     Home Search   What's New Index Books Links Q & A Newsletter Banners   Feedback Tip Jar     MSDN Visual Basic Community   Title Calculate a polygon's area polygon, area Graphics

Add the areas of the trapezoids defined by the polygon's edges dropped to the X-axis. When the program considers a bottom edge of a polygon, the calculation gives a negative area so the space between the polygon and the axis is subtracted, leaving the polygon's area. This method gives odd results for non-simple polygons (where edges cross).

The total calculated area is negative if the polygon is oriented clockwise. See also the HowTo Find the orientation of a polygon (clockwise or counterclockwise).

```Private Function SignedPolygonArea() As Single
Dim pt As Integer
Dim area As Single

' Add the first point to the end.
ReDim Preserve m_Points(1 To m_NumPoints + 1)
m_Points(m_NumPoints + 1) = m_Points(1)

' Get the areas.
For pt = 1 To m_NumPoints
area = area + _
(m_Points(pt + 1).X - m_Points(pt).X) * _
(m_Points(pt + 1).Y + m_Points(pt).Y) / 2
Next pt

' Return the result.
SignedPolygonArea = area
End Function

' Find the polygon's area.
Public Function PolygonArea() As Single
' Return the absolute value of the signed area.
PolygonArea = Abs(SignedPolygonArea())
End Function```

For more information on graphics programming in Visual Basic, see my book Visual Basic Graphics Programming.  Copyright © 1997-2010 Rocky Mountain Computer Consulting, Inc.   All rights reserved.  Updated 