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The following article attempts to explain the various inputs on the Overlay Vertical Adjument command.
"Specifies the thickness (vertical distance) from the profile grade line (PGL) elevation to the bottom of the backbone components." This value will be added to the centerline elevation calculated by finding the Minimum Milling/Overlay control point. Each of the 4 images below have a Backbone Thickness defined.
Minimum Mode = Minimum Overlay
"When this option is specified, the surface is evaluated for the point that is, vertically, nearest to the bottom of the backbone." The elevation of this control point is then used to calculate the elevation at the centerline, following along the bottom of the backbone.
In the image below, a red circle represents the point on the surface nearest the backbone. The yellow dashed line represents the bottom of the template placed on that controlling point. The Elevation of the new Adjusted Vertical is created by adding the Minimum Overlay value plus the Backbone thickness, at the location of the centerline.
The image below shows a Minimum Overlay value of 0.0', with a backbone thickness defined.
Minimum Mode = Minimum Milling
"When this option is specified, the surface is evaluated for the point that is, vertically, farthest from the backbone. The vertical adjustment is then made to place the backbone directly on that control point."
Minimum Milling with "Use Maximum Milling" toggle turned ON.
The image below shows a template placed using Minimum Milling with a Maximum Milling value specified. Without the Maximum Milling value, the template would be placed on the red control point, however, placing the template at that point would violate the Maximum Value at the location shown (and to the area to the right). Therefore the template is placed instead at the point of Maximum Milling depth.
Maximum Vertical Difference
If this value is greater than 0.0, the program will compare the elevation of the Adjusted Vertical at the current station against the elevation calculated at the previous station, and will ensure the difference in the two elevations does not exceed this value.