diff --git a/c/transform.c b/c/transform.c
index 5814ca8..9de2d89 100644
--- a/c/transform.c
+++ b/c/transform.c
@@ -24,43 +24,44 @@ INLINE static int outOfChina(double lat, double lng) {
 	return 0;
 }
 
-#define EARTH_R 6378137.0
+#define EARTH_R 6371000
 
 void transform(double x, double y, double *lat, double *lng) {
 	double xy = x * y;
-	double absX = sqrt(fabs(x));
+	double sqrtX = sqrt(fabs(x));
 	double xPi = x * M_PI;
 	double yPi = y * M_PI;
-	double d = 20.0*sin(6.0*xPi) + 20.0*sin(2.0*xPi);
+	double d = 2.0*sin(6.0*xPi) + 2.0*sin(2.0*xPi);
 
 	*lat = d;
 	*lng = d;
 
-	*lat += 20.0*sin(yPi) + 40.0*sin(yPi/3.0);
-	*lng += 20.0*sin(xPi) + 40.0*sin(xPi/3.0);
+	*lat += 2.0*sin(yPi) + 4.0*sin(yPi/3.0);
+	*lng += 2.0*sin(xPi) + 4.0*sin(xPi/3.0);
 
-	*lat += 160.0*sin(yPi/12.0) + 320*sin(yPi/30.0);
-	*lng += 150.0*sin(xPi/12.0) + 300.0*sin(xPi/30.0);
+	*lat += 16.0*sin(yPi/12.0) + 32.0*sin(yPi/30.0);
+	*lng += 15.0*sin(xPi/12.0) + 30.0*sin(xPi/30.0);
 
-	*lat *= 2.0 / 3.0;
-	*lng *= 2.0 / 3.0;
+	*lat *= 20.0 / 3.0;
+	*lng *= 20.0 / 3.0;
 
-	*lat += -100.0 + 2.0*x + 3.0*y + 0.2*y*y + 0.1*xy + 0.2*absX;
-	*lng += 300.0 + x + 2.0*y + 0.1*x*x + 0.1*xy + 0.1*absX;
+	*lat += -100.0 + 2.0*x + 3.0*y + 0.2*y*y + 0.1*xy + 0.2*sqrtX;
+	*lng += 300.0 + x + 2.0*y + 0.1*x*x + 0.1*xy + 0.1*sqrtX;
 }
 
 static void delta(double lat, double lng, double *dLat, double *dLng) {
 	if ((dLat == NULL) || (dLng == NULL)) {
 		return;
 	}
+	const double a = 6378245.0;
 	const double ee = 0.00669342162296594323;
 	transform(lng-105.0, lat-35.0, dLat, dLng);
 	double radLat = lat / 180.0 * M_PI;
 	double magic = sin(radLat);
 	magic = 1 - ee*magic*magic;
 	double sqrtMagic = sqrt(magic);
-	*dLat = (*dLat * 180.0) / ((EARTH_R * (1 - ee)) / (magic * sqrtMagic) * M_PI);
-	*dLng = (*dLng * 180.0) / (EARTH_R / sqrtMagic * cos(radLat) * M_PI);
+	*dLat = (*dLat * 180.0) / ((a * (1 - ee)) / (magic * sqrtMagic) * M_PI);
+	*dLng = (*dLng * 180.0) / (a / sqrtMagic * cos(radLat) * M_PI);
 }
 
 void wgs2gcj(double wgsLat, double wgsLng, double *gcjLat, double *gcjLng) {
@@ -94,35 +95,22 @@ void gcj2wgs(double gcjLat, double gcjLng, double *wgsLat, double *wgsLng) {
 }
 
 void gcj2wgs_exact(double gcjLat, double gcjLng, double *wgsLat, double *wgsLng) {
-	double initDelta = 0.01;
-	double threshold = 0.000001;
-	double dLat, dLng, mLat, mLng, pLat, pLng;
-	dLat = dLng = initDelta;
-	mLat = gcjLat - dLat;
-	mLng = gcjLng - dLng;
-	pLat = gcjLat + dLat;
-	pLng = gcjLng + dLng;
+	double dLat, dLng;
+	// n_iter=2: centimeter precision, n_iter=5: double precision
+	const int n_iter = 3;
 	int i;
-	for (i=0; i<30; i++) {
-		*wgsLat = (mLat+pLat) / 2;
-		*wgsLng = (mLng+pLng) / 2;
-		double tmpLat, tmpLng;
-		wgs2gcj(*wgsLat, *wgsLng, &tmpLat, &tmpLng);
-		dLat = tmpLat - gcjLat;
-		dLng = tmpLng - gcjLng;
-		if ( (fabs(dLat)<threshold) && (fabs(dLng)<threshold) ) {
-			return;
-		}
-		if (dLat > 0) {
-			pLat = *wgsLat;
-		} else {
-			mLat = *wgsLat;
-		}
-		if (dLng > 0) {
-			pLng = *wgsLng;
-		} else {
-			mLng = *wgsLng;
-		}
+	if ((wgsLat == NULL) || (wgsLng == NULL)) {
+		return;
+	}
+	*wgsLat = gcjLat;
+	*wgsLng = gcjLng;
+	if (outOfChina(gcjLat, gcjLng)) {
+		return;
+	}
+	for (i = 0; i < n_iter; i++) {
+		delta(*wgsLat, *wgsLng, &dLat, &dLng);
+		*wgsLat = gcjLat - dLat;
+		*wgsLng = gcjLng - dLng;
 	}
 }