Overview 

The Complement Arc tool is designed to simplify a common drafting and geometric modeling task: completing a full circle from an existing arc segment. 

In standard AutoCAD workflows, drawing the remaining part of an arc to form a circle requires manually calculating the center, radius, and angular span. This can be tedious and error-prone, especially when dealing with complex geometry or curved features extracted from field data. 

The Complement Arc tool automates this process. When a user selects an existing arc, it instantly draws the complementary arc segment—the portion needed to complete a full 360° circle using the same center and radius as the original. 

Use Cases

This tool is especially useful when: 

• Completing circular objects from partial arc data (e.g., manholes, tanks, wells) 
• Recovering full geometry from imported or scanned partial arcs 
• Designing closed circular shapes for mechanical or civil components 
• Working with geometric layouts that involve arc-based templates 

 Workflow Steps 

1. Launch the Tool 
   Start the Complement Arc command from the SPCAD Ribbon or type the command COMARC_SP in the command line. 

2. Select Input Arc 
   Click on the existing arc (Blue arc as illustrated below) that represents a partial circle. 

3. Automatic Completion 
   The tool calculates the missing angular span and draws the complementary arc (e.g., red arc in illustration below) to complete the circle. 

4. Result 
   You now have two arc entities (original + complement) that together form a perfect circle. 

How It Works 

• The tool reads the center, radius, and start/end angles of the selected arc. 
• It computes the remaining angular segment required to complete a 360° circle. 
• A new arc is created using the same center and radius, but with start and end angles opposite the original arc. 

Tips 

• The resulting circle is not a single object but two arcs totaling 360°. 
• You can use the JOIN command afterward to convert them into a single circle-like polyline if needed. 
• Works best on planar arc segments in the XY plane or parallel planes.