1. Alignment

An alignment is a three dimensional refernce line consisting of a series of straight and curved segments. It  serves as a reference for linear projects such as roads, pipelines, transmission lines, etc. Most alignments have horizontal and vertical components, Figures A-1 and A-2, each related but having their own geometric elements and specific design constraints.

 gen 01

 Figure A-1
Horizontal Component

 gen 02
 Figure A-2
Vertical Component


Although most contemporary alignment design is done digitally in three dimensions, it is still important to understand the geometry of the individual components.
The remainder of the chapter will concentrate on geometry for road alignments.

2. Stationing

A station is both a dimension and a position. As a dimension it has a fixed length with associated unit. As a position it is the cumulative horizontal distance along the alignment from its beginning.

In the English system, a station is defined as the length of a standard surveyor's tape making it 100.00 ft long. A position is expressed as the number of full stations (100 ft intervals) plus a partial station (less than 100 ft) and is written in the full+partial format. Station 8+35.67 is 835.67 feet along the alignment from its origin. Alignments do not usually start at station 0+00 because subsequent redesign can result in negative stations: -1+45.67 looks confusing, doesn't it?

In the metric system, a station may be 100 m or 1000 m with a corresponding partial station. A point 12,56.02 m along the alignment would be at station 12+26.02 or 1+256.02. We'll use the English system although the same basic logic applies to metric stationing.

Alignments are traditionally staked at full station (100 ft) intervals and changes in horizontal direction, Figure A-3. In some cases, such as trenching, stakes can be placed at half- or quarter-station intervals (50 ft and 25 ft intervals, respectively).

 gen 03
Figure A-3


The horizontal distance between two stations along the alignment is their stationing difference.

 gen 04
Figure A-4


In Figure A-4, the distance from sta 12+38.28 to sta 16+56.85 is:

gen 04b

The distance can be expressed in feet or stations.

The position of a point off the alignment is expressed by a station and an offset to the right or left (defined looking upstation). The position of the CP in Figure A-5 is 11+68.26 82.40R

gen 05
Figure A-5
Station and Offset


A single point can sometimes have two different stations values. The most common situation is where two different alignments cross, Figure A-6.

gen 06
Figure A-6
Station Equation


Point A is at station 12+83 along one alignment and station 26+45 along a second. A Station Equation can be written for point A: Sta 12+83 = Sta 26+45. This may be brought to the readers' attention on a set of plans to ensure the appropriate station value be used for alignment calulations. Another station equation situation can occur in horizontal curve computations and will be covered in that section.

3. Grade

a. Definition

Grade is the slope of a straight line and is computed from:

eqn 01       Eqn A-1
elev diff: elevation difference
horiz dist: horizontal distance


If the elev diff and horiz dist are both in feet (or meters) Equation A-1 results in grade as a dimensionless ratio; multiplying it by 100 converts it to a percentage (%).

If elev diff is feet and hori dist is stations, Equation A-1 returns grade as a percentage.

Grade is positive (+) going uphill in the direction of stationing, negative (-) going downhill, Figure A-7.

gen 07
Figure A-7


b. Examples

(1) The elevation at sta 10+25.00 is 1214.80 ft; at sta 12+75.00 it is 1193.50 ft.

What is the grade of the line between the two stations?

First, draw a sketch:

gr ex1

Using Equation A-1:

gr ex1b

(2) At sta 16+50.00 the elevation is 867.50 ft. If the grade through 16+50.00 is +3.00%, what is the elevation at sta 19+00.00?


gr ex2

We need to rearrange Equation A-1 to solve for the elevation difference between the two stations:


 The horizontal distance is  gr ex2c 
 and elevation difference is   gr ex2d
 and elevation at 19+00.00   gr ex2e


According to the sketch, 19+00 is higher than 16+50 so the computed elevation looks correct

(3) Using the data in example (2), what is the elevation at station 14+00?


gr ex3

The horizontal distance is:

gr ex3 dist

Since we're going backwards along the alignment, the distance is negative.

The elevation is:

gr ex3 elev

From the sketch we see that 14+00 is lower than 16+50, so the computed elevation looks correct.




1. Nomenclature

A vertical curve is used to provide a smooth transition between two different grade lines, Figure B-1.



Figure B-1
Adding a Vertical Curve


The parts of the curve, Figure B-2, are:

Figure B-2
Curve Parts


PVI Point of Vertical Intersection (aka PI)
BVC Begin Vertical Curve (aka, BC, PVC, PC)
EVC End Vertical Curve (aka EC, PVT, PT)
L Curve Length


Distances, including the curve length, are horizontal, not along the grade lines or curve.

An equal tangent vertical curve is used. This places BVC and EVC equidistant from the PVI, Figure B-3.

