diff -Nur a/pyproj/datadir.py b/pyproj/datadir.py
--- a/pyproj/datadir.py	2020-03-24 12:53:39.417440608 +0100
+++ b/pyproj/datadir.py	2020-03-24 12:56:19.870089479 +0100
@@ -66,9 +66,7 @@
     if _VALIDATED_PROJ_DATA is not None:
         return _VALIDATED_PROJ_DATA
     global _USER_PROJ_DATA
-    internal_datadir = os.path.join(
-        os.path.dirname(os.path.abspath(__file__)), "proj_dir", "share", "proj"
-    )
+    internal_datadir = "@proj@/share/proj"
     proj_lib_dirs = os.environ.get("PROJ_LIB", "")
     prefix_datadir = os.path.join(sys.prefix, "share", "proj")
 
diff -Nur a/setup.py b/setup.py
--- a/setup.py	2020-03-24 12:53:39.415440624 +0100
+++ b/setup.py	2020-03-24 12:52:05.311232522 +0100
@@ -11,7 +11,7 @@
 PROJ_MIN_VERSION = parse_version("6.2.0")
 CURRENT_FILE_PATH = os.path.dirname(os.path.abspath(__file__))
 BASE_INTERNAL_PROJ_DIR = "proj_dir"
-INTERNAL_PROJ_DIR = os.path.join(CURRENT_FILE_PATH, "pyproj", BASE_INTERNAL_PROJ_DIR)
+INTERNAL_PROJ_DIR = "@proj@"
 
 
 def check_proj_version(proj_dir):
@@ -146,7 +146,7 @@
     # By default we'll try to get options PROJ_DIR or the local version of proj
     proj_dir = get_proj_dir()
     library_dirs = get_proj_libdirs(proj_dir)
-    include_dirs = get_proj_incdirs(proj_dir)
+    include_dirs = get_proj_incdirs("@projdev@")
 
     # setup extension options
     ext_options = {
diff -Nur a/test/conftest.py b/test/conftest.py
--- a/test/conftest.py	2020-03-24 12:53:39.417440608 +0100
+++ b/test/conftest.py	2020-03-24 23:16:47.373972786 +0100
@@ -1,6 +1,7 @@
 import os
 import shutil
 import tempfile
+import stat
 
 import pytest
 
@@ -17,6 +18,15 @@
     with tempfile.TemporaryDirectory() as tmpdir:
         tmp_data_dir = os.path.join(tmpdir, "proj")
         shutil.copytree(data_dir, tmp_data_dir)
+
+        # Data copied from the nix store is readonly (causes cleanup problem).
+        # Make it writable.
+        for r, d, files in os.walk(tmp_data_dir):
+            os.chmod(r, os.stat(r).st_mode | stat.S_IWUSR)
+            for f in files:
+                fpath = os.path.join(r, f)
+                os.chmod(fpath, os.stat(fpath).st_mode | stat.S_IWUSR)
+
         try:
             os.remove(os.path.join(str(tmp_data_dir), "ntv2_0.gsb"))
         except OSError: