
Reference Information
This command computes parameters of a circular curve, a simple spiral curve, or a compound spiral curve based on two known parameters. This command is used primarily in association with other commands to compute the length and radius of circles or spirals when given known parameters. For example, if you wanted to determine the length of a circle based on a specified radius and a specified angle, you would define the first parameter (radius) and the second parameter (angle) by toggling on the Lock option for both. When you select Compute, the software computes the length of the circle (as well as the chord, tangent, and so on) based on the radius and angle.
Various books are available for the definition of transitions. As a common point of discussion the following are recommended:
Hickerson, Thomas F., Route Location and Design (ISBN 070286809). 5th Edition. New York: McGrawHill;1964.
Hennecke, Fritz; Muller, Gerhard; and Werner, Hans., Handbuch Ingenieurvermessung (ISBN 387907204X). Germany: Wichmann; 1993.
If you activated the Curve Calculator dialog box from the Define Curve or the Edit Element command, the curve definition (arc or chord) used in the Curve Calculator is determined based on the curve definition of the active horizontal alignment. If you activated the Curve Calculator from the Fit Curve command, the curve definition comes from the Geometry tab under the Tools > Options command, from the Horizontal parameter in the Curve Definition group box. 
Spiral/Compound Spiral Grouping (left side)
Caution The software is limited to spiral angles of less than 180 degrees.
Radius 1
specifies the length of the first radius.
Radius 2
specifies the length of the second radius, in case of a compound curve. If the Radius 2 Lock is on, this field specifies the radius value to use in the calculation. If the Radius 2 Lock is off, this field displays the calculated radius value.
Length
displays or specifies the total length of the spiral arc (from TS to SC). If the Length Lock is on, this field specifies the length value to use in the calculation. If the Length Lock is off, this field displays the calculated length value.
Length Lock
toggle on to specify that the length value is to be used in the calculation. Toggle off to specify that the length value is to be calculated.
Angle
displays or specifies the central angle of the spiral arc (Qs). If the Angle Lock is on, this field specifies the angle value to use in the calculation. If the Angle Lock is off, this field displays the calculated angle value. For more information about entering an angle, see the Angles (ang, va, defl, delta) topic.
Angle Lock
toggle on to specify that the angle value is to be used in the calculation. Toggle off to specify that the angle value is to be calculated.
Constant
displays or specifies the spiral constant, which is commonly called "A.” A = square root of ( length_of_spiral * minimum_radius ). If the Constant Lock is on, this field specifies the constant value to use in the calculation. If the Constant Lock is off, this field displays the calculated constant value.
Constant Lock
toggle on to specify that the constant value is to be used in the calculation. Toggle off to specify that the constant value is to be calculated.
P
displays or specifies the offset from the initial tangent to the point of curvature of the shifted circle. If the P Lock is on, this field specifies the P value to use in the calculation. If the P Lock is off, this field displays the calculated P value.
P Lock
toggle on to specify that the P value is to be used in the calculation. Toggle off to specify that the P value is to be calculated.
K
displays the abscissa of the shifted point of curvature relative to TS.
Xs
displays the tangent distance for the SC.
Ys
displays the tangent offset at the SC.
Long Tangent
displays the long tangent length.
Short Tangent
displays the short tangent length.
Long Chord
displays the long chord length.
Curve Grouping (center)
Data that cannot be computed (such as tangent, middle ordinate, and external) when the central angle is greater than or equal to 180 degrees is given a value of 1.0 in the dialog box. 
Radius
displays or specifies the radius of the circle (R). If the Radius Lock is on, this field specifies the radius value to use in the calculation. If the Radius Lock is off, this field displays the calculated radius value.
Radius Lock
toggle on to specify that the radius value is to be used in the calculation. Toggle off to specify that the radius value is to be calculated.
Degree of Curve (DOC)
displays or specifies the degree of curvature (D), which is defined using the arc definition circular curve definition method. This method defines D as the central angle which subtends a 100 foot arc. If the Degree of Curve Lock is on, this field specifies the DOC value to use in the calculation. If the Degree of Curve Lock is off, this field displays the calculated DOC value.
Degree of Curve (DOC) Lock
toggle on to specify that the degree of curve value is to be used in the calculation. Toggle off to specify that the degree of curve value is to be calculated.
