Horizontal Curve Dialog

There are many ways to compute a horizontal curve using this dialog box. Knowing a few rules about how it works will help you learn to use it.

Horizontal Curve Rules

• A depressed/held button for a field means that field will not be modified by any other curve computations unless you first release it.
• If you try to enter data that conflicts with other entered data, you will be given a warning message.

The following steps are used to compute a curve:

• Step 1: Compute Delta from Azimuths
• Step 2: Compute Radius or Delta from Middle Ordinate or External
• Step 3: Compute Radius, Tangent, Delta, ArcLength & ChordLength
• Step 4: Compute curve orientation
• Step 5: Compute PC, PT, PI & PR if not already known

PI, PR

PI = Point of Intersection. PR = Point of Radius.

To recall an existing point for the PI or PR, enter the point label of the existing point.

To save the computed position of the PI and/or PR to a survey point, enter unique point labels then choose Save PI/PR.

Plotting the PI and PR in the Current Drawing

Once you have saved a PI or PR, choose Edit PI or Edit PR do display the Survey Point Properties dialog. Choose Plot from this dialog.

PI, PC->PI, PI, PI->PT

Enter any combination of points on tangents or tangent bearings to establish the central angle. If you highlighted a PI in the Traverse View, the curves dialog box will automatically pick the nearest PIs to establish the tangents. If no other PIs are found in the traverse, it will use the closest control points.

Select Save PI to save it to the survey.

PC/PT, PC, PC->PR, PR, PR->PT, PC->PT, Pt

You cannot edit the PC and PT point labels in this dialog box. You must edit them in the Traverse View or Points Manager. They are displayed here only to show which points are being used for this curve.

If you are entering a curve at a PI that doesn't yet have a PC and PT, these boxes will be blank.

PC->PT

The bearing/azimuth from the PC to the PT. This information is sometimes given on plats and deed descriptions and can be used to establish the position of the PT from a known PC.

PC->PR

The bearing/azimuth of the PC to the PT. This information, along with the Radius and Central Angle, is sometimes given on plats and can be used to establish the position of the PT from a known PC.

PR->PT

Entering both the PC->PR and PR->PT computes the central angle and the rest of the curve parameters.

Note: Of the four curve points (PC, PT, PI & PR), only two can be depressed/held at any one time. To edit a third, you must first release one of two that is held.

Radius of the curve. If the PC and PT are held, the radius you enter must be at least 1/2 of the chord length established by the PC and PT.

To turn off the curve, enter a radius of 0 or choose Clear.

Degree of Curve

Degree of Curvature is based on curve definition (Arc vs. Chord) selected. The Degree of Curve is the central angle subtended by a 100 unit arc (Arc definition) or 100 unit chord (Chord definition).

Delta

Delta (central angle) of the curve. It can be computed from the tangent bearings or entered separately.

Chord Length

Horizontal distance from the PC to the PT. It is not affected by the Distance Factor specified as part of the Traverse View format.

The chord length is displayed as Horizontal Distance in the Traverse View and is affected by the Distance Factor there.

Arc Length

The length of the curve from the PC to the PT.

Tangent

The length of either tangent from the PC to the PI. Since this dialog box handles only simple curves and not spirals, both tangent lengths must be equal.

Middle Ordinate

The horizontal distance from the center of the long chord to the center of the arc length. You cannot enter this if both the Radius and Delta are held. The middle ordinate can sometimes be measured on an existing curve to establish its radius.

External

The horizontal distance from the center of the arc length to the PI. You cannot enter this if both the <Radius> and <Central Angle> are held. The external can sometimes be measured on an existing curve to establish its radius.

Direction, Counter Clockwise, Clockwise

The direction of the curve as seen from the PC to the PT. In the Traverse View, a Clockwise curve is displayed as a positive radius and a Counter Clockwise curve is displayed as a negative radius.

Note: If you enter a Clockwise curve and then recall the same curve starting with the PT into another traverse, the second curve will be Counter Clockwise, since it is seen from the other end of the curve.

Curve Definition

Select Arc or Chord. You have the option of making the Arc the Sum of Chords.

Delta > 180 Degrees

Turn this option on if you want to convert a simple curve to a cul-de-sac. By combining this field with the Direction field, you can put a simple curve or cul-de-sac on either side of the long chord.

Clear

Sets the Radius to zero, turning the curve into a tangent. In the case of a PI curve, the PC and PT that were created and added to the traverse will automatically be deleted.

OK

Return to the Traverse View, inserting the computed curve into the traverse at the highlighted point.

Cancel

Return to the Traverse View without changing the status of the highlighted point in any way.