# Station Offsets

TPC is just as comfortable working with stations offsets as it is working with coordinates. Once an alignment is established, points can be both located  and computed by station and offset based on that alignment.

## OFS Point Type

Traverses can utilize the OFS (Offset) point type to locate station/offset points.

Offset points always follow a point on the alignment that establishes the distance along the alignment per the stationing of the alignment. The distance to the offset point is then the prependicular (shortest) distance from the offset point to the corresponding alignment point. You can have any number of offset points following an alignment point, but each one originates fromt he shared alignment point.

Offsets points are also unique in that TPC completely controls the direction of the offset based on the alignment. It does this by computing a direction that is perpendicular to the alignment at the alignment point or by bisecting the angle if the offset if from an alignment angle point.

## Offsets From Points of Tangency

TPC examines points of tangency (before and after curves and spirals) to determine if they are tangent. If so, the offset is perpendicular and lies in the direction of the corresponding radius point. If not, TPC computes the offset-intersection of the alignment for the offset position. This can involve some pretty serious mathematics in the case of spirals, but TPC does a good job of locating the offset points.

## COGO Station Offset Command

The Station Offsets command inserts two points into the current traverse at the requested station. One is the station point along the alignment. The other is the offset point from that station.

• Enter the Traverse View for the alignment you want and choose COGO | Station Offsets.
• Enter the Station at which you want to create an offset.
• Enter an offset Point to create and its Offset Distance. To create an offset to the left, enter a negative distance. To create an offset to the right, enter a positive distance.
• Use the center line elevation for the offset or specify an elevation (Z) value.
• Choose Insert to add the computed station and offset points to the traverse.
• Enter as many offset points as you need, changing the point label and station as needed. When you are finished, choose Close.

The Tolerance field tells TPC not to create a new station point if the survey already has a point within the tolerance of the specified station.

## Offsets from an Existing Point

You can add an offset point to an existing station (any control point on an alignment). You can enter any combination of horizontal distance, slope distance, etc. TPC computes the bearing from the position of the alignment station, and uses the information you enter to compute the position of the offset point.

• Insert a point so that it follows an existing station.
• Highlight the point, choose Type | OFS=Offset.
• Enter the information you have. To create an offset to the left, enter a negative distance. To create an offset to the right, enter a positive distance.

## Grid vs Ground / Geodetic

The stations are computed based on the distance type of the traverse.

For example, if an alignment traverse is formatted for ground distance at project elevation, the computed station and offset will be ground distance at project elevation.