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Location systems and on-vehicle computers are available today. I built SITE CONTROLLER, an example of the critical missing component, in order to underline the fact that computer-aided earthmoving is feasible today. SITE CONTROLLER is a Common Lisp program that assists civil engineers in the design, estimation, and construction phases of earthmoving projects. The core of SITE CONTROLLER is a site modelling system that represents the following: existing and prospective terrain shapes; road and property boundaries; hydrology; important features such as trees and utility lines; and general user notations. Around this core are analysis, simulation, and vehicle control tools.
Terrain is represented by surfaces interpolated through triangulated sets of points of known elevation. The most basic surface model simply postulates a plane for each triangle, resulting in a faceted surface like that of a cut gemstone. When smooth contour lines are desired, a C1 surface model is computed by making each triangle a nine-parameter polynomial surface and matching up first derivatives at triangle borders. Existing topography can be entered in the form of surveyed points while prospective topography is captured from user-sketched contour lines.
Analysis software consists primarily of procedures that calculate the interaction between existing and prospective surfaces: Where would one have to excavate and fill to achieve a specified roadbed? How much earth would have to be trucked onto the site to build this foundation? What would the site look like after certain design ideas have been adopted?
Simulation software allows the user to plan earthmoving activities, then watch them progress in simulation and see the evolution of terrain shape. This provides a basis for accurate estimation of construction costs. Hand-in-glove with the simulation software is real-time vehicle communication and control. Site model information to aid equipment operators is sent over radio modems while vehicle location and operation information (e.g., engine temperature) comes back. SITE CONTROLLER keeps a record of activity so that any particular operation may be replayed on-screen in the future.
My experience building and using SITE CONTROLLER provides valuable lessons for the future:
* a substantial percentage of bugs may be traced to the use of floating-point arithmetic and untyped geometric operations. Reliable systems will require formal methods of error control or exact arithmetic and modern approaches to geometric programming.
* high-level programming languages, such as Lisp, and the best available software development tools allow a practical system to be built with only moderate effort.
* reasonably intelligent software decomposition with a modern object system provides enough flexibility for extensions into unforeseen areas of civil engineering.
* state-of-the-art computational geometry algorithms are barely adequate for coping with the constantly evolving surfaces created by operating bulldozers. I present one solution to the problem of representing surfaces cut by bulldozers.
* comprehensive site models are the key to making a useful system and one must be careful to avoid the pitfalls of computer-aided design (CAD) systems where raw geometry is used to represent real-world objects that have important non-geometrical characteristics.
* integrated computer-aided civil engineering systems will strain object database systems to their limits in the areas of performance, version control, synchronization and integrity.
Civil engineering and construction are enormous areas of human endeavor that have remained almost completely unsupported by computers. SITE CONTROLLER proves that a revolution affecting hundreds of billions of dollars of projects might be achieved by a small group of computer scientists.