Index: src/uk/me/parabola/mkgmap/osmstyle/actions/HeightFilter.java
===================================================================
--- src/uk/me/parabola/mkgmap/osmstyle/actions/HeightFilter.java	(revision 3782)
+++ src/uk/me/parabola/mkgmap/osmstyle/actions/HeightFilter.java	(working copy)
@@ -14,6 +14,7 @@
 package uk.me.parabola.mkgmap.osmstyle.actions;
 
 import uk.me.parabola.mkgmap.reader.osm.Element;
+import uk.me.parabola.mkgmap.scan.SyntaxException;
 
 /**
  * A {@code HeightFilter} transforms values into Garmin-tagged elevations.
@@ -24,9 +25,15 @@
  */
 public class HeightFilter extends ConvertFilter {
 
-    public HeightFilter(String s) {
-		super(s);
-    }
+	public HeightFilter() {
+		super("m=>ft");
+	}
+	
+	public HeightFilter(String s) {
+		super(s == null ? "m=>s" : s);
+		if (s != null && !(s.endsWith("ft") || s.endsWith("feet")))
+			throw new SyntaxException(String.format("height filter reqires ft (feet) as target unit: '%s'", s));
+	}
 
 	public String doFilter(String value, Element el) {
 		String s = super.doFilter(value, el);
Index: doc/styles/rules-filters.txt
===================================================================
--- doc/styles/rules-filters.txt	(revision 3782)
+++ doc/styles/rules-filters.txt	(working copy)
@@ -14,7 +14,7 @@
 
 | conv | `m=>ft` |
 Use for conversions between units.
-With the argument +m=>ft+ the value is converted into feet, with the
+With the argument +m\=\>ft+ the value is converted into feet, with the
 value being assumed to be in meters, unless the value includes a unit
 already.
 If any of the units are not recognised then the value is unchanged.
@@ -94,7 +94,10 @@
 
 | height | `m=>ft` |
 This is exactly the same as the +conv+ filter, except that it prepends a special
-separation character before the value which is intended for elevations.
+separation character before the value which is intended for elevations so that 
+the Garmin software can convert it to the unit configured by the user.
+If no argument is given the default is `m=>ft`, else the target unit 
+must be ft (foot).
 
 `${ele\|height:"m=>ft"}`