Image caption Tap o'Noth hill fort in Aberdeenshire. There are more than 5,000 of these ancient structures in Britain and Ireland but they have been little researched until now

Archaeologists are drafting a volunteer army to help map every ancient hill fort across Britain and Ireland.

It is part of a project to create an online atlas of around 5,000 of these Iron Age monuments.

Prehistory enthusiasts are being asked to identify and record features such as ramparts, ditches and entrances.

Prof Gary Lock, of Oxford University, said: "We want to shed new light on why they were created and how they were used."

Despite their large numbers there has been little academic work on hill forts, how they were used and how they varied across Britain and Ireland, the researchers say.

Market days

Prof Lock, who has studied and excavated a number of the forts in England, said that despite their name archaeological evidence suggests they were not primarily used for military purposes.

"We have found pottery, metalwork and evidence of domestic activities like spinning and weaving, also of agriculture, crops like wheat and barley and of keeping pigs, sheep and cattle," he told BBC News.

Researchers believe they may have been meeting places for religious festivals or market days.

The oldest hill forts are in Ireland and Wales and are up to 3,000 years old. Many were abandoned after the Romans arrived in Britain, but in areas that the Romans did not occupy they were used for longer.

The research team want information not only on well-preserved forts but also on sites where only crop marks indicate their existence. The idea is to build a free online database.

"We are hoping that local archaeology societies will get involved," said Prof Lock.

Citizen science

"Rather than going to a hill fort on your own, it would be better, with a group of people, to talk about what you are looking at, which should make it easier to identify the various details," he said.

Image caption Segsbury Camp, in Oxfordshire, has large ramparts around it but others only exist as crop marks

Dr Jon Murden, director of the Dorset museum in Dorchester, which is owned and run by the county's natural history and archaeological society, told BBC News: "We would love to be involved.

"There are at least 50 hill forts to explore and understand on the South Dorset Ridgeway alone."

Volunteers will be able to feed information on their local hill fort into an online form on the Atlas of Hillforts project website from Monday.

"We are keen to see what the citizen science approach may reveal," said Prof Ian Ralston, of Edinburgh University, the project co-director.

"We hope that the public, including archaeological societies, will get behind this project as it should lead to the discovery of new sites and new information about sites that are considered to be well known. We expect the results of this project to change our vision of these iconic monuments."

The four-year project is funded by the Arts and Humanities Research Council. The maps will be freely available to the public, searchable by region and linked to Google Earth to show the hillforts in the context of the landscape.