面积测量

在Mapcontrol的mousedown中实现

IGeometry pBuffer = m_pMapControl.TrackPolygon();
IPolygon pPoly = pBuffer as IPolygon;
if (pBuffer == null || pBuffer.IsEmpty)
       return;
IArea pArea = pBuffer as IArea;
double area = pArea.Area;
if (pArea.Area < 0)
{
        area = System.Math.Abs(pArea.Area);
}