Figure B-3
Equal Tangent Vertical Curve


The curve is tangent to the grade lines at both ends to provide a smooth transition between the grade lines and curve.

The stations and elevations of the BVC and EVC are determined from Equationa B-1 and B-2:

BVC EVC sta   BVC EVC elev
Equation B-1        Equation B-2


Because grade ratio and percent differ by a factor of 100 as do distances in feet or stations, care must be taken to use the correct form of each in Equations B-1 and B-2. If g is in %, then L must be in stations, if g is a ratio, then L is in feet.



2. Grade Change Rate

A vehicle enters the curve at g1 and after a distance of L it departs the curve at g2. The total grade change is (g2-g1). The grade change rate, k, is the total grade change divided by the distance, Equation B-3.

Equation B-3


For example, a -2.00% grade into a +1.00% grade connected with a 400.00 ft curve has a k of:


This tells us that the grade changes +0.75% per each 100 ft of curve. If we go 100 ft past the BVC, the curve grade is -1.00%+0.75% = +0.25%

Increasing the curve length to 900.00 ft changes k to +0.33%/sta.

That second smaller k means the longer curve is flatter since is spreads the total grade change over a longer distance, Figure B-4.

Figure B-4
Different Curve Lengths


Note that the example was a sag curve with a positive k: we went from a negative grade to a positive grade. A crest curve, on the other hand, would have a negative grade change rate, Figure B-5.

vcurv05 vcurv06
Figure B-5
Crest and Sag Curves


What about curves whose grades at both ends are different but have the same mathematical sign? Figure B-6 shows four different curve situations where the incoming and outgoing grades have the same mathematical sign.

vcurv06 c
Figure B-6
Grades with Same Mathematical Sign


The two curves on the left are both crest curves having a negative k since the outgoing grades are less than the incoming grades.

The two curves on the right are sag curves having a positive k since their outgoing grades are mathematically greater (less negative or more positive) than the incoming grades.

3. Curve Equation

A constant grade change rate means the curve does not have a fixed radius - it's continually changing. This provides a smoother transition when vertical travel direction is changed. A circular arc isn't generally used as a vertical curve because it has constant curvature with a fixed radius. While acceptable at lower speeds, at higher speeds there is a tendency for a vehicle to "fly" at the highest point of a crest curve or "bottom out" at the lowest point of a sag curve. While desirable for roller coaster design, upsetting passenger stomachs is generally frowned upon in road design.

Instead, a parabolic arc is used. A parabola tends to flatten at the vertical direction change making for a more comfortable transition.

The general equation for a parabolic curve, Figure B-7, is Equation B-4.

parabola eqn 
 Equation B-4
Figure B-7


The equation consists of two parts, Figure B-8:

  • a straight line, bx+c, which is tangent to the curve at its beginning (b is the slope, c is the y intercept), and,
  • an offset, ax2, which is the vertical distance from the tangent to the curve.
Figure B-8
Tangent and Offset


Figure B-9 shows the parabolic arc in terms of vertical curve nomenclature.

Figure B-9
Curve Terms


The curve begins at the BVC station and elevation.
The tangent line slope is g1, the incoming grade.

At distance d:

  • the tangent elevation is g1d+ElevBVC
  • the tangent offset is ad2, where a is a function of k

The parabolic equation written in curve terms is the Curve Equation, Equation B-5.

CurveEqn       Equation B-5

(For a complete graphic derivation of the Curve Equation see the Family of Curves section.)

di is the distance from the BVC to any point i on the curve. It is computed from Equation B-6 and ranges from 0 to L.

dist eqn       Equation B-6

Recall the previous warning on grade and distance units. In Equation B-5 the combination should be either grades in percent with distances in stations, or grades as a ratio with distances in feet. Remember that curve length, L, is a distance and must play by the same rules.

The grade at any point on the curve is a function of the beginning grade along with k and the distance from the BVC, Equation B-7.

grade eqn       Equation B-7


Using Equations B-5 through B-7 allows us to compute the elevation and grade at any point along the curve.


4. High or Low Point

When asked where a curve's highest, or lowest, point is, most people assume it is at the middle of the curve. This is true only under a specific condition. The high or low point occurs where the curve changes direction vertically. This happens where the curve tangent has a 0% grade, Figure B-10.


a. Crest Curve


b. Sag Curve

Figure B-10
High and Low Point Conditions


On a crest curve, the tangent grade begins at g1 and deceases uniformly (by k). As long as the grade is positive, the curve elevation keeps increasing. At 0%, the curve reaches its maximum elevation and after that the grade decreases, causing curve elevations to drop.

The opposite is true for a sag curve. As the grade decreases, while it is still negative curve elevations decrease. When the grade is 0%, the elevation decrease stops and as the tangent slope increases so do curve elevations.

The only condition under which the high or low point occurs at the center of the curve is when incoming and out going tangents are numerically equal but have opposite mathematical signs (eg, +2% into -2%, -3% into +3%).

The distance from the BVC to the high/low point, dp, is determined from Equation B-8:

Equation B-8 
Figure B-11
Distance to High/Low Point

The high/low point elevation can be computed by substituting dp in Equation B-5.

In order for Equation B-8 to return a value that makes sense, the grades must have opposite mathematical signs. Consider a +3.00% grade connected to a +1.00% grade with a 500.00 ft long curve. Using Eqation B-8 the distance from the BVC to the high/low point is:


The distance exceeds the curve length. What's up with that? Let's sketch it, Figure B-12.

Figure B-12
High Point After the EVC

Remember, we are using only part of the parabolic curve. 7.5000 stations is the distance to the curve's highest point, it's just on that part of the curve we're not using. Our curve's actual highest point is at the EVC.

Similarly for a +0.50% grade connected to a +2.00% with a 400.00 ft curve:


This means the high/low point occurs before the BVC, Figure B-13.

Figure B-13
Low Point Before the BVC

The lowest point for this curve is at the BVC.



5. Example Problems

a. Example 1

A +3.00% grade intersects a -2.40% at station 46+70.00 and elevation 853.48 ft. A 400.00 ft curve will be used to connect the two grades. Compute:

(1) Station and elevation for the curve's endpoints
(2) Elevations and grades at full stations
(3) Station and elevation of the curve's highest and lowest points

First, draw a sketch:

Figure B-14
Example 1 Sketch


For our computations, we'll use grades in percent and distances in stations.

(1) Use Equations B-1 and B-2 to compute the BVC and EVC

 Ex1 BVC Sta
 Ex1 EVC Sta
 Ex1 BVC Elev
 Ex1 EVC Elev


(2) Set up equations B-3, and B-5 to B-7 to compute curve elevations.

Equation B-3         

 Ex1 k
 Equation B-5  Ex1 Elev
 Equation B-6  Ex1 d
 Equation B-7  Ex1 g


 Set up the Curve Table:

Curve Table
      Equation B-6         Equation B-5         Equation B-7    
Station, i Dist, di (sta) Elevi (ft) Grade, gi (%)
 48+70.00 EVC       
 44+70.00 BVC       


Why is the table upside down? We'll get to that in a second.

For each station, starting at the BVC, compute each column using the equation identified at the column top. Elevations will be computed to an additional decimal place to minimize rounding error.

sta 44+70.00:

Ex1 4470

sta 45+00:

Ex1 4500

and so on to the EVC.At the EVC each computed value has a math check indicated in red.

sta 48+70.00:

Ex1 4570

The completed Curve Table:

Curve Table
      Equation B-6         Equation B-5         Equation B-7    
Station, i Dist, di (sta) Elevi (ft) Grade, gi (%)
 48+70.00 EVC  4.0000  848.680 -2.40
48+00 3.3000 850.029 -1.46
47+00 2.3000 850.809 -0.10
46+00 1.3000 850.239 +1.24
45+00 0.3000 848.319 +2.60
 44+70.00 BVC  0.0000 847.480 +3.00


There are a few other math checks in addition to those at the EVC.

In the Dist column, the difference between successive d's at full stations should be 1.0000

In the Grade column, the difference between successive g's at full stations should be k=-1.35

Some texts make reference to Second Differences or Double Differences as another check. When computations were all done manually, as many checks as possible were used. We'll include it here just to show the reader how it worked.

A Second Difference is the difference between the elevation differences at full stations. Difference of a difference, got it? For this problem, we compute second differences this way:


Elevi (ft)

First Difference

Second Difference

48+70.00 EVC




















44+70.00 BVC





Note that values in the Second Difference column are equal to k, that's the math check. For you calculus buffs, the second difference is the second derivative of the Curve Equation.

It looks like a lot of computing for only two checks against k (and it is). But for those two checks to be valid requires that 5 of the 7 elevations be correct. The remaining two are the BVC and EVC.

(3) High and low point

Because this is a crest curve it will have a high point somewhere along it. The lowest point of the curve is at the BVC: 847.48 ft at 44+70.00.

Examining elevations in the completed Curve Table, it looks like the curve tops out between stations 46+00 and 48+00.

Use Equation B-8 to compute the distance from the BVC:

Ex1 Hdist

Substitute into the Curve Equation to determine its elevation:

Ex1 HEl

Add the distance to the BVC station to get the point's station:

Ex1 HS

In summary



Elevation (ft)





44+70.00 BVC



(4) Misc

You're not restricted to computing elevations only at full stations along the curve. Using the equations in step (2) developed for this curve, you can compute the elevation and grade for any point between 0.00' and 400.00' feet from the BVC. If you exceed 400.00', the equations will give you values (after all, they're just equations), but for part of the curve beyond.

