Extracting trends (V)

One final post (for now) on extracting trends [see: (1)(2)(3)(4)]…

As I suggested I would in my last post on trend surfaces, I have been experimenting today with constructing individual trend surfaces for the four main broad periods of interest to our project in the NRHE data.  I have also been experimenting with some alternative colour schemes after talking to our project artist, Miranda.  So, without further ado, here are the logistic presence / absence trend surfaces for the Bronze Age (excluding specifically Early Bronze Age), Iron Age, Roman, and early medieval periods (blue being low likelihood and red being high):

1 BA
Logistic trend surface (12th power) for NRHE data for Bronze Age
2 IA
Logistic trend surface (12th power) for NRHE data for Iron Age
3 RO
Logistic trend surface (12th power) for NRHE data for Roman period
4 EM
Logistic trend surface (12th power) for NRHE data for early medieval period

These patterns all look intuitively sensible, albeit with some possible edge effects along the coastlines (as a trend surface becomes more unreliable towards its edges, due to comparative lack of data).  To take this further, we can then compare the difference between each surface and its preceding period (not including Bronze Age, as we are not so interested in the Neolithic / EBA):

5 BAtoIA
Difference between Iron Age and Bronze Age trend surfaces
6 IAtoRO
Difference between Roman and Iron Age trend surfaces
7 ROtoEM
Difference between early medieval and Roman trend surfaces

Again, these results do appear to make sense, with some changes of focus between the Bronze Age and Iron Age (with continuing focus in Wessex), a massive expansion in activity / visibility between the Iron Age and Roman periods, excluding the far north (beyond the Wall) and the south west (with a particular increase in the east of England, north of London), and then a large reduction in activity / visibility across the peak of Roman activity moving into the early medieval.

The next stage would be to start building in data from our other data sources into these models, but that will be something for the future.

Chris Green

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:

trend_AMIE_all
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):

trend_AMIE_englaid
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_AMIE_compare
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