Scholars had been misled by two maps of the battlefield in which the perspective is distorted. Now painstaking research by David Smurthwaite, a historian at the National Army Museum, has identified the major landmarks of the battle and shown how the maps have been misinterpreted.
The battle in 1645 was of crucial importance for the development of parliamentary democracy in England, yet there has been no general agreement on the exact location of the armies; the battlefield is marked by two memorials more than a mile apart.
Opponents of the A1-M1 link which passes just north of Naseby, Northamptonshire, had believed that the road, opened a few weeks ago, ran through the site.
But after an investigation by English Heritage it has been possible to pinpoint the precise location and so to understand more fully the disposition of the battle.
On 14 June 1645, Charles I's army of some 8,000 men clashed with the parliamentary army of 14,000 men just north of the village of Naseby. The parliamentary army, which had just been reformed on the 'New Model', scattered the royalist army capturing all their artillery, baggage and 4,000 infantry. It was the decisive battle of the Civil War, assuring the supremacy of Parliament and the defeat of the monarch's claim to absolute power. A pictorial map of the battle was published in 1647 by Joshua Sprigge in his history of the war, Anglia Rediviva. This shows roads which have proved difficult to identify because of the foreshortened perspective.
In 1968, Colonel H C B Rogers argued in his book, Battles and Generals of the Civil War, that historians had underestimated the frontages of armies in civil war battles. He argues that armies were deployed with three feet between each man and a further 18 inches allowed for the man himself. On this basis the front of the parliamentarian army at Naseby was about two miles long. Opponents of the link road adopted Rogers's theory which fitted in with the second map of the battlefield, published in the 17th century by the royalist engineer, Sir Bernard de Gomme.
The parliamentarian lines are shown on this map to be intersected by the road to Clipston. If this were the case then the A1-M1 link would cross the battlefield.
However, Mr Smurthwaite has discovered from a contemporary military drill book that the measurement of three feet between foot soldiers was made from the centre of each man without adding on an extra 18 inches, so reducing the length of the parliamentarian line to one and a third miles.
He also argues that de Gomme's map cannot be overlaid on a modern map because it is contrived from the distorted perspective of the earlier map published by Sprigge.
Mr Smurthwaite has produced a corrected version of Sprigge's illustration of the battle by enlarging the foreground. The altered illustration is now in agreement with Sprigge's own account of the battle which makes clear that the fighting occurred on a large fallow field north-west of Naseby, called Turnmoore field. The identification of the precise location of the troops before the battle enables the course of the battle to be fully understood. A rabbit warren, which lay just east of the Naseby to Sibbertoft Road, directly in front of Cromwell's cavalry, caused him serious problems.
Mr Smurthwaite has been able to identify the road passing through the royalist lines on the horizon in Sprigge's illustration as the road to Sibbertoft, although opponents of the A1-M1 link had argued it was the road to Clipston. However, Mr Smurthwaite concludes that 'Naseby remains one of the best preserved of all England's battlefields. The building of the new A1- M1 link road, although it separates the site of the parliamentarian baggage train west of Naseby from the rest of the battlefield, is screened from the main area of fighting by two ridgetops'.