> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/cadd/arc-cadd.md).

# Arc

An **Arc** is a portion of a circle defined by a center, radius, start angle, and sweep angle. In bridge drawings you'll use it for fillets between intersecting members, return arcs on parapet ends, curved abutment wingwall outlines, drilled-shaft outlines, and any place where you need a true circular sweep rather than a polyline approximation.

To draw: pick the center, then the start point (which sets the radius and start angle), then a final point (which sets the sweep angle).

## Geometry

**Name:** Identifier within the parent CAD document. Must be unique.

**Level:** The layer controlling color, line weight, and dash style.

**Center X / Center Y:** Coordinates of the arc's center.

**Radius:** Distance from the center to the arc.

**Start Angle:** Angle (in degrees) where the arc begins, measured counter-clockwise from the horizontal axis.

**Arc Angle:** Sweep — total angle the arc covers from start to end. Positive values sweep counter-clockwise, negative clockwise.

**Start X / Start Y / End X / End Y:** Endpoint coordinates (read-only — derived from center, radius, and angles).

## Editing

**Move the center:** drag the center handle to translate.

**Resize:** drag any of the four quadrant handles to change the radius. Center stays fixed.

**Adjust the sweep:** drag either endpoint handle to change the start angle or sweep angle while keeping the radius fixed.

**Snap:** other shapes can snap to the center, the start, the end, and the midpoint of the arc.

## Notes

For full circles use a Circle, not an Arc with sweep 360°.

For very flat arcs (sweep angle below \~5°) the result is nearly indistinguishable from a straight line at typical drawing scales — use a Line instead unless the arc is drawn at large scale and the curvature matters visually.

When the parent CAD document is alignment-driven, the arc's center is placed onto the alignment, but the arc itself stays a true circular arc (it does not warp into an elliptical curve over a horizontal curve).


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