Fixed a numerical issue at tile exit.

The low point at tile minimum elevation was computed with a tiny
positive error, meaning the point really was above min elevation (at
micrometer level). The line segment between entry and exit stopped just
before traversing the Digital Elevation Model, and an error was
triggered.

The solution was to take some margin when computing the segment
endpoints at entry and exit: we now start above max elevation and end
below min elevation so the intersection should be really on the line. 

rugged-core/src/main/java/org/orekit/rugged/core/duvenhage/DuvenhageAlgorithm.java

@@ -72,7 +72,7 @@ public class DuvenhageAlgorithm implements IntersectionAlgorithm {
             while (current == null) {
 
                 // find where line-of-sight crosses tile max altitude
-                final Vector3D entryP = ellipsoid.pointAtAltitude(position, los, tile.getMaxElevation());
+                final Vector3D entryP = ellipsoid.pointAtAltitude(position, los, tile.getMaxElevation() + STEP);
                 if (Vector3D.dotProduct(entryP.subtract(position), los) < 0) {
                     // the entry point is behind spacecraft!
                     throw new RuggedException(RuggedMessages.DEM_ENTRY_POINT_IS_BEHIND_SPACECRAFT);
@@ -267,7 +267,7 @@ public class DuvenhageAlgorithm implements IntersectionAlgorithm {
         throws RuggedException, OrekitException {
 
         // look for an exit at bottom
-        Vector3D exitP = ellipsoid.pointAtAltitude(position, los, tile.getMinElevation());
+        Vector3D exitP = ellipsoid.pointAtAltitude(position, los, tile.getMinElevation() - STEP);
         GeodeticPoint exitGP = ellipsoid.transform(exitP, ellipsoid.getBodyFrame(), null);
 
         switch (tile.getLocation(exitGP.getLatitude(), exitGP.getLongitude())) {

rugged-core/src/test/java/org/orekit/rugged/core/AbstractAlgorithmTest.java

@@ -149,7 +149,7 @@ public abstract class AbstractAlgorithmTest {
 
     }
 
-    private void setUpMayonVolcanoContext()
+    protected void setUpMayonVolcanoContext()
         throws RuggedException, OrekitException {
 
         // Mayon Volcano location according to Wikipedia: 13°15′24″N 123°41′6″E
@@ -196,7 +196,7 @@ public abstract class AbstractAlgorithmTest {
 
     }
 
-    private void setUpCliffsOfMoherContext()
+    protected void setUpCliffsOfMoherContext()
         throws RuggedException, OrekitException {
 
         // cliffs of Moher location according to Wikipedia: 52°56′10″N 9°28′15″ W
@@ -267,8 +267,8 @@ public abstract class AbstractAlgorithmTest {
         state     = null;
     }
 
-    private ExtendedEllipsoid earth;
-    private TileUpdater       updater;
-    private SpacecraftState   state;
+    protected ExtendedEllipsoid earth;
+    protected TileUpdater       updater;
+    protected SpacecraftState   state;
 
 }

rugged-core/src/test/java/org/orekit/rugged/core/duvenhage/DuvenhageAlgorithmTest.java

@@ -17,6 +17,12 @@
 package org.orekit.rugged.core.duvenhage;
 
 
+import org.apache.commons.math3.geometry.euclidean.threed.Vector3D;
+import org.junit.Assert;
+import org.junit.Test;
+import org.orekit.bodies.GeodeticPoint;
+import org.orekit.errors.OrekitException;
+import org.orekit.rugged.api.RuggedException;
 import org.orekit.rugged.core.AbstractAlgorithmTest;
 import org.orekit.rugged.core.raster.IntersectionAlgorithm;
 
@@ -26,4 +32,19 @@ public class DuvenhageAlgorithmTest extends AbstractAlgorithmTest {
         return new DuvenhageAlgorithm();
     }
 
+    @Test
+    public void testNumericalIssueAtTileExit() throws RuggedException, OrekitException {
+        setUpMayonVolcanoContext();
+        final IntersectionAlgorithm algorithm = createAlgorithm();
+        algorithm.setUpTilesManagement(updater, 8);
+        GeodeticPoint intersection = algorithm.intersection(earth,
+                                                            new Vector3D(-3787079.6453602533,
+                                                                          5856784.405679551,
+                                                                          1655869.0582939098),
+                                                            new Vector3D( 0.5127552821932051,
+                                                                         -0.8254313129088879,
+                                                                         -0.2361041470463311));
+        Assert.assertEquals(16.0, intersection.getAltitude(), 1.0e-10);
+    }
+
 }