OK, so why the upside down table? It's traditional. When standing on the alignment and looking up station, the layout of the table visually matches the stations in front of the surveyor, Figure B-15.

Figure B-15
Upside Down Curve Table


b. Example 2

A -3.50% grade intersects a +2.00% at station 12+17.53 and elevation 634.25 ft. A 400.00 ft curve will be used to connect the two grades. Compute:

(1) Station and elevation for the curve's endpoints
(2) Elevations and grades at half stations
(3) Station and elevation of the curves highest and lowest points

The computations are left to the reader. The answers are shown for checking your work.

Ex2 table

Low point at sta 12+72.07 and elev 636.80 ft.


6. Fitting Vertical Curves

a. Introduction

There are many different combinations of fitting a vertical curve to meet design conditions. In this section we will concentrate on two examples of fitting an equal tangent curve between fixed grade lines. Holding grade lines fixed isolates curve selection effects. When a grade is changed, whole sections of the alignment are affected and must be recomputed, Figure B-16. Although software can easily handle this, it's the designer's responsibility to ensure solving a problem in one area doesn't adversely affect another.


(a) Preliminary Vertical Alignment


(b) Holding Grades Fixed


(c) Changing a Grade

Figure B-16
Fitting Vertical Curves


In these examples we will compute simple equal tangent curves to meet some elevation condition between fixed grade lines. The next section deals with unequal tangent vertical curves which give additional design flexibility but at the cost of more complex computations.

When computing a curve passing through an elevation, the computed length may be a maximum or minimum. It depends if the specified elevation is a maximum or minimum and whether a sag or crest curve is involved. Increasing the length of a sag curve raises the entire curve; increasing a crest curve's length lowers it, Figure B-17.


(a) Increasing Sag Curve Length


(b) Increasing Crest Curve Length

Figure B-17
Curve Length and Elevations


To solve lengths requires rearranging and solving the Curve Equation, Equation B-5, for the unknown L.

Equation B-5

It looks simple until you realize that di is dependent on the location of the BVC which is in turn based on the curve length we're tying to compute. Hmmm....

b. No part of a curve may go below {above} a specific elevation

This criteria could come from the elevation of area groundwater, bedrock, or an existing overhead pass. The critical curve part of for this situation is the curve's highest or lowest point, Figure B-18.

Figure B-18
Low Point Condition


The distance to the high/low point was given in Equation B-8:

Equation B-8


Substituting Equation B-8 into Equation B-5 and solving for L, we arrive at Equation B-9:

Equation B-9


Remember: grades in % and L in stations.


g1 & g2 are -5.00% and +2.00% respectively; the PVI is at 10+00.00 with an elev of 800.00 ft.

What is the length of curve which does not go below 805.00 ft elev? Is this a maximum or minimum curve length?

Draw a sketch:


Substitute known data into Equation B-9 and solve:


The curve should be at least 700.00 ft long. Using longer lengths is fine since they pull the curve up from 805.00 ft.

Are there any math checks?

Using L=700.00 ft, set up the Curve Equation and compute the low point's elevation.


c. Curve must pass thru a specific elevation at a specific station

In this case the curve has to pass through a particular elevation at a particular station. For example, an existing road crossing may dictate the station and elevation for an at-grade intersection.

This situation appears to be easier since, unlike the previous example, we know the station at which the elevation must be met, Figure B-19.

Figure 19
Elevation at a Station


Since we know the station of the fixed elevation, we can determine its distance from the PVI. We can also relate the BVC location to the PVI.
If we substitute known and related quantities into Equation B-5, we get:

Equation B-10


OK, maybe it's not as simple as it seemed. But the only unknown in the equation is L; unfortunately, it appears as numerator and denominator in the various terms. If we combine and sort terms we see that the equation takes on the form of a (surprise!) parabolic equation, a second degree polynomial. A second degree polynomial has two solutions and can be solved using the quadratic solution.



Second Degree Polynomial
Equation B-13 

Quadratic Solution
Equation B-12 


 To simplify Equation B-10 and solve it using the quadratic solution we can create the following two sets of equations:

FitCurve13   FitCurve14
Quadratic Coefficients
Equation B-13
     Curve Length Solutions     
Equation B-14


Equation B-14 will return two values for the curve length, one which makes sense in the context of the problem, the other which doesn't.


g1 & g2 are -4.00% and +1.00% respectively, the PVI is at 14+00.00 with an elev of 900.00 ft.
The curve must go through an elevation of 902.65 at station 15+60.00.
What curve length meets this condition?

