What is UAV mapping?
The ability to generate mapping from overlapping aerial photographs through a process known as photogrammetry has been a core technique in mapping for many years. Recent developments in photographic processing using a technique known as Structure from Motion (SfM) allows photogrammetric techniques to be used on basic workstations and laptops using low cost software. SfM analyses overlapping images and uses pixel matching techniques to extract the geometry of the subject of the photography. When this is combined with the ability to capture aerial photographs from an Unmanned Aerial Vehicle (UAV) or drone it creates a powerful and flexible mapping system.
The product of the Structure from Motion processing is a Digital Surface Model (DSM) of the local area with a remarkably high resolution. The survey resolution is generally in the range of 2 – 5cm, even working with photography from an entry-level UAV.
A critical element of this process, particularly if change detection is the primary objective, is the ability to accurately place the DSM in its true geographical position. This is most probably a national mapping system such as Irish Transverse Mercator (ITM) in Ireland. Global Navigation Satellite Systems (GNSS) are needed, either through built-in sensors on the UAV or through the use of accurately surveyed ground markers. When this is done a baseline survey has been established against which a future survey, carried out to the same specification, can be compared. Change can then be identified and quantified.
Glascarrig Motte & Bailey, County Wexford
The site of Glascarrig motte and bailey is located on a slight promontory overlooking the coast. In 1167, Diarmuid Mac Murchada landed at Glascarrig on his return to Ireland, having requested the help of King Henry II to recover his kingdom of Leinster. The motte and bailey castle were probably constructed by William de Caunteton at the end of the 12th century. In 1311, Glascarrig was destroyed by MacMurchadas. At this time a substantial settlement consisting of 48 burgages is recorded at Glascarrig and the site may have been abandoned after this attack.
The motte, a grass-covered flat topped mound, almost 6m in height and 36m in diameter, is defined by a flat bottomed fosse. To the south of the motte is an enclosed area or bailey which is outlined by an earthen bank. The site is located in an area of glacial drift making it particularly susceptible to erosion. Erosion of the eastern extent of the bailey and the fosse has produced a rich collection of pottery and animal bones.
UAV Survey at Glascarrig Motte
The motte and its landscape setting have been mapped by UAV survey twice so far for the CHERISH project (June 2018 and February 2019). The 2018 survey established the baseline, against which future surveys could be precisely compared to detect change.
The surveys to date have used the same UAV, and produced over 400 images, ready to be processed through SfM software. To ensure the survey could be precisely mapped to ITM, GNSS control was achieved. A DSM and orthoimage of the motte were produced, with a resolution of 2.5cm, and positional accuracy, in ITM, to a survey-grade of better than 2cm.
Glascarrig GIS analysis
To assess whether any change has taken place the two DSMs are interrogated in our Geographical Information System (GIS). A shaded relief model of each DSM allows a visual comparison to be made, which suggests no dramatic change has taken place.
A visual inspection is highly subjective, it is not particularly scientific and could potentially miss small but significant changes in the landscape. GIS allows us to do much better than this through mathematical calculations to compute the differences and graphically display them on a deviation map.
What does this tell us?
The deviation map confirms that very little has changed at Glascarrig over the time period June 2018 – February 2019; the site has remained stable. This is perhaps not a great surprise as this is a relatively short time period, and without a major storm event. The majority of the site is within the +/-0.1m range but there are some areas, the blue tones, which were higher in 2018. This can be explained by differences in seasonal vegetation growth, a June survey in 2018, and a February survey in 2019. This is a lesson learned that when planning repeat surveys where possible the time of year should be matched. The red tones at the eastern edge of the area is a pebble beach, which appears to be higher in 2019, indicating the dynamic nature of the foreshore.
This analysis, although not revealing any significant damage at the site on this occasion, has been incredibly informative in testing the value of the technique. It gives confidence that repeat UAV mapping will reveal the extent of any erosion at our monitoring sites should any occur in the future.