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The purpose of this article is to show you how to use alignments and profiles to apply your design criteria in the first stages of grading a site. In the end you will create finished ground points along an alignment.
Contents
Editing the Vertical Alignment
Your site grading strategy usually starts by establishing
high and low points in some linear fashion. You could be grading a road, a
ditch, or a parking lot. This linear technique for grading can be applied to
any of these situations. Just because I use the term linear does not
necessarily restrict you to straight lines. Linear in this context means
following a 
path
such as an alignment. From this alignment you create a profile of the existing
ground. Using profile commands you draw the finished ground geometry on your
profile and use this geometry to define a vertical alignment. Refine your
design in the Vertical Alignment Editor then automatically update your profile
with the new data. Labels are automatically created when you new design is
drawn on the profile.
On your site plan draw centerline geometry and define a horizontal alignment. Place drainage structures where you anticipate having low points. Use the Alignments > Station/Offset > Labels command to label the stations of the low points.
Create an existing ground profile for that alignment sampling the existing ground terrain model. (I’m assuming you already know the processes previously described. I will go into more detail from here on.)
FG Centerline Tangents
The goal for this leg of the journey is to apply your design parameters to produce high points and low points in a profile. Draw the geometry for the FG Profile, referred to as a Vertical Alignment using the Profiles > FG Centerline Tangents > Create Tangents command. This command will ultimately just draw AutoCAD lines on your profile. The Create Tangents command uses the command line to give you feedback in terms of station, elevation, and grade; something you won’t get if you use the AutoCAD LINE command.
When you are doing a conceptual layout of your vertical alignment speed is often more important than accuracy. While you are in the Create Tangents command your cursor position has a rubber band tied to the previous point. You can quickly pick a location on your screen that is near your desired design distance from the previous point by watching the coordinate display (lower left corner). To toggle your coordinate display to polar coordinates use the F6 key. This command has an Undo option which will delete the last segment drawn without canceling the command.
When you pick a point the command line displays the station, elevation, and last grade. You can use the Station option by typing “s” as shown in the command line history below.
| Select point [Station/eXit/Undo/Length]: |
| Station: 4+99.72 Elevation: 22.5589 Last Grade: 0.6244 |
| Select point [Station/eXit/Undo/Length]: s |
| Enter station: 650 |
| Select point [Grade/Elevation/Undo/eXit]: g |
| Grade (%): -0.3 |
| Station: 6+50 Elevation: 22.1081 Last Grade: -0.3000 |
| Select point [Station/eXit/Undo/Length]: l |
| Length: 150 |
| Select point [Grade/Elevation/Undo/eXit]: g |
| Grade (%): 0.3 |
| Station: 8+00 Elevation: 22.5581 Last Grade: 0.3000 |
There are several strategies you can use to create these tangents. They all produce the same result: a profile drawn between the highs and lows. Read the station values from the labels you placed on your plan view to get the low points. Split the difference between low points to establish high points. The command history above shows the use of the Length option with a Grade of 0.3%. Using the Length option is sometime easier than trying to figure out the station of the next point. Enter a positive value when your next PVI (Point of Vertical Intersection) is higher. Enter a negative value when the next PVI is lower.
The Create Tangents command draws lines on the …PFGC layer. You will see why this in important when we get to the next step, Defining a Vertical Alignment.
Use the Profiles > FG Vertical Alignments > Define FG Centerline command to create a vertical alignment from your FG Centerline Tangents.
Immediately you will notice all the layers are shut off except the PFGC layer for the current alignment. This helps you to choose the correct entities for your vertical alignment definition.
The first command line prompt tells you to “Select the starting point:” Pick the first line of your FG Centerline Tangents. Make sure you see the yellow X at the endpoint of this first object. The second command line prompt tells you to “Select objects:” Window the remaining objects to complete the selection of all objects you wish to use for the vertical alignment.
Press ENTER when you are done selecting the objects. Layer visibility is restored and the command line reports the number of CL PVI’s found.
Choose the Profile > Edit Vertical Alignment command to open the Vertical Alignment Editor. Select the Finish Ground Tab and the Center offset. The Vertical Alignment Editor is a feature rich tool. The online help file does a good job of explaining the various parts. At this point you only need to know how to apply your design parameters to produce the high and low points.
Suppose your design parameters call for a 0.3% grade between your highs and lows. Simply edit the values in the Grade Out column. The Station column is where you can refine your station values if they need adjustment.
When you are satisfied with your edits close the Vertical Alignment Editor, save your changes, and import the results back into your profile.
