Complement #77o7

doc/rst/source/project.rst

@@ -259,10 +259,10 @@ Optional Arguments
 Units
 -----
 
-For map distance unit, append *unit* **d** for arc degree, **m** for arc
-minute, and **s** for arc second, or **e** for meter [Default unless stated otherwise], **f**
-for foot, **k** for km, **M** for statute mile, **n** for nautical mile,
-and **u** for US survey foot.
+For map distance unit, append *unit* **d** for arc degree, **m** for arc minute, and **s** for arc second,
+or **e** for meter [Default unless stated otherwise], **f** for foot, **k** for km, **M** for statute mile,
+**n** for nautical mile, and **u** for US survey foot. We compute such distances using a spherical
+approximation with great circles using the authalic radius (see PROJ_MEAN_RADIUS).
 
 .. include:: explain_help.rst_