# Spirals

TPC works with spirals much like it works with horizontal curves. You can include spirals in area computations, right-of-way offsets, stationing, etc. There are two basic spiral commands in TPC - the Combination and Single spiral dialogs.

## Combination Spirals

Combination spirals consist of an incoming spiral, a simple curve, and an outgoing spiral. TPC lets you enter a combination spiral at any PI and takes care of computing and inserting the spirals and curve. Combinations spirals can have different incoming and outgoing spiral lengths. In fact, you can even omit either spiral and just have a spiral - curve or curve - spiral combination.

To enter a combination spiral

• Highlight a point and choose Type | PI. TPC will place a PI in the type column.
• Choose COGO | Spiral to display the Combination Spiral dialog.
• Enter the appropriate curve and spiral information and choose OK.

TPC will insert the appropriate points into the traverse.

## Single Spirals

Single spirals consist of a single spiral segment with beginning and ending radii and some length. A radius of zero indicates the flat end of the spiral or the start/end of the spiral.

To enter a single spiral

• Highlight any control point. If you highlight a new point added to the traverse, the spiral will compute the position of the added point. If you have already computed the position of the highlighted point, TPC will try to fit the spiral to the current position of the highlighted point or allow you to release it and recompute its position.
• Choose COGO | Spiral to display the Single Spiral dialog.
• Enter the spiral information and choose OK.

TPC will insert the spiral information into the traverse and recompute the highlighted point if appropriate.

## Spiral Point Types

TPC uses the following point types to identify spiral points:
TS - tangent to spiral (flat end of an incoming spiral)
SC - spiral to curve (sharp end of an incoming spiral)
CS - curve to spiral (sharp end of an outgoing spiral)
ST - spiral to tangent (flat end of an outgoing spiral)

A typical combination spiral might look something like this in the Traverse View.

 Point Type 12TS TS 12SC SC 12 PI 12CS CS 12ST ST

Notice that TPC uses the original PI point label plus the spiral point types to label the spiral points it creates.

## Spiral Data in the Traverse View

The Traverse View will display certain information about the spiral if you turn on the corresponding columns.

Horizontal Distance column (H) - long chord (point-to-point) of the spiral.
Bearing/Azimuth column (B/A) - bearing/azimuth of long chord.
Arc length (C) - spiral length.
Delta (Q) - spiral angle.

## Grid vs Ground / Geodetic

The spiral distances and directions displayed in the Traverse Veiw and the COGO dialogs are based on the distance type of each alignment traverse. For example, if an alignment traverse is formatted for ground distance at project elevation, radii and spiral lengths will be ground distance at project elevation.

This gives you the option of working with alignment parameters at ground (use a ground distance type) or grid (use grid distance type).