Cannot convert tag type "rational/byte" to string

I am trying to retrieve the `GPSAltitude` (rational) as integer and `GPSAltitudeRef` as byte but when I use `StringVal()` on the `Tag` data I get that error that those values cannot be converted to string. It is strange because that's what that method is supposed to do.

I first use `v,err := x.Get(Field name(tagname))` followed by `vStr,err := v.StringVal()`
