Tree ring dating problems
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The degree of cross-matching, that is the measure of similarity between sample and reference, is denoted by a "t-value"; the higher the value the greater the similarity.The greater the similarity the greater is the probability that the patterns of samples and references have been produced by growing under the same conditions at the same time.
In essence, a short period of growth, anything less than 54 rings, is not reliable, and the longer the period of time under comparison the better.
Apart from general commercial undertakings, the Laboratory has also been involved in several major research projects on buildings, the environment and ecology, and, most recently, climate change.
The Laboratory maintains academic links with a number of University units, is involved with public lecture programmes, and provides advice and training to other dendrochronologists.
Tree-ring dating relies on a few simple, but quite fundamental, principles.
Firstly, as is commonly known, trees (particularly oak trees, the most frequently used building timber in England) grow by adding one, and only one, growth-ring to their circumference each, and every, year.
Tree-ring dating relies on obtaining the growth pattern of trees from sample timbers of unknown date by measuring the width of the annual growth-rings.
This is done to a tolerance of 1/100 of a millimetre.
Where a sample retains complete sapwood, that is, it has the last or outermost ring produced by the tree before it was cut, the last measured ring date is the felling date of the tree.
Depending on the date of the last sapwood ring on the other dated samples, and the amount of sapwood they have, the felling date of the other timbers represent can then be deduced.
When samples from the same phase do cross-match with each other they are combined at their matching positions to form what is known as a "site chronology".
As with any set of data, this has the effect of reducing the anomalies of any one individual (brought about in the case of tree-rings by some non-climatic influence) and enhances the overall climatic signal.
As stated above, it is the climate that gives the growth pattern its distinctive pattern.