diff --git a/sbapp/sideband/geo.py b/sbapp/sideband/geo.py
index 17279d0..77ad157 100644
--- a/sbapp/sideband/geo.py
+++ b/sbapp/sideband/geo.py
@@ -418,7 +418,10 @@ class GeoidHeight(object):
                 raise Exception("File has the wrong length")
 
             self.headerlen = headerlen
-            self.raw = mmap.mmap(fd, fullsize, mmap.MAP_SHARED, mmap.PROT_READ)
+            if RNS.vendor.platformutils.is_windows():
+                self.raw = mmap.mmap(fd, fullsize, access=mmap.ACCESS_READ)
+            else:
+                self.raw = mmap.mmap(fd, fullsize, mmap.MAP_SHARED, mmap.PROT_READ)
 
         self.rlonres = self.width / 360.0
         self.rlatres = (self.height - 1) / 180.0