Index: /issm/trunk-jpl/src/c/Container/DataSet.cpp
===================================================================
--- /issm/trunk-jpl/src/c/Container/DataSet.cpp	(revision 14237)
+++ /issm/trunk-jpl/src/c/Container/DataSet.cpp	(revision 14238)
@@ -178,4 +178,5 @@
 	/*Check index in debugging mode*/
 	_assert_(this!=NULL);
+	_assert_(offset>=0);
 	_assert_(offset<this->Size());
 
Index: /issm/trunk-jpl/src/c/Container/Observations.cpp
===================================================================
--- /issm/trunk-jpl/src/c/Container/Observations.cpp	(revision 14237)
+++ /issm/trunk-jpl/src/c/Container/Observations.cpp	(revision 14238)
@@ -129,9 +129,17 @@
 	Observation *observation  = NULL;
 
-	/*If radius is not provided or is 0, return all observations*/
+	/*First, find closest point in Quadtree (fast but might not be the true closest obs)*/
+	this->quadtree->ClosestObs(&index,x_interp,y_interp);
+	_assert_(index>=0);
+	observation=dynamic_cast<Observation*>(this->GetObjectByOffset(index));
+	hmin2 = (observation->x-x_interp)*(observation->x-x_interp) + (observation->y-y_interp)*(observation->y-y_interp);
+
+	/*Compute radius square*/
 	if(radius==0) radius=this->quadtree->root->length;
-
-	/*Compute radius square*/
 	radius2 = radius*radius;
+	if(hmin2<radius2){
+		radius =sqrt(hmin2);
+		radius2=hmin2;
+	}
 
 	/*Find all observations that are in radius*/
@@ -140,14 +148,7 @@
 		observation=dynamic_cast<Observation*>(this->GetObjectByOffset(indices[i]));
 		h2 = (observation->x-x_interp)*(observation->x-x_interp) + (observation->y-y_interp)*(observation->y-y_interp);
-
-		if(i==0){
+		if(h2<hmin2){
 			hmin2 = h2;
 			index = i;
-		}
-		else{
-			if(h2<hmin2){
-				hmin2 = h2;
-				index = i;
-			}
 		}
 	}  
@@ -175,5 +176,8 @@
 	for(int i=0;i<n;i++){
 		this->ClosestObservation(&xi,&yi,&obs,x[i],y[i],radius);
-		distances[i]=sqrt( (x[i]-xi)*(x[i]-xi) + (y[i]-yi)*(y[i]-yi) );
+		if(xi==UNDEF && yi==UNDEF)
+		 distances[i]=UNDEF;
+		else
+		 distances[i]=sqrt( (x[i]-xi)*(x[i]-xi) + (y[i]-yi)*(y[i]-yi) );
 	}
 }/*}}}*/