[Therion] redefine altitude symbol

[Thomas Holder]

Hi,

how to redefine the altitude point symbol? Defining a metapost 
p_altitude macro within layout/endlayout has no effect.

I'd like to have a symbol like this:
http://www.carto.net/neumann/caving/cave-symbols/cave_symbol_detail.php?symbolChoice%5B%5D=altitude_sealevel&planSymbol=Symbol+Plan&profileSymbol=Symbol+Profile&explanations=Explanations&translations=Translations&languageSelection=english

Regards,
   Thomas