diff --git a/src/goesproc/map_drawer.cc b/src/goesproc/map_drawer.cc
index f5f6a69a..d8f96c0b 100644
--- a/src/goesproc/map_drawer.cc
+++ b/src/goesproc/map_drawer.cc
@@ -5,12 +5,12 @@ using namespace nlohmann;
 namespace {
 
 Proj longitudeToProj(float longitude) {
-  std::stringstream args;
-  args << "+proj=geos ";
-  args << "+h=35786023.0 ";
-  args << "+lon_0=" << std::to_string(longitude) << " ";
-  args << "+sweep=x";
-  return Proj(args.str());
+  std::map<std::string, std::string> args;
+  args["proj"] = "geos";
+  args["h"] = "35786023.0";
+  args["lon_0"] = std::to_string(longitude);
+  args["sweep"] = "x";
+  return Proj(args);
 }
 
 } // namespace
diff --git a/src/goesproc/proj.cc b/src/goesproc/proj.cc
index 50634c51..bc491101 100644
--- a/src/goesproc/proj.cc
+++ b/src/goesproc/proj.cc
@@ -5,6 +5,85 @@
 
 namespace {
 
+std::vector<std::string> toVector(const std::map<std::string, std::string>& args) {
+  std::vector<std::string> vargs;
+  vargs.reserve(args.size());
+  for (const auto& arg : args) {
+    std::stringstream ss;
+    ss << arg.first << "=" << arg.second;
+    vargs.push_back(ss.str());
+  }
+  return vargs;
+}
+
+}
+
+#if PROJ_VERSION_MAJOR == 4
+
+namespace {
+
+std::string pj_error(std::string prefix = "proj: ") {
+  std::stringstream ss;
+  ss << prefix << pj_strerrno(pj_errno);
+  return ss.str();
+}
+
+} // namespace
+
+// Forward compatibility.
+double proj_torad (double angle_in_degrees) {
+  return angle_in_degrees * DEG_TO_RAD;
+}
+
+Proj::Proj(const std::vector<std::string>& args) {
+  std::vector<char*> argv;
+  for (const auto& arg : args) {
+    argv.push_back(strdup(arg.c_str()));
+  }
+  proj_ = pj_init(argv.size(), argv.data());
+  if (!proj_) {
+    throw std::runtime_error(pj_error("proj initialization error: "));
+  }
+  for (auto& arg : argv) {
+    free(arg);
+  }
+}
+
+Proj::Proj(const std::map<std::string, std::string>& args)
+  : Proj(toVector(args)) {
+}
+
+Proj::~Proj() {
+  pj_free(proj_);
+}
+
+std::tuple<double, double> Proj::fwd(double lon, double lat) {
+  projUV in = { lon, lat };
+  projXY out = pj_fwd(in, proj_);
+  return std::make_tuple<double, double>(std::move(out.u), std::move(out.v));
+}
+
+std::tuple<double, double> Proj::inv(double x, double y) {
+  projXY in = { x, y };
+  projUV out = pj_inv(in, proj_);
+  return std::make_tuple<double, double>(std::move(out.u), std::move(out.v));
+}
+
+#elif PROJ_VERSION_MAJOR >= 5
+
+namespace {
+
+std::string toString(const std::vector<std::string>& vargs) {
+  std::stringstream ss;
+  for (auto it = vargs.begin(); it != vargs.end(); it++) {
+    ss << "+" << *it;
+    if (std::next(it) != std::end(vargs)) {
+      ss << " ";
+    }
+  }
+  return ss.str();
+}
+
 std::string pj_error(std::string prefix = "proj: ") {
   std::stringstream ss;
   ss << prefix << proj_errno_string(proj_errno(NULL));
@@ -13,13 +92,18 @@ std::string pj_error(std::string prefix = "proj: ") {
 
 } // namespace
 
-Proj::Proj(const std::string& args) {
+Proj::Proj(const std::vector<std::string>& vargs) {
+  const auto args = toString(vargs);
   proj_ = proj_create(NULL, args.c_str());
   if (!proj_) {
     throw std::runtime_error(pj_error("proj initialization error: "));
   }
 }
 
+Proj::Proj(const std::map<std::string, std::string>& args)
+  : Proj(toVector(args)) {
+}
+
 Proj::~Proj() {
   proj_destroy(proj_);
 }
@@ -37,3 +121,5 @@ std::tuple<double, double> Proj::inv(double x, double y) {
   PJ_COORD out = proj_trans(proj_, PJ_INV, in);
   return std::make_tuple<double, double>(std::move(out.uv.u), std::move(out.uv.v));
 }
+
+#endif
diff --git a/src/goesproc/proj.h b/src/goesproc/proj.h
index 5c5a86c6..b8cd5edd 100644
--- a/src/goesproc/proj.h
+++ b/src/goesproc/proj.h
@@ -1,17 +1,28 @@
 #pragma once
 
-#if PROJ_VERSION_MAJOR < 5
-#error "proj version >= 5 required"
+#if PROJ_VERSION_MAJOR == 4
+#include <proj_api.h>
+#elif PROJ_VERSION_MAJOR >= 5
+#include <proj.h>
+#else
+#error "proj version >= 4 required"
+#endif
+
+#if PROJ_VERSION_MAJOR == 4
+// Forward compatibility.
+double proj_torad (double angle_in_degrees);
 #endif
 
+#include <map>
 #include <string>
 #include <tuple>
-
-#include <proj.h>
+#include <vector>
 
 class Proj {
 public:
-  explicit Proj(const std::string& args);
+  explicit Proj(const std::vector<std::string>& args);
+
+  explicit Proj(const std::map<std::string, std::string>& args);
 
   ~Proj();
 
@@ -20,5 +31,9 @@ class Proj {
   std::tuple<double, double> inv(double x, double y);
 
 protected:
+#if PROJ_VERSION_MAJOR == 4
+  projPJ proj_;
+#elif PROJ_VERSION_MAJOR >= 5
   PJ *proj_;
+#endif
 };