Answer yes to the next two prompts to remove the old linework and import the new profile with labels.
| Label tangents and vertical curves [Yes/No] <Yes>: y |
| Delete finished ground profile layer [Yes/No] <Yes>: y |
You may choose to run through several iterations of edits to your vertical alignment to further refine your design.
Consider how you will be using the finished ground points. You want these points to be convenient to work with as a group. You want these points to be consecutively numbered so you can easily create a breakline from a point number range. You may also wish to snap to these points while creating 3D geometry.
Use the Points > Point Utilities > List Available Point Number command to determine a range of available point numbers. You will see a range of point numbers like the list shown below. These are un-used points.
Command: 139-147,149-499,1071-1099,1213+
You will be using these points for grading purposes so set the Point Settings to insert points to their Actual elevation. Choose the Points > Point Settings command. Select the Insert Tab and pick the radio button next to Actual Elevation. Points will be inserted at their true elevation. You do not need to change the current point number at this time.
Choose the FG Vertical Alignments > Create COGO Points command to create points along your current alignment.
The Centerline Point Output dialog box is displayed. Set the Centerline increment to your desired level of detail. Change the Start station value to a whole number so your increments will fall on even station values and not something like 53.61…103.61…153.61. You may also wish to change the Point description field to a value which matches a Description Key code rather than using the default Point description CLPT. This also has the advantage of automatically assigning the new points to a pre-defined Point Group.
Choose a starting point number to allow for a consecutive range of point numbers.
Current point number <1235>: 1500
Points are created in the drawing and in the project where their elevations are derived from the vertical alignment.
You may wish to use the Points > Create Points – Slope > High/Low Point command to set points at the exact high/low location which may occur at odd stations.
A key ingredient to grading is the breakline. Breaklines are 3d polylines used to control the location of triangle edges in a terrain model. These 3d polylines are also useful when creating grading objects.
To quickly create a breakline from the profile points you created earlier open Terrain Model Explorer. Right click on Breaklines under the Surface of your choice and choose Define by Point Number. Enter your point number sequence at the command prompt. At this stage of the grading process the breakline definition in the surface is not as useful as the resulting 3d polyline.
As a side note to creating breaklines in the previous step: The additional points you created using the High/Low command will not be in the numbered sequence as the rest of the profile points are. Here are a few options which will include the High/Low points in your resulting 3d polyline.
The Alignments > Station/Offset > Display Points can help you develop a point number list sorted by increasing station number. Just open a Notepad session and edit the output from this command to create a point list which you can paste onto the command line during the Define by Point Number command for the breakline. In the example below you see the results from the Display Points command. The point number range 1550-1555 would become 1550-1552,1558,1553-1555 to include the High/Low point added later.
Command:
Alignment Name: CL01 Number: 2 Descr: Perimeter
Starting Station: 0.00 Ending Station: 2846.67
Sort the points by station [Yes/No] <Yes>:
List points by [Selection/Number] <Selection>:
Select objects: Specify opposite corner: 7 found
Select objects:
Point Station Offset Elevation Description
--------------------------------------------------------------------------------
1550 25+00 0.00 22.62 (PCL) PCL
1551 25+50 0.00 22.47 (PCL) PCL
1552 26+00 0.00 22.32 (PCL) PCL
1558 26+24.85 0.00 22.25 (PCL) PCL
1553 26+50 0.00 22.33 (PCL) PCL
1554 27+00 0.00 22.48 (PCL) PCL
1555 27+50 0.00 22.63 (PCL) PCL
Who needs all those points anyway? When you use the Create COGO Points command there will be several points all defining the same grade along a tangent. Why not Erase one of those points and renumber the High/Low in its place? There is a good chance that there will be an expendable point very near your High/Low point. Now you still have an easy to use point number range in the correct sequence.
As much as I dread tedious tasks you may find that just picking the points in the correct sequence causes the least amount of stress. In this case you can just use the 3DPOLY command, set your running OSNAP to NODE, and pick away. If you did not want to create a breakline in the first place you have avoided that wasted effort.
The grading process requires you to start somewhere on your site and then sequentially build your design elements from that starting point. Creating design points from a profile is a quick way to establish a baseline and cover a large distance across your site. In one process you can incorporate changes in slope and direction along this baseline. It is easy to edit the profile and run another iteration to explore another design option without abandoning the first scheme. Although originally intended to design roads the profile features in Civil Design can be creatively implemented to help grade other site features all because of this one command Profiles > FG Vertical Alignments > Create COGO Points.
Brian Morse is a certified Autodesk instructor specializing in Land Desktop. He does training, consulting, contract drafting and design for Land Surveyors, Civil Engineers, Planners and Architects.
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