1.  Field of the Invention
This invention relates generally to railroad maintenance and, more particularly, to methods of surveying railroad track beds in conjunction with position recording and performing track maintenance operations, such as ballast replacement, based on the survey results. 
2.  Background & Description of the Related Art
Conventional railroads in the United States and elsewhere are formed by a compacted sub-grade, a bed of gravel ballast, wooden cross-ties positioned upon and within the ballast, and parallel steel rails secured to the ties.  Variations of construction occur at road and bridge crossings, at switch points, and in other circumstances.  The ballast beneath and between the ties stabilizes the positions of the ties, keeps the rails level, and provides some cushioning of the composite structure for loads imposed by rail traffic.  Vibrations from the movement of tracked vehicles over the rails and weathering from wind, rain, ice, and freeze and thaw cycles can all contribute to dislodging of some of the ballast over time.  Thus, in addition to other maintenance activities, it is necessary to replace ballast periodically to maintain the integrity and safety of railroads. 
Ballast is usually spread using specially configured ballast hopper cars which include a hopper structure holding a quantity of ballast, a ballast chute communicating with the hopper, and a motorized ballast discharge door in the chute.  The door can be controlled to selectively open or close to control the discharge of ballast.  In some designs, the discharge door can be controlled to open outboard toward the outside of the rails, to close, or to open inboard toward the inside between the rails.  In other configurations, a center door and outer doors may be provided for each hopper.  Typical ballast hopper cars have a front hopper and a rear hopper, and each hopper has two transversely spaced doors, one to the left and one to the right.  Thus, each hopper door can be controlled to discharge ballast outside the rails on the left and/or the right or between the rails.  A typical configuration of a ballast hopper car is described in more detail in U.S. Pat.  No.  5,657,700, which is incorporated herein by reference. 
In general, ballast spreading has been controlled manually in cooperation with spotters who walk alongside the moving ballast cars to open or close the ballast doors as necessary.  A more recent ballast spreading control technique is by the use of a radio linked controller carried by an operator who walks alongside the moving ballast cars.  Both conventional control methods are so slow as to disrupt normal traffic on the railroad section being maintained, thereby causing delays in deliveries and loss of income.  Because railroad companies typically maintain hundreds or thousands of miles of track on a recurring schedule, the ballast replacement component of track maintenance alone can be a major undertaking in terms of equipment, materials, traffic control, labor, and management.  In the past, estimations of the amount of ballast to be spread along tracks were based on inspection and experience.  More recently, automated systems for scanning a track bed and determining amounts of ballast to be replaced have been developed, such as described in U.S. Pat.  No.  6,976,324, which is incorporated herein by reference.  Methods for spreading railroad ballast with location control based on data received from the global navigation satellite system (GNSS), also commonly known as the global positioning system or GPS, are disclosed in U.S. Pat.  Nos.  6,526,339 and 7,152,347, which are incorporated herein by reference.