# Tangent-Tangent Intersections

• Given two arcs, TPC can compute two tangent intersects.
• Give two circles, TPC can compute four tangent intersects.

## Intersecting Curves (arcs)

For the example on the right, TPC computed tangent intersects at both ends of the red tangent line.

• INT1R which is tangent to the  curve from 3PC to 3PT
• INT2R which is tangent to the curve from 7PC to 7PT

Here is the dialog used to compute this example.

• We selected the Curve [>>] for Curve 1 then selected the arc from 3PC to 3PT in the drawing.
• Then we selected the Curve [>>] for Curve 2 then selected the arc from 7PC to 7PT.
• TPC activated the Right fields, indicating that it had computed these intersections.
• We choose Save and Plot for each of them.

When you select a curve for either Point 1, Point 2 or both, TPC limits the possible intersections to the length of that curve. In the example shown above, INT1R was restricted to the arc between 3PC and 3PT. INT2R was restricted to the arc between 7PC and 7PT.

This restriction is common in surveying because you want the intersections to fall on existing survey lines.

You can remove this restriction by turning on the [x] Accpet intersects beyond ends of the line.

## Intersecting Circles

For the example on the right, we have computed intersects 20' from PR1 and 40' from PR2.  We've drawn the resulting circles around both PR1 and PR2. The tangent intersects must lie on these circle.

Without the restriction of an existing survey line, TPC can compute two intersections on each circle, using L for Left and R for right.

Here is the dialog used to compute this example.

• We entered a From point and Radius for Point 1.
• We entered a From point and Radius for Point 2.
• TPC activated all four Left and Right fields for the computed intersections.
• We choose Save and Plot for each of them.