Extracting trends (VIII)

This is yet another short post about trend surfaces, following on from previous (I)(II)(III)(IV)(V)(VI)(VII), but with a new dataset.  After this, I think I have probably exhausted the possibilities for getting information out of our data using trend surface modelling, which is best thought of as an initial exploratory technique in any event.

This time, I have been looking at spatial trends present in English Heritage’s Excavation Index, which has been kindly supplied to us by Tim Evans at the ADS, who recently wrote an excellent journal article on the potential of the Index as a research tool.  This is a record of excavations and investigations that have taken place in England since around the mid nineteenth century.  I do not think that it pretends in any way to be comprehensive, but it is another way of filling in gaps in our data, especially for archaeological work that took place before 1990.

In any event, here are the trend surfaces that I have created based upon the Excavation Index (to different scales [the values being records per sq.km], but the broad picture is the important thing):

1 eh_excind_trend all
12th power linear trend surface for all data in the Excavation Index.
2 eh_excind_trend englaid
12th power linear trend surface for EngLaID period data in the Excavation Index.
3 eh_excind_trend PR
12th power linear trend surface for unspecified prehistoric data in the Excavation Index.
4 eh_excind_trend BA
12th power linear trend surface for Bronze Age data in the Excavation Index.
5 eh_excind_trend IA
12th power linear trend surface for Iron Age data in the Excavation Index.
6 eh_excind_trend RO
12th power linear trend surface for Roman data in the Excavation Index.
7 eh_excind_trend EM
12th power linear trend surface for early medieval data in the Excavation Index.

So, what can we see from looking at these maps?  Overall, the Index shows greatest density of work in the south, particularly around Bristol, London and Kent.  For the EngLaID period as a whole, the pattern is similar, but with the area around Dorset becoming more important.  The unspecified prehistoric is biased towards London and Kent, but there are too few of these records to say that this is particularly meaningful.  The Bronze Age stands out as very distinct from all other periods, with clear peaks in Wessex, eastern Yorkshire and the Peak District: my assumption is that this represents particular research projects undertaken by EH.  The Iron Age shows peaks north of London and stretching down to Kent and towards Wessex.  The Roman trend is similar to the overall pattern for all periods, which is not surprising due to the high numbers of Roman records in the database.  The early medieval peaks around Hampshire, Kent and London, with greater emphasis also on East Anglia than the other periods.

Overall, most of these trends are fairly similar to those seen with previous datasets, at least when considered on a broad brush basis.  The major exception is for the Bronze Age, where the high trend surface peaks previously seen in south west England are no longer as dramatic.  London is also standing out more strongly in the Index than it had in most previous datasets, I think (although this is less pertinent when comparing with the NRHE, as we did not receive NRHE data for London).

Chris Green