diff --git a/include/mapnik/map.hpp b/include/mapnik/map.hpp
index 0140201ae..face79aca 100644
--- a/include/mapnik/map.hpp
+++ b/include/mapnik/map.hpp
@@ -131,7 +131,7 @@ private:
     boost::optional<std::string> font_directory_;
     freetype_engine::font_file_mapping_type font_file_mapping_;
     freetype_engine::font_memory_cache_type font_memory_cache_;
-    proj_cache_type proj_cache_;
+    mutable proj_cache_type proj_cache_;
 public:
 
     using const_style_iterator = std::map<std::string,feature_type_style>::const_iterator;
@@ -287,6 +287,12 @@ public:
      */
     layer const& get_layer(size_t index) const;
 
+    /*! \brief Get a layer.
+     *  @param index layer number.
+     *  @return Non-constant layer.
+     */
+    layer& get_layer(size_t index);
+
     /*! \brief Remove a layer.
      *  @param index layer number.
      */
@@ -297,6 +303,11 @@ public:
      */
     std::vector<layer> const& layers() const;
 
+    /*! \brief Get all layers.
+     *  @return Non-constant layers.
+     */
+    std::vector<layer> & layers();
+
     /*! \brief Remove all layers and styles from the map.
      */
     void remove_all();
diff --git a/src/map.cpp b/src/map.cpp
index 43e121e93..a0ca76bf5 100644
--- a/src/map.cpp
+++ b/src/map.cpp
@@ -333,8 +333,10 @@ proj_transform * Map::get_proj_transform(std::string const& source, std::string
     auto itr = proj_cache_.find(key, compatible_hash{}, compatible_predicate{});
     if (itr == proj_cache_.end())
     {
-        throw std::runtime_error("Failed to initialise projection transform:" +
-                                 key.first.to_string() + " -> " + key.second.to_string());
+        mapnik::projection srs1(source, true);
+        mapnik::projection srs2(dest, true);
+        return proj_cache_.emplace(std::make_pair(source, dest),
+                                   std::make_unique<proj_transform>(srs1, srs2)).first->second.get();
     }
     return itr->second.get();
 }
@@ -387,11 +389,21 @@ layer const& Map::get_layer(size_t index) const
     return layers_[index];
 }
 
+layer& Map::get_layer(size_t index)
+{
+    return layers_[index];
+}
+
 std::vector<layer> const& Map::layers() const
 {
     return layers_;
 }
 
+std::vector<layer> & Map::layers()
+{
+    return layers_;
+}
+
 unsigned Map::width() const
 {
     return width_;