May : Greenways survey CADD operators create a “Surface file”, a commonly used name for a DTM (digital terrain model). The DTM is a representation of an actual three-dimensional surface. It is created as a TIN (triangulated irregular network)—a mesh of flat triangles with sides of varying length. When creating the TIN surface, the computer program looks at entities that contain x, y, and z position information. These are generally contours on a grading plan because it is the current convention for expressing a grading concept. Point data and spot elevations are also included in the file.
This model is then used in modern GPS controlled grading machinery to more efficiently and more accurately perform a grading job. Satellites send positioning data to a GPS antenna/receiver base station. The base is located at a known stationary point on the site. At the same time, positioning data are also sent to the “rover” on the machine—a rugged GPS antenna mounted to a shock-absorbing, vibration-damping pole and a receiver box mounted in a secure location. The stationary base and mobile rover work together to provide RTK (real-time kinematic) position information and accurately determine the machine’s three-dimensional location on the site.
Software in the control box processes these position data and compares them to the design grade at that specific location. Data files, provided by Greenways GPS professionals, is loaded in the control box on a compact flash card, providing the design grade information.