diff --git a/include/GSkyRegionRect.hpp b/include/GSkyRegionRect.hpp
index d57eb2c..2ce274d 100644
--- a/include/GSkyRegionRect.hpp
+++ b/include/GSkyRegionRect.hpp
@@ -93,18 +93,24 @@ public:
     bool              overlaps(const GSkyRegion& reg) const;
     std::string       print(const GChatter& chatter = NORMAL) const;
 
+    GSkyDir           transform_to_local(const GSkyDir& skydir) const;
+
 protected:
     // Protected methods
     void init_members(void);
     void copy_members(const GSkyRegionRect& region);
     void free_members(void);
     void compute_solid_angle(void);
+    void update_cache(void);
 
     // Protected members
-    GSkyDir	m_centre;   //!< Centre or reference point of the region
-    double 	m_halfwidth;  //!< Half width of the region [deg]
+    GSkyDir m_centre;     //!< Centre or reference point of the region
+    double  m_halfwidth;  //!< Half width of the region [deg]
     double  m_halfheight; //!< Half height of the region [deg]
     double  m_posang;     //!< Position angle, counterclockwise from North [radians]
+
+    double m_posang_cos; //!< Cache for transform_to_local
+    double m_posang_sin; //!< Cache for transform_to_local
 };
 
 
@@ -153,44 +159,6 @@ double GSkyRegionRect::posang_deg(void) const
 
 
 /***********************************************************************//**
- * @brief Set position angle of rectangular region
- *
- * @param[in] posang Position angle [radians].
- *
- * Sets the position angle of the rectangular sky region. The position angle
- * is counted counterclockwise from North.
- ***************************************************************************/
-inline
-void GSkyRegionRect::posang(const double& posang)
-{
-    // Set the position angle
-    m_posang = posang;
-
-    // Return
-    return;
-}
-
-
-/***********************************************************************//**
- * @brief Set position angle of rectangular region
- *
- * @param[in] posang Position angle [degrees].
- *
- * Sets the position angle of the rectangular sky region. The position angle
- * is counted counterclockwise from North.
- ***************************************************************************/
-inline
-void GSkyRegionRect::posang_deg(const double& posang)
-{
-    // Set the position angle
-    m_posang = posang * gammalib::deg2rad;
-
-    // Return
-    return;
-}
-
-
-/***********************************************************************//**
  * @brief Return region width extension (in degrees)
  *
  * @return Region width [deg].
diff --git a/pyext/GSkyRegionRect.i b/pyext/GSkyRegionRect.i
index d30633f..63867b0 100644
--- a/pyext/GSkyRegionRect.i
+++ b/pyext/GSkyRegionRect.i
@@ -72,6 +72,8 @@ public:
     bool              contains(const GSkyDir& dir) const;
     bool              contains(const GSkyRegion& reg) const;
     bool              overlaps(const GSkyRegion& reg) const;
+
+    GSkyDir           transform_to_local(const GSkyDir& skydir) const;
 };
 
 
diff --git a/src/sky/GSkyRegionRect.cpp b/src/sky/GSkyRegionRect.cpp
index f0dbfd8..67f7eb5 100644
--- a/src/sky/GSkyRegionRect.cpp
+++ b/src/sky/GSkyRegionRect.cpp
@@ -250,6 +250,48 @@ GSkyRegionRect* GSkyRegionRect::clone(void) const
 
 
 /***********************************************************************//**
+ * @brief Set position angle of rectangular region
+ *
+ * @param[in] posang Position angle [radians].
+ *
+ * Sets the position angle of the rectangular sky region. The position angle
+ * is counted counterclockwise from North.
+ ***************************************************************************/
+void GSkyRegionRect::posang(const double& posang)
+{
+    // Set the position angle
+    m_posang = posang;
+
+    // Update the cache
+    update_cache();
+
+    // Return
+    return;
+}
+
+
+/***********************************************************************//**
+ * @brief Set position angle of rectangular region
+ *
+ * @param[in] posang Position angle [degrees].
+ *
+ * Sets the position angle of the rectangular sky region. The position angle
+ * is counted counterclockwise from North.
+ ***************************************************************************/
+void GSkyRegionRect::posang_deg(const double& posang)
+{
+    // Set the position angle
+    m_posang = posang * gammalib::deg2rad;
+
+    // Update the cache
+    update_cache();
+
+    // Return
+    return;
+}
+
+
+/***********************************************************************//**
  * @brief Set width of rectangular region
  *
  * @param[in] width Width [deg].
@@ -586,6 +628,38 @@ bool GSkyRegionRect::overlaps(const GSkyRegion& reg) const
 }
 
 
+/***********************************************************************//**
+ * @brief Transform a sky direction to the local coordinate system.
+ *
+ * @param[in] skydir Sky direction in global coordinate system.
+ * @return Sky direction in local region object coordinates.
+ *
+ * Transform the sky direction :skydir: to the local coordinate system. The
+ * origin of the local coordinate system is fixed to the center of the
+ * rectangle and aligned in +ra (width) and +dec (height).
+ ***************************************************************************/
+GSkyDir GSkyRegionRect::transform_to_local(const GSkyDir& skydir) const
+{
+    // Compute separation (in radians)
+    double dx = skydir.ra()  - m_centre.ra();
+    double dy = skydir.dec() - m_centre.dec();
+
+    // Correct for spherical coordinate system (global dx -> local dx)
+    dx *= std::cos(skydir.dec());
+
+    // Rotate with neg. PA
+    double new_x = m_posang_cos*dx - m_posang_sin*dy;
+    double new_y = m_posang_sin*dx + m_posang_cos*dy;
+
+    // Create output sky direction
+    GSkyDir transformed;
+    transformed.radec(new_x, new_y);
+
+    // Return
+    return transformed;
+}
+
+
 
 /*==========================================================================
  =                                                                         =
@@ -623,6 +697,10 @@ void GSkyRegionRect::copy_members(const GSkyRegionRect& region)
     m_halfheight = region.m_halfheight;
     m_posang     = region.m_posang;
 
+    // Copy cache
+    m_posang_cos = region.m_posang_cos;
+    m_posang_sin = region.m_posang_sin;
+
     // Return
     return;
 }
@@ -650,3 +728,17 @@ void GSkyRegionRect::compute_solid_angle(void)
     // Return
     return;
 }
+
+
+/***********************************************************************//**
+ * @brief Update the cache.
+ ***************************************************************************/
+void GSkyRegionRect::update_cache(void)
+{
+    // Precompute cos and sin of position angle
+    m_posang_cos = std::cos(m_posang);
+    m_posang_sin = std::sin(m_posang);
+
+    // Return
+    return;
+}