qtdragon_hd -docs: add info on workpiece height probing

Author: c-morley

docs/src/gui/images/qtdragon_hd_workpiece_probe.png

@@ -482,6 +482,9 @@ setp axis.z.eoffset-scale 1.0
 QtDragon_hd can be set up to probe and compensate for Z level height changes 
 by utilizing the external program 'Gcode Ripper'.
 
+[NOTE]
+This is only available in the QtDragon_hd version.
+
 Z level compensation is a bed levelling/distortion correction function 
 typically used in 3D printing or engraving. It uses a HAL user space 
 component which utilizes the external offsets feature of LinuxCNC. The 
@@ -508,6 +511,7 @@ image::images/qtdragon_hd_gcoderipper.png["QTDragon gcode Ripper"]
 
 [NOTE]
 Gcode Ripper offers many functions that we will not go in to here.
+This is only available in the QtDragon_hd version.
 
 * In Qtdragon_hd switch to the file tab and press the load Gcode Ripper button.
 * set origin to match the origin of the gcode file to be probed
@@ -673,6 +677,63 @@ There are several videos on you tube that demonstrate the technique using
 Gmoccapy.
 The Gmoccapy screen pioneered the technique.
 
+=== Work Piece Height Probing
+
+.QtDragon_hd - Work piece Height probing
+image::images/qtdragon_hd_workpiece_probe.png["QtDragon_hd height probing"]
+
+This program probes 2 user specified locations in the Z axis and calculates the difference in heights.
+
+[NOTE]
+This is only available in the QtDragon_hd version.
+
+Enable Probe Position Set Buttons
+
+* when checked, the SET buttons are enabled
+* this allows the user to automatically fill in the X, Y and Z parameters with the current position as displayed on the DROs.
+
+Autofill Workpiece Height on Main Screen
+
+* when checked, the calculated height is automatically transferred to the Workpice Height field in the main screen.
+* otherwise, the main screen is not affected
+
+Workpiece Probe At
+
+* the X, Y and Z coordinates specify where the first probing routine should start, in current WCS
+
+Machine Probe At
+
+* the X, Y and Z coordinates specify where the second probing routine should start, in current WCS
+
+Z Safe Travel Height
+
+* the machine is raised to the Z safe travel height before jogging to the X and Y coordinates
+* the spindle then lowers to the specified Z coordinate
+* it should be selected so that the tool clears all obstructions while jogging
+
+START button
+* the machine will jog to the first location and then probe down
+* the machine then jogs to the second location and probes down again
+* the difference in probed values is reported as Calculated Workpiece Height
+* the parameters for search velocity, probe velocity, maximum probe distance and return distance are read from the main GUI Settings page.
+
+ABORT button
+
+* causes all jog and probe routines currently executing to stop
+
+HELP button
+
+* displays this help file
+
+[NOTE]
+Any 2 points within the machine operating volume can be specified.
+        If the first point is higher than the second, the calculated height will be a positive number.
+        If the first point is lower than the second, the calculated height will be a negative number.
+        Units are irrelevant in this program. The probed values are not saved and only the difference is reported.
+
+[CAUTION]
+Setting incorrect values can lead to crashes into fixtures on the machine work surface. Initial testing with no tool and safe heights is recommended.
+
 === Tool Measurement Pins
 
 Versaprobe offers 5 pins for tool measurement purpose. The pins are used