Extracting trends (IV)

I have been having another little play around today with extracting trends from our data [previous: (1)(2)(3)], this time from English Heritage’s National Record of the Historic Environment (NRHE).  We have this data for all time periods for all of England, except London (as such, London is masked out in the maps below).  I was wondering how the broad trends in this data for our period would compare to the broad trends for all time periods.  This time, I created logistic trend surfaces, which vary between 0 and 1 to reflect a binary record of presence or absence.

The result for all time periods showed that there was a very consistent presence of NRHE records across pretty much all of England, with the exception of southern Cumbria and the Scottish Borders:

Logistic trend surface (12th power) for NRHE data for all time periods

However, the picture for our overall time period of interest was very different (Bronze Age, Iron Age, Prehistoric, Roman, early medieval):

Logistic trend surface (12th power) for NRHE data for Bronze Age through to early medieval (plus prehistoric)

Here, we can see that there is a clear peak across England from Wessex across the Home Counties and up towards North Yorkshire, with clear troughs in the Weald, most of the south west, and most of the West Midlands, north east and north west.  Smaller peaks exist in north Northumbria, north Cumbria, and south west Cornwall.  The fact that data clearly exists in great quantities for later periods across some of the troughs in this trend surface could, perhaps, suggest that this represents a genuine absence of activity during our time period of interest (as archaeology has clearly been found for other time periods)?

As these are both logistic trend surfaces that vary across the same numerical scale from 0 to 1, we can also perform some simple mathematical calculations using the rasters as algrebraic terms:

Trend surface for all time periods subtracted from trend surface for EngLaId time periods

On this surface a value of -1 shows a strong trend across all periods but a weak trend within our time period, a value of 0 shows similar trends in both, and a value of 1 shows a strong trend in our period with a weak trend across all periods as a whole.  However, as should be expected, because the trend surface for all periods is distinctly high value for most of the country (and because it includes our time period), no areas have come out with a strong trend in our period and a weak trend across all periods.  As such, this result is not particularly interesting, but might be made more interesting by removing the data for the EngLaId time periods from the “all periods” data or by comparing two specific time periods (e.g. Roman against early medieval or Roman against Iron Age).  I shall continue to experiment.

Happy new year!

Chris Green

Author: Chris Green

Postdoctoral Researcher (GIS)

2 thoughts on “Extracting trends (IV)”

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