[Corpora-List] Standard ontology for document classification?

[Xabier Saralegi Urizar]

Dear all,
I want to classify many scientific documents among different categories 
based on their knowledge area, such as health, geography...
My question is whether there is a standard ontology for such a 
classification.
Regards,

-- 
Xabier Saralegi Urizar
Elhuyar I+G+B
Zelai Haundi kalea, 3
Osinalde industrialdea
20170 Usurbil
(+34) 943 36 30 40
xabiers at elhuyar.com / www.elhuyar.org