diff --git a/cartographer/io/image.cc b/cartographer/io/image.cc
index 693973e..3b4db48 100644
--- a/cartographer/io/image.cc
+++ b/cartographer/io/image.cc
@@ -31,8 +31,6 @@ cairo_status_t CairoWriteCallback(void* const closure,
   return CAIRO_STATUS_WRITE_ERROR;
 }
 
-constexpr cairo_format_t kCairoFormat = CAIRO_FORMAT_ARGB32;
-
 int StrideForWidth(int width) {
   const int stride = cairo_format_stride_for_width(kCairoFormat, width);
   CHECK_EQ(stride % 4, 0);
diff --git a/cartographer/io/image.h b/cartographer/io/image.h
index db27189..8512a29 100644
--- a/cartographer/io/image.h
+++ b/cartographer/io/image.h
@@ -29,6 +29,9 @@
 namespace cartographer {
 namespace io {
 
+// The only cairo image format we use for Cartographer.
+constexpr cairo_format_t kCairoFormat = CAIRO_FORMAT_ARGB32;
+
 // std::unique_ptr for Cairo surfaces. The surface is destroyed when the
 // std::unique_ptr is reset or destroyed.
 using UniqueCairoSurfacePtr =
diff --git a/cartographer/io/submap_painter.cc b/cartographer/io/submap_painter.cc
index 27f1ef2..2498d43 100644
--- a/cartographer/io/submap_painter.cc
+++ b/cartographer/io/submap_painter.cc
@@ -26,12 +26,12 @@ Eigen::Affine3d ToEigen(const ::cartographer::transform::Rigid3d& rigid3) {
 
 void CairoPaintSubmapSlices(
     const double scale,
-    std::map<::cartographer::mapping::SubmapId, SubmapSlice>* submaps,
+    const std::map<::cartographer::mapping::SubmapId, SubmapSlice>& submaps,
     cairo_t* cr, std::function<void(const SubmapSlice&)> draw_callback) {
   cairo_scale(cr, scale, scale);
 
-  for (auto& pair : *submaps) {
-    auto& submap_slice = pair.second;
+  for (auto& pair : submaps) {
+    const auto& submap_slice = pair.second;
     if (submap_slice.surface == nullptr) {
       return;
     }
@@ -57,13 +57,12 @@ void CairoPaintSubmapSlices(
 }  // namespace
 
 PaintSubmapSlicesResult PaintSubmapSlices(
-    std::map<::cartographer::mapping::SubmapId, SubmapSlice>* submaps,
+    const std::map<::cartographer::mapping::SubmapId, SubmapSlice>& submaps,
     const double resolution) {
   Eigen::AlignedBox2f bounding_box;
   {
     auto surface = ::cartographer::io::MakeUniqueCairoSurfacePtr(
-        cairo_image_surface_create(PaintSubmapSlicesResult::kCairoFormat, 1,
-                                   1));
+        cairo_image_surface_create(::cartographer::io::kCairoFormat, 1, 1));
     auto cr =
         ::cartographer::io::MakeUniqueCairoPtr(cairo_create(surface.get()));
     const auto update_bounding_box = [&bounding_box, &cr](double x, double y) {
@@ -90,7 +89,7 @@ PaintSubmapSlicesResult PaintSubmapSlices(
 
   auto surface =
       ::cartographer::io::MakeUniqueCairoSurfacePtr(cairo_image_surface_create(
-          PaintSubmapSlicesResult::kCairoFormat, size.x(), size.y()));
+          ::cartographer::io::kCairoFormat, size.x(), size.y()));
   {
     auto cr =
         ::cartographer::io::MakeUniqueCairoPtr(cairo_create(surface.get()));
diff --git a/cartographer/io/submap_painter.h b/cartographer/io/submap_painter.h
index 93473d9..8a20389 100644
--- a/cartographer/io/submap_painter.h
+++ b/cartographer/io/submap_painter.h
@@ -27,9 +27,6 @@ namespace cartographer {
 namespace io {
 
 struct PaintSubmapSlicesResult {
-  // Data format for 'surface'.
-  static constexpr cairo_format_t kCairoFormat = CAIRO_FORMAT_ARGB32;
-
   PaintSubmapSlicesResult(::cartographer::io::UniqueCairoSurfacePtr surface,
                           Eigen::Array2f origin)
       : surface(std::move(surface)), origin(origin) {}
@@ -59,8 +56,8 @@ struct SubmapSlice {
 };
 
 PaintSubmapSlicesResult PaintSubmapSlices(
-    std::map<::cartographer::mapping::SubmapId, SubmapSlice>* submaps,
-    const double resolution);
+    const std::map<::cartographer::mapping::SubmapId, SubmapSlice>& submaps,
+    double resolution);
 
 }  // namespace io
 }  // namespace cartographer