its efficiency in receiving a signal of a certain shape and frequency. There are now many instruments available in the general frequency ranges of 60 Hz, 512 Hz, 1 kHz, 8 kHz, 29 kHz, 33 kHz, 80 kHz, 83 kHz, 110 kHz, 250 kHz, 300 kHz and 480 kHz. It may be necessary to have equipment in each one of these available frequency ranges in order to effectively detect and trace any particular utility, although in some cases, a single frequency might be all that is needed, depending on the complexity of the local geology, congustion and the attributes of the particular utilites one is attepting to locate. Pipe and cable locators are used for both detection and tracing of utilities. Depending upon frequency and method of signal generation, these methods can be effective up to 20 feet in depth, although, more frequently, their maximum depth of effective detection is morelimited. One method of increasing effective operating depth is the pulling of a sonde through the utility. While increasing the effective depth of detection to 50 feet or more, drifting of the peak signal at the surface from the true alignmet of the utility occur with depth. Depth estimation with pipe and cable locators is possible, but fraught with the possibility of error due to site conditions. Under ideal conditions (a single conductor in a relatively homogeneous soil with a recently calibrated instrument), depths can be quite precise and accurate. Some manufacturer specifications state a 2.5% - 5% accuracy up to 10 feet of depth. In the congested utility arena of an urban or suburban street, depth estimations are frequently in error by a significant amount.