Calculating Area

Does anybody have an algorithm for calculating the area defined by an array of coordinates (or points)? I need something independent of AutoCAD ActiveX (mean the Area member of AcadEntity class). An analytic approach should be better. Thanks, if anybody...

 

Check here for a formula:

http://mathforum.org/dr.math/

Good Luck,

Scott

 

found this at http://www.foundrysearch.com/downloads/polygonarea.zip . I
haven't checked it. It looks like it is for a 1-based array. Also, it uses
an open polygon, since the first 3 lines close the poly.

'Greene's Theorem area calculation
pointCount = pointCount+1
pointX(pointCount) = pointX(1)
pointY(pointCount) = pointY(1)
#main.gb "line ";pointX(pointCount-1);" ";pointY(pointCount-1);"
";pointX(pointCount);" ";pointY(pointCount);
for i = 1 to pointCount-1
x1 = pointX(i)
x2 = pointX(i+1)
y1 = pointY(i)
y2 = pointY(i+1)
TrapArea(i) = (x2 - x1)*y1 + (1/2)*(x2 - x1)*(y2 - y1)
next i
x1 = pointX(pointCount)
y1 = pointY(pointCount)
x2 = pointX(1)
y2 = pointY(1)
TrapArea(pointCount) = (x2 - x1)*y1 + (1/2)*(x2 - x1)*(y2 - y1)
TotalArea = 0
For i = 1 to pointCount
TotalArea = TotalArea + TrapArea(i)
Next i
#main.area abs(TotalArea)
wait

array of coordinates (or points)? I need something independent of AutoCAD
ActiveX (mean the Area member of AcadEntity class). An analytic approach
should be better. Thanks, if anybody...

 

This is one way of doing it.

pts is an array of doubles such that
pts(0,0) = x1
pts(0,1) ) = y1
pts(1,0) = x2
pts(1,1) = y2


the last point of the array should be equal to the first (closed polygon)


Public Function AreaCalc(pts() As Double) As Double
Dim j As Long
Dim i As Long
Dim k As Long
Dim M As Double
Dim N As Double

j = UBound(pts)
k = LBound(pts)

For i = k To j - 1
M = M + pts(i, 0) * pts(i + 1, 1)
N = N + pts(i + 1, 0) * pts(i, 1)
Next i
AreaCalc = Abs(M - N) / 2#

End Function

--
Saludos, Ing. Jorge Jimenez, SICAD S.A., Costa Rica

array of coordinates (or points)? I need something independent of AutoCAD
ActiveX (mean the Area member of AcadEntity class). An analytic approach
should be better. Thanks, if anybody...

 

if you take away the 'abs' from
AreaCalc = Abs(M - N) / 2#
it will give you a +ve or -ve area,
this is good for subtracting 'holes',

just make sure your polylines
are counterclockwise for areas,
and clockwise for holes,
or viceversa.

cheers

 

Thanks to everybody !

 

Mihai, the example I posted is simple.
It lacks error handling and does not include the normalizing of the
coordinates to minimize rounding errors.