Draw a sketch


Compute the quadratic coefficients using Equation B-13.


Compute the curve lengths using Equation B-14.


Which curve length is correct?

Compute the BVC and EVC stations for each curve length and see if station 15+60.00 falls between them.

Length (sta) BVC Sta EVC Sta 15+60 bewteen?
1.5739 13+21.30 14+78.70 No
6.5061 10+74.70 17+25.30 Yes


The correct curve length is 650.61 ft.

Math check?

Set up the Curve Equation, Equation B-5, and solve for the elevation at station 15+60.00. It should be the same as the specified elevation of 902.65 ft.

Why are there two possible curve lengths and why does only one fit our design situation?

Remember that these are mathematical formulae independent of physical constraint. We are using only part of a complex curve for our alignment design which does have physical constraints.

There are two parabolic curves which are tangent to the grade lines and pass through the design point. These are shown plotted in Figure B-20.


Figure 20
Two Curve Solutions


Both curves are tangent to the two grade lines and pass through sta 15+60 and elev 902.65. Only one of the curves has the design point between the tangent (BVC and EVC) points on the grade lines. Mathematically both curves are correct, but only one meets our design criteria.

d. Multiple design points

It may not be possible to design an equal tangent curve to pass through multiple design points, Figure B-21. A single curve can be fit if each of the points can be missed by a specified amount. Least squares could be applied in this case to create a best-fit curve.

Figure B-21
Multiple Design Points


Or one of the points could be treated as the most critical and used to design a curve, Figure B-22.

Figure B-22
Critical Point Fit


Another possible approach is to use a compound vertical curve which will be covered next.


7. Compound Curves

a. Geometry

Sometimes a simple equal tangent vertical curve cannot be fit to a particular design condition. For example, the vertical curve in Figure B-23 must start at an existing intersection at sta 20+00 elev 845.25 ft and end at a second intersection at sta 28+00 elev 847.75 ft. To minimize earthwork an incoming grade of +2.50% is followed by an outgoing grade of -1.00%. This places the PVI at sta 23+00 elev 852.75 ft.

Figure B-23
Fitting a Curve


This combination results in a PVI that is not midway between the BVC and EVC. If a 600.00 ft equal tangent curve were used, it would begin at 20+00 and end at 26+00, 200.00 ft back along the -1.00% grade from the specified EVC.

This situation requires an unequal tangent vertical curve. While a mathematical curve other than a parabolic arc could be used, the traditional method is to use two back-to-back equal tangent curves. This is referred to as a compound curve, Figure B-24.

Figure B-24
Compound Curve


A compound curve isn't as complex as it looks: the key is to break it into two successive equal tangent veridical curves.

PVIs are created for each curve at midpoint on the grade lines. A new grade, g3, is created by connecting the new PVIs. This is the outgoing grade for the first curve and the incoming grade for the second.

The CVC is the Curve to Vertical Curve point which is the EVC of the first curve and the BVC of the second; its station is the same as the overall PVI station. Once we have sufficient information for each curve to set up its Curve Equation, we can compute each curve independently.

The closer the individual curve's k values, the smoother the transition between them, particularly when there is a large grade change over a short distance.

b. Curve Equation

For an equal tangent vertical curve, we set up and solved Equations B-4 and B-5 through B-7. When setting these up for the curves in a compound curve, care must be taken to use the correct grades and lengths in each. Using subscripts i and m for the first and second curves respectively, their equations are:

  Curve 1 Curve 2
Equation B-3     compound7  compound11
Equation B-5        compound8       compound12   
Equation B-6     compound9  compound13
Equation B-7     compound10  compound14


c. High/Low Point

A compound curve has the same high/low point condition as an equal tangent curve: it occurs where the curve grade passes through 0%.

For the compound curve shown in Figure B- 24: the first curve has two positive grades so it never tops out, the second  begins with a positive and ends with a negative grade. That places the highest point of this compound curve on the second curve.

When using the equation to compute distance to the low/high point, remember:

  • use the grades for the curve on which the point occurs, and,
  • the distance computed is from the BVC of the curve on which the point occurs.

For example, the distance equation Equation B-8 is:

Equation B-8


Setting up Equation B-8 and hi/low point station for each curve:

  Distance Station
Curve 1        compound3     compound5  
Curve 2        compound4     compound6  


Remember: the high/low point occurs only on one curve or the other. Should g3 be 0%, then the low/high point would be at the CVC. If all three grades are positive or negative, then Equation B-8 does not apply as neither curve tops/bottoms out.

d. Example

Using the data given in Figure B-24:

(1) Complete the following table below
(2) Setup the Curve Equations for both curves
(3) Determine the station and elevation of the highest curve point.



