> I am less sure. isnt <geo> just a specialization of <num>?

I don't think of it taht way, myself. <num> contains a number,
written in any form. <geo> contains two numbers, that I think TEI
intends should be expressed as numeric representations. (I.e.,
<num>two</num> and <num>3/4</num> are fine, but
<geo>twenty-three</geo> and <geo>4+7</geo> are not.)

If anything, <geo> is a specialized
      <measureGrp type="globalCoordinates">
        <measure type="latitude" unit="degrees" quantity="51.727103"/>
        <measure type="longitude" unit="degrees" quantity="1.2605"/>
      </measureGrp>
with the added bit that a <geoDecl> in the header tells you if your
using a geodetic system other than WGS.