Before plunging enthusiastically into this topic, the following preface of the case against, might be salutary.
A) Variant or Not?
In the 1960s, and using the GRO birth indexes for 1850, Francis Leeson mapped the distribution of his name and what he regarded as its variants, Lee, Lees, Leigh, Leigh, Lea, Ley, Leese, Leeson, Leason. The resulting plots revealed discrete areas, which remained the same even when compared with a 1960's telephone survey.
A reply was made to this article by Dr Reaney, who criticised the distributions on several fronts:
1) A plot of the modern spelling does not necessarily equate with the original form or distribution:
"Lee, Lea, Ley, Lay and Leigh are all one surname. They all go back ultimately to OE leah and both surnames and place-names have a variety of forms; the different modern spellings may be partly due to ME grammar, partly due to the local dialect or simply to mere chance...Parish Registers did not begin until long after surnames becam fixed; they are not necessarily proof of the original distribution."
It would have taken maybe just one fertile family to migrate in 1530, to give a false impression of the home of a name. Especially if that name did not appear elsewhere in mediaeval documents.
2) A plot cannot be made comparing a root name and its variants, unless one is totally sure that the supposed variant did derive etymologically from the root name. Reaney points out, that in his opinion, Leese (from OE laes, 'pasture') and Leeson, derived from 'son of Lece', are not variants of the root names Lee, Lea, Ley, Lay and Leigh. Reaney has subsequently been criticised for over-reliance on etymology, but I think his general points should be borne in mind by anyone plotting any kind of surname distribution.
B) Surname corruption
George Redmonds in his study of Yorkshire surnames has shown the amazing variability of surnames.
"In addition to the obvious variations associated with the distortion of vowel sounds and the confusion when pronouncing consonants, the author draws attention to the remarkably high incidence of elision and truncation, as well as the introduction of so-called prosthetic consonants such as Y, W or S to preface some surnames beginning with a vowel. He also notes that the final consonant of a first name may transfer to the surname, citing Thomas Anderson alias Saunderson and John Nellis alias Ellis."
Source: Surnames and Genealogy: A New Approach by George Redmonds, New England Historic Genealogical Society, Boston, USA. 1997. (Quoted by Derek Palgrave in a review in The Escutcheon Vol 3, No. 1, 1997)
Dramatic changes could also occur to the final syllable of surnames as for example in Whithalghe/Whitalk/Whitack and Astmough/Astmall/Asman/Asmond. Surnames such as these seem to have had very little stress on the final syllable; it was left to the listener to decide their own interpretation, often in perpetuity.
If you collect the occurrences of a name from say the Hearth Tax, how do you know that the name is what you think it is unless you investigate the genealogy of each bearer? Surname dictionaries will be of little help because they tend to ignore local corrupted forms. They concentrate on the earliest form of a name; surname corruption comes much later.
As George Redmonds says, each occurrence of a surname should be treated as being unique.
End of the cautionary preface