Length
displays or specifies the distance along the arc from A to B. If the Length Lock is on, this field specifies the length value to use in the calculation. If the Length Lock is off, this field displays the calculated length value.
Length Lock
toggle on to specify that the length value is to be used in the calculation. Toggle off to specify that the length value is to be calculated.
Angle
displays or specifies the angle at O from A to B (D). This is the central angle. A curve can deflect to the left or to the right. Note the central angle at the PI. If the Angle Lock is on, this field specifies the angle value to use in the calculation. If the Angle Lock is off, this field displays the calculated angle value. For more information about entering an angle, see the Angles (ang, va, defl, delta) topic.
Angle Lock
toggle on to specify that the angle value is to be used in the calculation. Toggle off to specify that the angle value is to be calculated.
Chord
displays or specifies the long chord length. If the Chord Lock is on, this field specifies the chord value to use in the calculation. If the Chord Lock is off, this field displays the calculated chord value.
Chord Lock
toggle on to specify that the chord value is to be used in the calculation. Toggle off to specify that the chord value is to be calculated.
Tangent
displays or specifies the tangent length (T). If the Tangent Lock is on, this field specifies the tangent value to use in the calculation. If the Tangent Lock is off, this field displays the calculated tangent value.
Tangent Lock
toggle on to specify that the tangent value is to be used in the calculation. Toggle off to specify that the tangent value is to be calculated.
External
displays or specifies the external distance (E), the distance from V to C. If the External Lock is on, this field specifies the external value to use in the calculation. If the External Lock is off, this field displays the calculated external value.
External Lock
toggle on to specify that the external value is to be used in the calculation. Toggle off to specify that the external value is to be calculated.
Ordinate
displays or specifies the middle ordinate (M), the distance from C to D. If the Ordinate Lock is on, this field specifies the ordinate value to use in the calculation. If the Ordinate Lock is off, this field displays the calculated ordinate value.
Ordinate Lock
toggle on to specify that the ordinate value is to be used in the calculation. Toggle off to specify that the ordinate value is to be calculated.
Spiral Grouping (right side)
Caution The software is limited to spiral angles of less than 180 degrees.
Radius
displays or specifies the radius of the spiral. If the Radius Lock is on, this field specifies the radius value to use in the calculation. If the Radius Lock is off, this field displays the calculated radius value.
Length
displays or specifies the total length of the spiral arc (from TS to SC). If the Length Lock is on, this field specifies the length value to use in the calculation. If the Length Lock is off, this field displays the calculated length value.
Length Lock
toggle on to specify that the length value is to be used in the calculation. Toggle off to specify that the length value is to be calculated.
Angle
displays or specifies the central angle of the spiral arc (Qs). If the Angle Lock is on, this field specifies the angle value to use in the calculation. If the Angle Lock is off, this field displays the calculated angle value. For more information about entering an angle, see the Angles (ang, va, defl, delta) topic.
Angle Lock
toggle on to specify that the angle value is to be used in the calculation. Toggle off to specify that the angle value is to be calculated.
Constant
displays or specifies the spiral constant, which is commonly called "A.” A = square root of ( length_of_spiral * minimum_radius ). If the Constant Lock is on, this field specifies the constant value to use in the calculation. If the Constant Lock is off, this field displays the calculated constant value.
Constant Lock
toggle on to specify that the constant value is to be used in the calculation. Toggle off to specify that the constant value is to be calculated.
P
toggle on to specify that the P value is to be used in the calculation. Toggle off to specify that the P value is to be calculated.
P Lock
toggle on to specify that the P value is to be used in the calculation. Toggle off to specify that the P value is to be calculated.
K
displays the abscissa of the shifted point of curvature relative to TS.
Xs
displays the tangent distance for the SC.
Ys
displays the tangent offset at the SC.
Long Tangent
displays the long tangent length.
Short Tangent
displays the short tangent.
Long Chord
displays the long chord length.
Compute
specifies what you are calculating. You can select from: Simple Spiral, Simple Curve, Spiral Curve Spiral, Spiral Curve, Curve Spiral, or Compound Spiral.
Compute (button)
starts the computation process using the locked values you have defined.
OK
initiates the command using the information defined in the dialog box and then dismisses the dialog box.
Results
opens the Results dialog box which displays the output of this command. This button is activated only after the command is finished processing.
Help
displays help.
Cancel
dismisses the dialog box and exits the command. Any changes you have made to the dialog box are ignored.