在d3中计算多边形的面积

Question

I cannot calculate the area of a polygon in D3 using the function path.area()

I have tried feeding it a list of coordinates as follows:

var d = [
      [-1, 415.44],
      [146.93, 304.47],
      [195.45, 152.13],
      [-1, 134.64]
    ]

path.area(d)

I have also tried to feed it (what I think is) a TopoJSON object, as follows:

path.area({        
        "type": "Feature",
        "geometry": {
            "type": "Polygon", 
            "coordinates": d},            
        });

The first attempt gives you value 0 . The second gives you 'NaN' . Do you guys know what I am doing wrong? As an FYI, I need to calculate the area of a polygon that I reference as follows:

d3.select("#IDofpolygon");

Answer 1

You have to use d3.polygonArea , which:

Returns the signed area of the specified polygon. If the vertices of the polygon are in counterclockwise order (assuming a coordinate system where the origin ⟨0,0⟩ is in the top-left corner), the returned area is positive; otherwise it is negative, or zero.

Here is the demo:

 var d = [ [-1, 415.44], [146.93, 304.47], [195.45, 152.13], [-1, 134.64] ]; var area = d3.polygonArea(d); console.log(area) 

 <script src="https://d3js.org/d3.v4.min.js"></script>