Transformation one projection to another in one step should not skip the datum shift

core/src/main/java/org/locationtech/proj4j/BasicCoordinateTransform.java

@@ -185,16 +185,33 @@ private void datumTransform(ProjCoordinate pt) {
         /* -------------------------------------------------------------------- */
         /*      Short cut if the datums are identical.                          */
         /* -------------------------------------------------------------------- */
-        if (srcCRS.getDatum().isEqual(tgtCRS.getDatum())
-                || srcCRS.getDatum().getTransformType() == Datum.TYPE_UNKNOWN
-                || tgtCRS.getDatum().getTransformType() == Datum.TYPE_UNKNOWN)
+        int srcCrsDatumTransformType = srcCRS.getDatum().getTransformType();
+        int tgtCrsDatumTransformType = tgtCRS.getDatum().getTransformType();
+        if (srcCRS.getDatum().isEqual(tgtCRS.getDatum())) {
+          return;
+        }
+
+        if (srcCRS.getDatum().getEllipsoid().isEqual(tgtCRS.getDatum().getEllipsoid())) {
+          if (srcCrsDatumTransformType == Datum.TYPE_UNKNOWN) {
             return;
+          }
+          if (tgtCrsDatumTransformType == Datum.TYPE_UNKNOWN) {
+            return;
+          }
+        }
+
+        if (srcCrsDatumTransformType == Datum.TYPE_WGS84 && tgtCrsDatumTransformType == Datum.TYPE_UNKNOWN) {
+          return;
+        }
+        if (srcCrsDatumTransformType == Datum.TYPE_UNKNOWN && tgtCrsDatumTransformType == Datum.TYPE_WGS84) {
+          return;
+        }
 
         /* -------------------------------------------------------------------- */
         /*	If this datum requires grid shifts, then apply it to geodetic    */
         /*      coordinates.                                                    */
         /* -------------------------------------------------------------------- */
-        if (srcCRS.getDatum().getTransformType() == Datum.TYPE_GRIDSHIFT) {
+        if (srcCrsDatumTransformType == Datum.TYPE_GRIDSHIFT) {
             srcCRS.getDatum().shift(pt);
         }
 
@@ -227,7 +244,7 @@ private void datumTransform(ProjCoordinate pt) {
         /* -------------------------------------------------------------------- */
         /*      Apply grid shift to destination if required.                    */
         /* -------------------------------------------------------------------- */
-        if (tgtCRS.getDatum().getTransformType() == Datum.TYPE_GRIDSHIFT) {
+        if (tgtCrsDatumTransformType == Datum.TYPE_GRIDSHIFT) {
             tgtCRS.getDatum().inverseShift(pt);
         }
     }

core/src/test/java/org/locationtech/proj4j/datum/DatumShiftTest.java

@@ -0,0 +1,73 @@
+package org.locationtech.proj4j.datum;
+
+import org.junit.Test;
+import org.locationtech.proj4j.CRSFactory;
+import org.locationtech.proj4j.CoordinateReferenceSystem;
+import org.locationtech.proj4j.CoordinateTransformFactory;
+import org.locationtech.proj4j.ProjCoordinate;
+
+import static org.junit.Assert.assertTrue;
+
+public class DatumShiftTest {
+
+  private final CRSFactory crsFactory = new CRSFactory();
+  private static final CoordinateTransformFactory transformerFactory = new CoordinateTransformFactory();
+
+  @Test
+  public void gk3ToUtm32Zn() {
+
+    String epsg31467Parameters = "+proj=tmerc +lat_0=0.0 +lon_0=9.0 +k_0=1.0 +x_0=3500000.0 +y_0=0.0 +datum=potsdam +a=6377397.155 +f=299.1528128 +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 +pm=greenwich +units=m +no_defs";
+    String utm32znParameters = "+proj=tmerc +lat_0=0.0 +lon_0=9.0 +k_0=0.9996 +x_0=3.25E7 +y_0=0.0 +a=6378137.0 +f=298.257222 +pm=Greenwich +units=m +no_defs";
+
+    CoordinateReferenceSystem etrsCrs = crsFactory.createFromName("EPSG:4258");
+    CoordinateReferenceSystem wgs84Crs = crsFactory.createFromName("EPSG:4326");
+    CoordinateReferenceSystem dhdnCrs = crsFactory.createFromName("EPSG:4314");
+    CoordinateReferenceSystem gk3Crs = crsFactory.createFromParameters("Anon", epsg31467Parameters);
+    CoordinateReferenceSystem utm32znCrs = crsFactory.createFromParameters("Anon", utm32znParameters);
+
+    ProjCoordinate coordinate = new ProjCoordinate(9.0, 50.0);
+
+    ProjCoordinate dhdnToGk3Coordinate = transform(dhdnCrs, gk3Crs, coordinate);
+    ProjCoordinate gk3ToUtm32znCoordinate = transform(gk3Crs, utm32znCrs, dhdnToGk3Coordinate);
+
+    ProjCoordinate gk3ToDhdnCoordinate = transform(gk3Crs, dhdnCrs, dhdnToGk3Coordinate);
+    ProjCoordinate dhdnToWgs84Coordinate = transform(dhdnCrs, wgs84Crs, gk3ToDhdnCoordinate);
+    ProjCoordinate wgs84ToEtrsCoordinate = transform(wgs84Crs, etrsCrs, dhdnToWgs84Coordinate);
+    ProjCoordinate etrsToUtm32znCoordinate = transform(etrsCrs, utm32znCrs, wgs84ToEtrsCoordinate);
+
+    double dx = Math.abs(gk3ToUtm32znCoordinate.x - etrsToUtm32znCoordinate.x);
+    double dy = Math.abs(gk3ToUtm32znCoordinate.y - etrsToUtm32znCoordinate.y);
+    double delta = Math.max(dx, dy);
+
+     assertTrue(delta < 0.001);
+  }
+
+  @Test
+  public void debug() {
+
+    String epsg31467Parameters = "+proj=tmerc +lat_0=0.0 +lon_0=9.0 +k_0=1.0 +x_0=3500000.0 +y_0=0.0 +datum=potsdam +a=6377397.155 +f=299.1528128 +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 +pm=greenwich +units=m +no_defs";
+    String utm32znParameters = "+proj=tmerc +lat_0=0.0 +lon_0=9.0 +k_0=0.9996 +x_0=3.25E7 +y_0=0.0 +a=6378137.0 +f=298.257222101 +pm=Greenwich +units=m +no_defs";
+
+    CoordinateReferenceSystem dhdnCrs = crsFactory.createFromName("EPSG:4314");
+    CoordinateReferenceSystem gk3Crs = crsFactory.createFromParameters("Anon", epsg31467Parameters);
+    CoordinateReferenceSystem utm32znCrs = crsFactory.createFromParameters("Anon", utm32znParameters);
+
+    ProjCoordinate coordinate = new ProjCoordinate(9.0, 50.0);
+
+    ProjCoordinate dhdnToGk3Coordinate = transform(dhdnCrs, gk3Crs, coordinate);
+    ProjCoordinate gk3ToUtm32znCoordinate = transform(gk3Crs, utm32znCrs, dhdnToGk3Coordinate);
+
+
+    assertTrue(true);
+  }
+
+  private ProjCoordinate transform(
+      CoordinateReferenceSystem sourceCrs,
+      CoordinateReferenceSystem targetCrs,
+      ProjCoordinate coordinate) {
+    ProjCoordinate result = new ProjCoordinate();
+    transformerFactory.createTransform(sourceCrs, targetCrs).transform(coordinate, result);
+    return result;
+  }
+
+}