When operating a 2D or 3D grader system, the slope is measured across the A-frame draw bar, which is highlighted by the two red dots in the image. The left and right lift cylinders are attached to the draw bar.
Since the slope is measured across the draw bar, it is important to keep the alignment of the draw bar perpendicular to the slope.
Articulating or swinging the A-frame to the right or left will change the alignment of the draw bar and create a grade error.
If the machine is articulated, the operator will need to shift the A-frame to align the draw bar with the desired direction of slope.
When grading in 2D mode, the draw bar alignment will create a slope error.
When grading in 3D mode, the draw bar alignment will create both a slope error and elevation error, as the machine will have a heading error.