Yesterday Dr Isobel Thompson, of the Historic Environmental Unit at Hertfordshire County Council, gave a very interesting talk on the Hertfordshire Historic Environment Record which holds details of archaeological finds and investigations, and other sites and monuments up to the time of the Second World War. In addition there is a comprehensive collection of air photographs of the county covering the last 40 years. While much of the detailed information is not available online, some is available on the Heritage Gateway web site where a map will show locations of the sites records (and for monuments and some buildings brief information) However you can also see a list of the Hertfordshire records and get a summary of the site record.