Draft: Allow to set 'arch' unit override on dimensions

src/Mod/Draft/draftutils/units.py

@@ -100,10 +100,16 @@ def display_external(internal_value,
 
             conversion = q.getUserPreferred()[1]
             uom = q.getUserPreferred()[2]
+        elif unit.lower() == "arch":
+            return App.Units.schemaTranslate(q,5)[0].replace("+"," ")
         else:
-            uom = unit
-            internal_value = q.getValueAs(unit)
-            conversion = 1
+            try:
+                uom = unit
+                internal_value = q.getValueAs(unit)
+                conversion = 1
+            except:
+                conversion = q.getUserPreferred()[1]
+                uom = q.getUserPreferred()[2]
     elif dim == 'Angle':
         return App.Units.Quantity(internal_value, App.Units.Angle).UserString
     else:

src/Mod/Draft/draftviewproviders/view_dimension.py

@@ -211,7 +211,8 @@ class ViewProviderDimensionBase(ViewProviderDraftAnnotation):
         if "UnitOverride" not in properties:
             _tip = QT_TRANSLATE_NOOP("App::Property",
                                      "A unit to express the measurement.\n"
-                                     "Leave blank for system default")
+                                     "Leave blank for system default.\n"
+                                     "Use 'arch' to force US arch notation")
             vobj.addProperty("App::PropertyString",
                              "UnitOverride",
                              "Units",