diff --git a/src/ifcgeom/mapping/IfcCurveSegment.cpp b/src/ifcgeom/mapping/IfcCurveSegment.cpp
index cd92d229bb..727032895d 100644
--- a/src/ifcgeom/mapping/IfcCurveSegment.cpp
+++ b/src/ifcgeom/mapping/IfcCurveSegment.cpp
@@ -40,11 +40,11 @@ namespace
 	double integrate(double a, double b, unsigned n, std::function<double(double)> fn)
 	{
 		double area = 0;
-		double h = (b - a) / (n + 1);
-		for (auto i = 0; i < n; i++)
+		double h = (b - a) / n;
+		for (auto i = 1; i <= n; i++)
 		{
-			auto x1 = a + h * i;
-			auto x2 = a + h * (i + 1);
+			auto x1 = a + h * (i - 1);
+			auto x2 = a + h * i;
 			auto f1 = fn(x1);
 			auto f2 = fn(x2);
 			area += h * (f1 + f2) / 2.0;