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CHAPTER 10

Horizontal Alignment Layout

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Civil Drafting Technology, 7thedition

Madsen, Shumaker, and Madsen

2010 Pearson Higher Education,

Upper Saddle River, NJ 07458. All Rights Reserved.2

Horizontal Alignment Layout

Geometrically designing the shape of a

roadway within the horizontal plane.

The configuration of a roadway in plan view.

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Plan Layout Defining the location, route, extents and characteristics of a

proposed highway or road. Typical layout steps:

1. Analysis of contour maps and aerial photos.

2. Road designers, engineers, and government officials determinethe location (centerline) of the proposed road using maps andfield studies.

3. Surveyors locate the centerline and extents of the proposedroad.

4. Drafters use field notes to plot the road on a contour map(plan).

5. Drafters generate a profile and additional details based on theplan view.

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Plan Layout Initial survey:

Surveyors use route survey to establish the centerline of theproposed road.

Route survey surveying for a linear feature such as a road,pipeline, or power lines.

Bearings, distances and station points are recorded. Additional surveys:

Right-of-way (easement). Width of the road.

Profiles (cross sections). Additional details.

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Plan Layout

Provides horizontal (plan view) information. Drawn on existing or new contour maps. Shows trees, fences, buildings, other roads, and

cultivated areas. Identifies the centerline, or transit line, of the proposed

road. Display the extents (width) of the proposed road. Distances and bearings of proposed road centerline.

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Plan Layout

Option 1: Station points

Curve data

Option 2: Points of intersection

Curve radius

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Plan Layout: Station Points and Curve Data Proposed road centerline. Station points:

The proposed road route is divided into 100 foot increments. Curve data:

Point of curve (P.C.) the point at which a curve begins. Point of tangency (P.T.) the point at which a curve ends. Point of reverse curve (P.R.C.) two intersecting curves. The

point where one curve ends and another begins.

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Plan Layout: Station Points and Curve Data

Curve data:

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More curve data: Radius (R) the curve radius. The radius center point is

perpendicular to the centerline of the road.

Delta angle (

) the included angle between the P.C.and P.T. Curve length (L) - the centerline length of the arc from

the P.C. and P.T.

Degree of curve (D) the angle between station pointsalong the centerline.

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Plan Layout: Points of Intersection and

Curve Radius. Proposed road centerline. Points of intersection (P.I.):

The junction of two centerlines of different bearings. Used to define the location of curves.

Curve construction: Draw a tangent, tangent, radius circle. Trim the circle to form an arc.

OR Offset centerlines using the curve radius as the offset distance. Draw an arc tangent to the centerlines, using the offset

intersection as the arc center point.

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Plan Layout: Points of Intersection and

Curve Radius.

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Plan Layout with a CADD System

Similar to the manual drafting process.

Curve lengths, bearings, and distances are entered

when drawing the transit line.

A digital terrain model is generated for the project

area. When the layout of the proposed alignment is

established, a profile of the existing conditions can

be automatically generated.

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Chapter 10 Activities

Chapter 10 Test

Chapter 10 Problem P10-4

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