Units Classes design

I make a class Meters to represent meters, a class Feet to represent feet, and a class Length to represent a length.

Meters "is a" Length.
Feet "is a" Length.
So it sounds like Meters should inherit from Length, and Feet should inherit from Length.

I would also like Meters to have a UnitSymbol property, and Feet to have a UnitSymbol property.

But I can't have a UnitSymbol property for Length, because units are specific to the subtype.

But I do want every subtype to have a UnitSymbol property.

So I'll make UnitSymbol a virtual property in class Length, and have Meters and Feet overrride that.

Now I would like to be able to compare Feet to Feet, and Meters to Meters, using comparison operators <, <=, ==, !=, >=, >.

I can add that to Feet, and I can add that to Meters.

Can I add that to Length?

Now I have a problem comparing Feet to Meters.

Length m = new Meters(12);
Length f = new Feet(36);
if (f > m)
{
    ...
}

Select allOpen in new window

Well now it starts to get complicated.
What if I later add a class for Inches and a class for Kilometers? And I want to be able to compare any length with any other length?
Is this possible? Or am I going off in the wrong direction here?