XGRD

Datum-adjust elevation of selected Cogo points, based on FG profiles in horizontal alignments and gradually-changing cross-slope.

You may also indicate a reference point by picking a point and keying in an elevation, if there is no Cogo point at the desired point.

Usage:

The XGRD command is similar to the PT2ALIGN command. Like the PT2ALIGN command, the XGRD command datum-adjusts existing Cogo points, based on a Finish Grade profile of an alignment. But while the PT2ALIGN command requires you to enter a specific cross-slope, the XGRD allows you to pick two points. These two points are used to calculate a starting station and start cross-slope, and an ending station and cross-slope.

For points before the starting station, the cross-slope at the starting station is used to calculate the elevation of the point. For points after the ending station, the cross-slope at the ending station is used. For points between the starting and ending stations, the cross-slope is pro-rated. Then the elevation of the point is calculated by applying the calculated cross-slope to the elevation of the FG profile at that station.

In the dialog box, select the alignment and FG profile to use to calculate the grade. The other values in the dialog box are as follows:

Datum Adj

This is a vertical datum adjustment applied to the profile grade for calculations. In the diagram below, the datum adjustment would be the sum of y + z.

Grading Offset

This is the distance between the Cogo point and the grading point. This distance is indicated by x in the diagram below.

The grade g% is projected out to the grading point from the profile line. In the example below, the Cogo point is an offset to the top back of curb. The grade at this point is a set distance above the lip of the curb (the grading point).

If the selection point is further from the profile line than the grading point, then x should be positive. This is true, even on the left side of the profile line. This lets you select points on both sides of a standard road without changing the settings.

If there is no Cogo point at the desired reference location and/or elevation, you may also type "P" at the prompt to specify a point. After typing "P", pick a point on the screen, and then type in the elevation to use.

Example 1:

Consider the diagram below. Assume the bottom line is an edge of pavement for a roadway, while the alignment runs down the roadway centerline. The FG profile for the roadway centerline is a stright grade from Station 0+00 to Station 5+00. The cross-slope of the roadway is falling at a 2% grade before Station 1+00, and the cross-slope of the roadway is flat at 0% after Station 4+00. The grade of the FG profile is indicated at various points along the alignment for reference.

Points 1 and 2 also have their grades already calculated, possibly with the PT2ALIGN command. We wish to calculate grades for the remaining points.

Start the XGRD command. Select the alignment and FG profile to use. For this example, we are using 0.00 for both the Datum Adjustment and the Grading Offset. Hit OK to exit the dialog box. At the prompts, pick Points 1 and 2. You may pick the points in either order. You should see the cross-slope at both starting and ending stations indicated in the command window, followed by a prompt to select the points to change. Select the points you wish to grade. The results are as follows:.

Notice that since Point 6 is not in between our selection points (Points 1 and 2), the cross-slope used to calculate its grade is the same for Point 1. Likewise, the cross-slope used for point 7 is the same as the cross-slope for point 2.

Example 2:

The following diagram indicates the same basic principle as Example 1, but the FG profile contains a vertical curve. The vertical curve can be noticed by inspecting the grades along centerline. However, the XGRD program works in basically the same way. The grade for Point 3 is still down at 1.45% from the centerline grade, except now the centerline grade is lower because of the vertical curve.

Example 3:

The following diagram is the same as Example 2, but now the points are 10-foot offsets to the edge of roadway. For this example, the Datum Adjustment in the XGRD offset is set to "10.00". Notice that all points have the same grade as in Example 2, even though they are now 10-foot offsets.

Example 4:

XGRD will also project grades to the opposite side of the road. Consider the following diagram. Again, the alignment is running down the centerline of the road, a straight-grade from Station 0+00 to Station 5+00. The roadway transitions from 2% cross-slope draining to the left at Station 1+50, to a 2% cross-slope draining to the right at Station 4+00.

The grades for the FG Profile at the centerline points (Points 100, 103, 108, and 110) are shown for reference. The elevation for Points 114 and 130 are set as indicated.

Now, to calculate grades for all points, start the XGRD routine. Select the alignment and FG profile, and enter 0.00 for both Datum Adjustment and Grade Offset. At the prompts to select your grading reference points, select Points 114 and 130 in either order. At the "Select points to change" prompt, select all the points. The result is as follows:

Note that the grade was projected across centerline.