Unit 3 – Lesson 3: Create Alignments Civil 3D 2010 Student Workbook ▪ 1 Horizontal Alignments Overview In this lesson, you learn how to create subdivision road alignments from AutoCAD® entities such as lines, arcs, and polylines. Alignments are a critical component of all subdivision and roadway projects that have linear corridor design elements such as residential and collector roads. Alignments can also be used with creeks and rivers for floodplain analysis and channel design. The following illustration shows two intersecting alignments: Lesson 3 AutoCAD Civil 3D 2010 Education Curriculum Student Workbook Unit 3: Land DevelopmentUnit 3 – Lesson 3: Create Alignments Civil 3D 2010 Student Workbook ▪ 2 Objectives After completing this lesson, you will be able to: - Describe alignments and their properties. - Create alignments using objects. - Describe alignment tag labels. - Label alignments and create a table. Exercises The following exercises are provided in a step by step format in this lesson: 1. Create an Alignment 2. Label an Alignment About Alignments An alignment is a linear feature in the horizontal plane. An alignment is typically used for features such as road centerlines, pavement edges, and drainage lines. Alignments can represent existing features or proposed features. When used for roadway design, criteria exist to regulate the geometry of alignments based on a variety of factors including roadway classification, design speed, cross slope, pavement type, and many other items. The alignment also serves as the controlling geometry for the layout and construction of the road. Detailing the particular geometric points of alignments is critical in order for the design engineer to provide clear communication of the engineering data for the client, reviewers, and contractors. Alignment labels and styles are effective tools for providing this communication. Alignments are the first of the three primary design planes to be laid out and used by the designer. Profiles and cross sections provide the other two design planes necessary for a full three-dimensional description and modeling of proposed roadways. The Layout Process The subdivision layout process is an iterative process where a developer, or the developer's engineer, strives to maximize the use of the land based on zoning, parcel layout, and road design criteria. In many circumstances, the parcel outline for the subdivision is designed first, and then handed off to a designer who is tasked with designing the roads in the subdivision. Road designers often offset parcel right-of-way lines, polylines, and arcs to create the horizontalUnit 3 – Lesson 3: Create Alignments Civil 3D 2010 Student Workbook ▪ 3 alignment geometry for the subdivision roads. This geometry is converted to polylines, which are then used to create alignment objects for the subdivision roads. Alignments are a series of coordinates, lines, curves, and spirals used to represent the centerline of linear features such as roads, edges of pavement, sidewalks, and rights-of-way. Alignments can also be used to represent the centerline of a railway, channel, or stream. Alignments Example Horizontal alignments in subdivisions are usually not very complex and consist mostly of tangents and curves. In some instances, lane tapers are modeled using alignments to create acceleration, deceleration, and turn lanes at intersection locations. Subdivision road centerline alignments are most often created by offsetting right-of-way lines by half the width of the right-of-way. Common commands such as Trim, Edit, Extend, and Fillet are used to create the alignment geometry from AutoCAD entities. Once the geometry is in place, the Polyline Edit command can be used to join the lines and arcs together to form a continuous polyline representing the alignment. The direction of the polyline does not matter because once you create the alignment, you can reverse the direction of the alignment. You can also create alignments from AutoCAD line and arc entities. Alignments for residential subdivision roads are shown in the following illustration. Keep the following guidelines in mind when you create alignments: The direction of the polyline is not important, as you can reverse the direction of an alignment during the alignment creation process. You can also reverse the alignment direction after it has been created. When you create an alignment from a polyline with no curves, or from lines, you can automatically add curves between the tangents.Unit 3 – Lesson 3: Create Alignments Civil 3D 2010 Student Workbook ▪ 4 You can assign a value to the starting station of the alignment, which is the start point of the polyline, line, or arc. Alignment station reference points and base stationing values can be adjusted later. Alignments can either be independent or included in a site. Use alignments in a site if you want them to interact with other objects in the site, or if you want to use sites to organize the alignments. Labeling Alignments Horizontal alignments are made up of segments, which are lines, arcs, or spirals. There are a number of powerful labeling tools in AutoCAD® Civil 3D® software for labeling horizontal alignment geometry, either on the alignment itself or in a table. When you edit or change an alignment, associated labels and tables automatically update to reflect the new alignment geometry. The following illustration shows alignment geometry with segment labels. When plans become difficult to read because of too many geometry alignment labels, you can create tag labels for the alignment segments and show the geometry in a corresponding table. After adding tag labels, you create an alignment table that references the tags. You can create a line, curve, spiral, or segmental table that shows the geometry for the entire alignment. The table can be dynamic. When you edit the horizontal alignment or change the station reference point, the data in the table automatically updates to reflect the new geometry. To create tag labels, you select a tag label style from the Add Labels dialog box as shown.Unit 3 – Lesson 3: Create Alignments Civil 3D 2010 Student Workbook ▪ 5 Definition of Alignment Tag Labels There are a number of different label types that you can add to a horizontal alignment. To label alignment geometry, you can either label single segments or multiple
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