Guidance is achieved using the following process:
- The GPS receiver initially sends the information about the position of the machine to the Display.
- The software for the Display then transforms this GPS position to a position on the project design
- The software for the Display applies the measurements of the machine and the mast in order to compute the final positions at the edges of the base of the drum.
- The machine position is displayed on the design map.
- The actual elevation of the drum is also compared to the grade of the design at each location in order to provide the cut or the fill at the edges of the drum.
The following terms are associated with this method of guidance:
- Heading
- Orientation
- Mainfall
- Cross slope
The heading is the direction of travel of the machine. The heading is measured clockwise in degrees from "Grid North". This is North in the project design coordinate system. North equals Zero degrees and South equals 180 degrees.
The heading is calculated from successive positions of the antenna as the machine is moving. The machine must be moving in order for the heading to be calculated correctly. Guidance is not provided if there is not enough information to calculate the heading. A "Direction Unknown" message appears if the heading cannot be calculated.
Note: When the signal from the GPS is lost, or when there are insufficient GPS positions to compute the direction of movement, a "Low accuracy (Move)... "message flashes on the AccuGrade Display. If this message appears, ensure that there are sufficient satellites to compute a position. If there are sufficient satellites, then move the machine forward until the messages disappears.
Orientation is the "forward looking" direction that the drum is pointing. The orientation is shown in degrees clockwise from grid (project) North. When the machine is moving forward, the orientation is the same as the heading. When the machine is moving in reverse, the orientation is 180 degrees opposite from the heading.
The mainfall is the angle of the machine in the forward and aft direction. This angle is relative to level, or horizontal, which is considered to be zero degrees. Mainfall is also sometimes called the "pitch" of the machine or the "slope" of the machine.
The mainfall is expressed as a percentage of slope or as "Rise:Run". The displayed unit depends on the units that are configured for "Grade" in the installation menu.
The mainfall is considered to be positive when the angle is above the horizontal, or when the machine is pointing upward. The mainfall is considered to be negative when the angle is below the horizontal, or when the machine is pointing downward.
The mainfall may become inaccurate if one of the following occurs:
- The AccuGrade system has started, but the machine has not yet moved.
- The AccuGrade system is reporting "Low accuracy GPS".
- The compactor turns sharply.
- The slope of the compactor changes drastically over a short distance.
The cross slope is the "cross fall" of the drum perpendicular to the machine heading. A positive cross slope rises to the right. With respect to the left edge of the drum, the cross slope is considered to be positive when the right edge of the drum is high. The cross slope is considered to be negative when the right edge of the drum is low.