I looked up the semanticweb's definition of the "length" property, and I was happy to find most of the information there for making a height property in my own semantic wiki, but I have one slight problem that I'm not sure how to address.  Here's an example:

I've got a bunch of values already entered into this template that use one of two forms for defining the height property:  either 5'6" or 170cm or even 1.7m

Now for the last two CM and M I've got that covered, but it seems that since the first one is a combination of feet and inches, it doesn't quite work out.  Is there any easy way to fix this?  Or do I need to get working on converting all of those to some single value?