The drawing shows a probe in tilted casing. We can overlay the probe with a right triangle. The vertical side of the triangle is the gravity vector. The hypotenuse of the triangle is the gauge length - the distance between the top and bottom wheels of the probe. You can see the tilt angle at the bottom of the triangle. The tilt angle applies anywhere along the length of L. The Digitilt DataMate displays readings in sine units (the sine of the angle of tilt), which also apply anywhere along the length of L. The AT Reader records sine units, but when it displays a reading, it multiplies the sine value by L, the gauge length of the sensor (500mm or 24 inches) to display a reading in mm or inches. This value applies only at the top of the triangle, as in the illustration, which is the elevation of the top wheels of the probe. Thus we measure cable marks from the top wheels of the probe, so that the reading is shown at the proper depth.