(1) Complete the table

Curve 1


Curve 2


incoming grade (%)


outgoing grade (%)


Length (ft)




BVC Elev




PVI Elev




EVC Elev



Entries that can be made from the given criteria:

Curve 1


Curve 2


incoming grade (%)


outgoing grade (%)


Length (ft)




BVC Elev




PVI Elev




EVC Elev





Since the PVIs are halfway along the grade lines, their positions are the averages of the grade line ends.



g3 is the grade of the line connecting the two curve PVIs. There can be different ways to compute it depending on the given data. In this example, we can use the position of the PVIs.


Compute the CVC elevation from PVI1.


The CVC elevation can also be computed from PVI2. We'll do it as a math check.


Complete the table.

Curve 1


Curve 2


incoming grade (%)


outgoing grade (%)


Length (ft)




BVC Elev




PVI Elev




EVC Elev



(2) Set up curve equations

Curve 1


Curve 2


Once these are set up, a Curve Table can be computed for each curve.

(3) Station and elevation of highest curve point.

The highest point will occur on Curve 2 since its grades go through 0%.

Use the equations for Curve 2.




e. Reverse Curve

A reverse curve is a compound curve except that the two curves have opposite curvature. It consists of either a sag-crest or crest-sag curves sequence, Figure B-25. The CRC (Curve to Reverse Curve) is the EVC of the first curve and BVC of the second.

Figure B-25
Reverse Curve


Reverse vertical curves can be used to better approximate terrain to reduce earthwork. They also provide additional design flexibility if multiple design points are constrained. Their disadvantage is the vertical direction reversal which can be abrupt at higher speeds, particularly if the designer gets carried away and adds a third, fourth, etc, curve. Regardless, they are computed as equal tangent vertical curves once sufficient parameters for each is fixed or computed.

High/Low point determination can be interesting since both could exist on a reverse curve. For example, in Figure B-25, the first curve has a low point and the second has a high point. However, the lowest point on the entire curve is on the second curve at its EVC and the highest point on the entire curve is at the BVC of the first vertical curve.



Section 1. Basic Concepts

a. General

While some alignments like an electrical transmission line can be designed with angle points at changes in horizontal direction, alignments for moving commodities with mass must have less abrupt transitions. This is accomplished by linking straight line segments with curves, similar to that in vertical alignments. However in this case the lines and curves linking them are in a horizontal plane.

A simple tangent geometric curve is used to link the two lines. A curve is tangent to a line when its radius is perpendicular to the line, Figure C-1.


Figure C-1
Line-Curve Tangency Condition


The two lines bounding the curve are generically referred to as tangents or tangent lines. The direction of a curve is either right or left based on the deflection direction between the tangents, Figures C-2 (a) and (b).

(a) Deflection Angles
(b) Curves
Figure C-2
Curve Direction


In roadway design horizontal geometry differs from vertical because a driver is responsible for guiding a vehicle from the first tangent to the second one. This requires turning a steering wheel changing the car's direction. Ideally this would be a smooth action to cause minimal discomfort: once into a curve, the steering angle would be maintained until the tangent is reached.

Two different mathematical arcs can be used for horizontal curves, either singularly or in combination: a circular arc and a spiral arc.

b. Circular arc

A circular arc has a fixed radius which means a driver doesn't have to keep adjusting the steering wheel angle as the car traverses the curve. It is a simple curve which is relatively easy to compute, Figure C-3.

Figure C-3
Circular Arc


Its primary disadvantage is that the constant curvature must be introduced immediately at curve's beginning. That means a driver would have to instantaneously change the steering wheel angle from 0° to full or the car would overshoot the curve. A similar condition exists ate the curves's end. The higher the vehicle speed and the sharper the curve, the more pronounced the effect.

c. Spiral arc
A spiral has a constantly changing radius, Figure C-4. At the spiral's beginning, its radius is infinite; as the vehicle progreeses into the curve, the spiral radius decreases. A spiral provides a more natural direction transition - the driver changes the steering wheel angle uniformly as the car traverses the curve.

Figure C-4
Spiral Arc


Attaching a second sprial with reversed radius change creates an entrance-exit spiral condition where the driver gradually increases then gradually decreases steering wheel angle, Figure C-5.

Figure C-5
Combined Spirals


A circular arc can be combined with two spirals, Figure C-6.


Figure C-6
Spiralled Horizontal Curve


Combined spirals and spiralled horizontal curves, in conjunction with superelveation, help balance centrifugal forces. For a constant velocity, as the radius decreases the centrifugal force increases. A spiral allows superelevation to be introduced at a uniform rate allowing it to offset increasing centrifugal force. In theory, there exists an equilibrium velocity at which a vehicle could travel from one tangent through the spirals and curve to the second tangent safely even if the road were completely ice covered.

