A grandson who has set out to visit all 61 US National Parks with his 89-year-old grandma has revealed that the epic journey has given him 'a purpose in life'.

Veterinarian Brad Ryan and Joy Ryan, both from Zanesville, Ohio, have so far visited 29 national parks since they set off on their quest in 2015 and Grandma Joy, who had previously never glimpsed the sea or even a mountain, has now 'seen more of America than a majority of Americans ever will'.

Brad, 38, says he and Grandma Joy became estranged after his parents' divorce a decade ago and it was only after they started to reconnect that he learned how much she regretted never seeing the world – and an idea was born.

Brad explained that during a conversation about hiking, Grandma Joy said that she wished she had experienced more of the great outdoors throughout her life.