Differences in spelling of Rodvald

I am wondering if there is any difference between the last names Rotvald and Rodvald. Is it just a simple change of a letter from Norway to America? My great-great-grandfather's first wife was named Gisken Pederdatter Rotvald, born 1852 and died 1894. I am wondering if any info on her family can be found. They were married around Osakis, MN and I know there were many Rodvald's around this area 100 years ago. As I said they would have been married in the US. I would just like to find out somethings about the Rotvalds/Rodvalds. Thanks!