Helper geometry is used to create working references over the source.
Helper cross
Places the intersection of a horizontal and vertical helper line.
Helper line
A line is defined by two points and can have any direction. With Ortho enabled, you can easily create an exactly horizontal or vertical line.
Snapping
Calibration and measurement points can snap to helper lines and their intersections. Helper geometry can therefore act as a precise construction grid without becoming part of the measurement itself.