DemShape: Let tolerance bet 1/1000 pixel, as that is more robust

isis/src/base/objs/DemShape/DemShape.cpp

@@ -281,7 +281,9 @@ namespace Isis {
     // Secant method with at most 15 iterations. This method converges fast. 
     // If it does not converge in this many iterations, it never will. 
     bool converged = false;
-    double tol = resolution()/100;  // 1/100 of a pixel
+    // Use 1/1000 of a pixel as tolerance. Otherwise the results may be not 
+    // accurate enough for ground-level sensors with oblique views.
+    double tol = resolution()/1000;  
     for (int i = 1; i <= 15; i++) {
 
       if (std::abs(f1) * 1000.0 < tol) {