Railroad alignments typically use spiral horizontal curves because of their force balancing nature. Because of the wheel flange to rail connection, a train moving around a curve exerts a force directly to the rails (unlike a vehicle's tire-pavement connection which can devolve into a skid). A rail line laid out with a circular arc would shift to a spiral configuration after a train has run through it a number of time at transport speed.

The traditional disadvantage of a spiral is that it is complex to compute, although that has been largely negated by software. While still used for railways, in high speed highway design using long flat circular arcs minimizes the tangent to curve transition so spirals aren't as critical. They can be useful in low speed situations where there is a large direction transition. We'll examine spiral geometry and application in a later section.

Section 2. Nomenclature; Components

a. Degree of Curvature

For any given set of tangents, there are an infinite number of circular arcs which can be fit between them. The arcs differ only in their radii which relates to their "sharpness." Consider the two arcs in Figure C-7:

Figure C-7
Different Arcs


Both curves must accommodate a total direction change of Δ but since C1 is shorter it is sharper than C2.

Degree of curvature is a traditional way of indicating curve shaprness. There are two different definitions of degree of curvature: arc and chord.

Arc definition, Da, is the subtended angle for a 100.00 ft arc; Chord definition, Dc, is the subtended angle for a 100.00 ft chord, Figure C-8.

   (a) Arc Definition   
   (b) Chord Definition   
Figure C-8
Degree of Curvature


Street and road alignments use the arc definition; chord definition is used for railroad alignments. For the remainder of this Chapter we will use the arc defintion and refer to it simply as D.

Degree of curvature is inversely proportional to radius: as D increases, R decreases, Figure C-9. The larger D is, the sharper the curve.

Figure C-9
D and R Relationship


Note the importance of 100.00 in both Degree of Curvature versions. This goes back to a standard tape length and stationing interval. However, in the metric system there is no convenient base equivalent of 100.00 ft. While design criteria was traditionally expressed in terms of D, it is more common today to instead use R which works for both the English and metric systems.

The relationship between D and R is expressed by Equation C-1.

horiz21      Equation C-1


b. Curve Components; Equations

Figure C-10 shows a tangent circular arc with some basic components labeled. 

Figure C-10
Basic Components of a Circular Arc



Point of Instersection


Begin Curve (aka: PC - Point of Curve; TC - Tangent to Curve)


End Curve (aka: PT - Point of Tangent; CT - Curve to Tangent)


Defelction angle at PI; also the central angle of the arc


Arc radius


Arc Length


Long Chord length


 Figure C-11 includes additional curve components.

Figure C-11
Additional Curve Components



Tangent distance


External distance - from PI to midpoint of arc


Middle ordinate - distance between midpoints of arc and Long Chord


Equations for the curve components are:

horiz22       Equation C-2
 horiz23       Equation C-3
 horiz24       Equation C-4
 horiz25       Equation C-5
 horiz26       Equation C-6


Although it may look like it in Figure C-11, E and M are not equal.

c. Stationing

As mentioned in Chapter A, an alignment is stationed at consistent intervals from its beginning through its end. On a finished desgin, the stationing should be along the tangents and the fitted curves.

Traditionally, an alignment is stationed along the straight lines thru each PI, Figure C-12. Curve fitting comes later.

 Figure C-12
Stationing Along Tangents


A circular curve is fit and staked. The stationing along the tangents between curve ends would be replaced by the curve stations, Figure C-13.

There are two ways to get from BC to EC:

(1) Up and down the tangents, T+T
(2) Along the curve, L

The distance along the curve is shorter than up and down the tangents: L < T+T

Figure C-13
Curve Inserted


That means for a typical curve there are two stations for the EC:

One along the original tangents
One along the curve.


Figure C-14
Stationing Along Curve


The EC station with respect to the original tangents is the EC Ahead. If we are standing on the EC and take a step ahead (up-station), we are on the tangent and its original stationing.

The EC station with respect to the curve is the EC Back. If we are standing on the EC and take a step back, we are on the curve and its stationing.

Chapter A mentioned that a station equation is used where one point has two stations. In this case we have a station equation at the EC: EC Sta Ahead = EC Sta Back. Figure C-15 is an example of a station equation indicator on a set of WisDOT highway plans.

Figure C-15
Station Equation Indicator


A station equation represents a stationing discontinuity. We know the distance between two alignment points is their stationing difference. However if the points are on each side of an EC, the discontinuity must be taken into account. For example, the distance between stations 13+00 and 16+00 on the alignment shown in Figure C-14 is normally 300.00 ft but there are 48.88 ft "missing" at the EC. The 48.88 ft is the difference between the Ahead and Back stations: (14+82.97) - (14+34.09) = 48.88 ft. So the correct distance from 13+00 to 16+00 is 251.12 ft.

