Volunteers Liz and Roger Hamling have been surveying the snakes at the Pebblebed Heaths for 15 years.
Mrs Hamling said she had done the surveys so many times she knew the route by heart.
“Around the Pebblebed Heaths we have smooth snake refuges – a sheet of corrugated iron laid flat on the ground, under which smooth snakes like to hide and keep warm,” she said.
Mr Hamling said he had taken more than 150 photographs of the snakes since 2010 to help gauge numbers.
“The head of a smooth snake has a distinctive heart-shaped marking and combined with the first few segments of their bodies, which also have patterns, these markings are unique to each individual snake,” he said.