While place names are relatively stable they can vary, and places can merge - and "new" names emerge. Much Hadham and Great Hadham are the same place. Great Berkhampstead only finally decided on the name/spelling Berkhamsted early in the 20th century, and Chipping Barnet is now simply Barnet. Ardeley was once spelt Yardley - which can cause difficulty for the unaware. Harpenden was once called Harding. Market Street became Markyate.
One can also get "new" names. Welwyn Garden City is no great problem - but Dacorum (formerly the name of a Saxon hundred) is now used for the modern borough which includes Hemel Hempstead, Berkhamsted and Tring. Names "vanish" from maps - particularly parishes and hamlets which become part of bigger towns - such as Bengeo - which immediately adjoined Hertford and is now shown as part of Hertford - with only a street name to show where it is in the current Hertfordshire Street atlas.
Sometimes names are changed for "politically correct" reasons. Union Road in St Albans was given the name in the 19th century because it went to the Union Workhouse - and about 50 years ago it was changed to Normandy Road because no-one wanted to be reminded of the Workhouse. Hill End, St Albans, which is one of the many Hill End in Hertfordshire has been renamed Highfield because some people are afraid potential house buyers on the new estate might realise that the site was previously occupied by a Victorian Mental Asylum. Sadly many public houses, which have remained with the same name for hundreds of years are falling foul of brewery chains who ditch the old name with a trendy new name.
An example of the problems is appropriate. I am researching the area of Bernards Heath and the boundaries in the area are complex. There are three parishes Sandridge, St Michael's and St Peters. In addition there was the old borough of St Albans, with boundaries which had no relation to the parish boundaries. There were also three Manors - Sandridge manor appears to have corresponded to the parish of Sandridge - apart from a four acre field - but Newlane Squillers manor (which was mainly in St Peters and but for which the exact boundaries appear to have been lost) seems to have overlapped.
Since the latter part of the 19th century St Albans has expanded its boundaries on several occasions with no relation to the ancient parish or manor boundaries. Around 1900 a new parish, St Saviours, was carved out of the southern part of Sandridge (which has also lost the northern part for different reasons.) This means that my great grandfather moved into a farm in the parish of Sandridge in 1871. A few years later the farm house became part of the City of St Albans, while most of the fields, and part of the farmyard remained administratively in Sandridge. However both the farm house and the farm remained in the Church of England Parish of Sandridge. Later St Albans expanded to swallow all that was left of the farm (most of which was by then housing estates) and at a different date the Church of England parish of St Saviour's was formed.
This means that without moving house my great grandfather was transferred, in stages, from the church and administrative parish of Sandridge to the City of St Albans and the parish of St Saviours.
In genealogical research these changes must be coupled with the uncertainties of the records. For instance when someone was asked where they were born for the census did they give the name of the farm, the hamlet, the Church of England parish, the administrative district, or merely the county. (Most people born in the urban part of the parish of St Peter's probably said St Albans, while many born in the rural part probably gave the hamlet name.) The enumerator would then write down what he though he heard with the best spelling he could muster - and the result may have been transcribed by someone with little knowledge of the local geography. Add to this the problems of several places with the same name (at least 3 Water End, and I don't know how many Dane End) and places with similar names, such as Albury and Aldbury, and it is obvious that attempting serious genealogy without also researching the local geography and understanding place names is really not on.
See also Where is it? Evaluating "Erroneous" Place Names
If you have not already done so look at the information on Hertfordshire Maps
Page updated January 2008