savernake forest a report for the national mapping ... - English Heritage

of using lidar versus conventional aerial photography. To this end, rather than using the
lidar derived imagery as just another source to be examined simultaneously with the
conventional aerial photographs, each map sheet was mapped using aerial photographs
and lidar derived imagery independently of one another. Because mapping was carried
out by the same team members it is impossible to be certain that knowledge of sites
recorded via one medium did not influence the search for features using the other, but as
far as possible mapping was carried out separately without reference to other sources.
For each map sheet all features were recorded using standard NMP methodology, but
with the duplication of layers according to source (i.e. BANK and LIDAR_BANK etc). This
meant that it was possible to compare not only the number and type of features
recorded, but also their relative locational accuracy. Because the lidar derived imagery is
based on data with residual accuracy of 10 – 15cm it has been assumed that the mapping
based on this is always more accurate than that from rectified aerial photographs. The
detailed survey comparison described below confirmed this.
It had also been assumed that for those areas of actual woodland within the Forest there
would be very little visible on the conventional aerial photographs. In fact in the only
other project that has looked at a largely wooded landscape, the Forest of Dean NMP
(Small & Stoertz 2006), this had not been entirely true as a large number of features were
recorded from within the core area of the forest. However, these were almost exclusively
large features related to industrial activity such as extractive pits and spoilheaps and since
such activity was not expected to be found within the bounds of Savernake it was
believed there would be little recorded within the woodland; this was particularly thought
to be the case with regard to the prehistoric settlement enclosures that were assumed to
exist within the woodland, given their distribution in the arable land around the forest in
the general vicinity. In the Forest of Dean only a single probable prehistoric enclosure had
been found during the NMP survey, whereas many more were recorded from the lidar
data. This did not turn out entirely to be the case in Savernake. As well as the massive
number of features related to the wartime use of the Forest as a wartime ammunition
storage area (see above) a surprisingly large number of features of varied dates were
visible on conventional photographs through fortuitous flight dates in winter when the
trees were largely without leaves and ground cover was low.
It is difficult to calculate the precise number of monuments recorded by each source
because the final records do not record every source on which a given feature was
present, but rather the key source that best illustrates the feature. It is possible to
calculate the number of individual features recorded (as opposed to monument records)
based on the two separate mapping processes and of the 5803 features recorded from
the various sources 1765 were recorded from lidar data, as opposed to 4038 from
conventional photography. These figures however are heavily skewed by the military
remains as 2180 features were recorded from the 1944 USAAF photographs, the vast
majority of which were single military ammunition storage shelters. If, taking into account
the above problems, the actual monuments recorded are analysed based on the recorded
source then of the 350 monuments 166 (47.5%) were recorded from lidar, 131 (37.5%)
from traditional aerial photographs and 53 (15%) from both. As has been noted above
this figure cannot be considered an absolute representation of the source for any given
monument, but should preserve the general impression of sources, which suggests that
for the Savernake project aerial photographs and lidar were equally valuable sources. It is
true that the majority of features recorded from conventional aerial photographs within
©ENGLISH HERITAGE 46 29-2009