Where can I find isogloss maps for Mandarin? It seems that questions about resources are generally accepted on the site. If not, please let me know.

I was wondering if anyone knew where to find maps with isoglosses for dialects of Mandarin. I am especially interested in phonemic features, such as


erhua
mergers, such as eng/en, in/ing, x/sh etc.
regional differences in tones


but anything would be interesting.
The shapes of Isogloss maps are generally identical to those of dialect maps. So I think this could give you some clues.
 

Below is a sample for isogloss maps(from http://www.yupoo.com/photos/9919/7485900/)

More of them:





Suggestion: For more information about isogloss maps, visit http://image.baidu.com/ and type in "同言线". Further reading: http://book.kongfz.com/15325/226467644/ This is a book about chinese linguistic geography. For more books in this subject, search "汉语方言地理学".