With software, it is possible to avoid station equations by waiting until the curves are fitted before stationing the entire alignment. With computer assisted designs, the PIs could be computed positions, Figure C-16.

Figure C-16
Computed Tangents and PVI


A curve is then fit to the tangents, Figure C-17.


Figure C-17
A Curve is Fit


Then the alignment is stationed from its beginning to its end through the curves, Figure C-18. That way there are no station equations.

Figure C-18
Stationing Through Curve


So this latter method is simpiler and the one that should be used, right? Well, it does have some advantages, but it also has disadvatages. What happens if the alignment design must be changed at some point later in the process?

With traditional stationing, each curve has its own EC station equation. If one curve is altered, only its stationing is affected, no other stations on the alingment change, Figure C-19.

(a) Original Stationing with Station Equations

(b) Limited Stationing Changes with Curve Modification 
Figure C-19
Alignment with Station Equations


With continuous stationing, when a curve is altered it affects stations on it as well as all stations after it, Figure C-20.

(a) Original Continuous Stationing
(b) Stationing Changes on and after Modified Curve
Figure C-20
Alignment with Continuous Stationing


For example, if the redesigned curve is shorter, then all full (+00) station points after the curve increase. For example, 12+00 becomes 12+7.03, 13+00 becomes 13+07.03, etc. If the alignment is already staked then each stake could be renumbered or each could be each be moved back 7.03 feet. Hmm, odd stations or moving stakes.... Maybe station equations aren't so bad after all.

BC and EC stations are computed from the following formulae:

horiz47            Equation C-7 
 horiz48 Equation C-8 
 horiz49 Equation C-9 


d. Example: Curve Components, Stationing

A PI is located at station 25+00.00. The deflection angle at the PI is 55°00'00" R. A 500.00 ft radius curve will be fit between the tangents.

Compute curve components and endpoint stations.

Start with a sketch:


Use Equations C-2 through C-6 to compute curve components (carry an additional digit to minimize rounding errors):






Compute degree of curvature using Equation C-1:


Use Equations C-7 through C-9 to compute endpoint stations:






Section 3. Radial Chord Method

a. Circlular Geometry

For any circular arc, the angle between the tangent at one end of the arc and the chord is half the arc's central angle, Figure C-21.

Figure C-21 Deflection angle


 Angle a/2 is the deflection angle from one end of the arc to the other. The chord's length is computed from:


     Equation C-10


In terms of the degree of curvature, Figure C-22: 

Figure C-22 Full station deflection angle


The deflection angle for a full station is half the degree of curvature. Since the deflection angle is D/2 and it occurs over a 100.00 ft, the deflection rate can be computed from:

 horiz73         Equation C-11


Extending this geometry to the entire curve, Figure C-23, the total deflection angle at the BC from the PI to the EC is Δ/2.

Figure C-23 Deflection angle for entire curve


Since the deflection angle occurs across the curve's length, the deflection rate can also be written as Equation C-12.

 horiz73b         Equation C-12


b. Radial chords

One way to stake a horizontal curve is by the radial chord method, Figure C-24.

Figure C-24 Radial chord method


An instrument is set up on the BC and the PI sighted as a backsight. Then to stake each curve point, a defelction angle is turned and chord distance measured. For each point to be staked, we need to compute its deflection angle and chord distance.

The deflection angle to any point i on the curve, Figure C-25, can be computed from Equation C-13.

Figure C-25 Deflection angle and radial chord


horiz75        Equation C-13


li is the arc distance to the point from the BC and is computed using Equation C-14.

horiz76        Equation C-14


Equation C-10 can be re-written using the deflection angle:


horiz77         Equation C-15


Using Equations C-13 to C-15, the defelction angle and distance any curve point from the BC can be computed.


c. Example

Determine the radial chord stakeout data at full stations for the example from Section 2.d.

Summary of given and computed curve data:

Δ = 55°00'00" R = 500.00 ft  
D = 11°27'33.0" L = 479.965 ft T = 260.284 ft
LC = 461.749 ft E = 63.691 ft M = 56.494 ft


Point Station
PI 25+00.00
BC 22+39.716
EC 27+19.681 Bk = 27+60.284 Ah


Use Equation C-11 to compute the curve's deflection rate:


Set up Equations C-13, C-14, and C-15 for the curve:


Set up the Curve Table:



Curve Point

Arc dist, li, (ft)

Defl angle,δi

Radial chord, ci

EC 27+19.681 Bk
























BC 22+39.716