Exploratory analysis of two Iron Age hillforts in Northern Galicia (Spain)

Airborne LiDAR (Light Detection and Ranging) or Airborne Laser Scanning (ALS) is one of the most innovative and promising active remote sensing techniques for Landscape Archaeology. However, it´s only by the combination of different techniques in an integrated perspective that we acquire a more complete overview of the hidden Archaeology, being fundamental to ground truth the results.

This case study integrates airborne LiDAR and vertical aerial imagery to investigate two Iron Age hillforts in Northern Galicia (Guitiriz, Lugo, Spain). In particular, we combine airborne LiDAR data with current orthophotos and historical aerial photos, namely those from the United States Air Force (USAF) 1950s flight.

With this complementary information we were able to analyze the recent past evolution and alteration and also the morphology of both settlements.

In the end, we discuss the methodology and the results and we also make some considerations about future prospects.

Aerial photos of hillfort 2 in 2007 (up) and the 50s

Source

João Fonte & Luis Gonçalves-Seco. An integration of airborne LiDAR and vertical aerial imagery to analyze two Iron Age hillforts in Northern Galicia (Spain). Poster presented at the International Aerial Archaeology Conference - AARG 2010 (Bucharest, Romania).

Material and Methods - LiDAR data colection

Parameters Values

Sensor ALTM 3025, Optech

Laser pulse frequency 25 kHz

Scanning frequency 200 Hz

Scanning angle +- 17º

Flight elevation 1300 m

Point density 4 pts/m2 (south zone) and 8 pts/m2 (north zone)

Block number (4 pts/m2) 133 (2971 ha)

Block number (8 pts/m2) 171 (3626 ha)

Planimetric accuracy H/2000

Altimeter accuracy 15 cm at 1200 m, 15 cm at 2000 m

Flight date July 2007

Raw Lidar data for hillfort 2

Results

The DSM was interpolated from the first LiDAR return. The Adaptive Morphological Filter (AMF) proposed by Gonçalves-Seco et al. was used for DTM extraction. This method was based on the approach suggested by Zhang et al. (2003), but it was adapted to rural areas. In addition, this algorithm enables the discrimination of low and high vegetation, which is important for the generation of vegetation properties. After applying the AMF, DTM generation merely consisted in interpolating the points classified as terrain. To produce a smooth topographic surface a kriging interpolation method with a 0.5 m cell size was used to generate the models.

Digital Surface Model (top) and Digital Terrain Model (bottom) for hillfort 2

Conclusions and future perspectives

Multi-echo discrete-pulse airborne LiDAR data poses some problems in the detection of anthropogenic micro-topographic reliefs in forested areas. We think that full-waveform airborne laser scanning (Doneus et al. 2008) is the way forward in archaeological prospection. An integrate perspective seems to be the best approach. Visualization techniques are currently experiencing a great improvement and so local relief models (Hesse 2010) to enhance the visibility of subtle topographic features. However, quite work is still to be done in the consideration of airborne LiDAR data within a landscape perspective.

Some extra analyses were carried out, as this Local Relief Model (following Hesse 2010).

References

Doneus, M., Briese, C., Fera, M., Janner, M. (2008). "Archaeological prospection of forested areas using full-waveform airborne laser scanning", 35: 882-893.

Gonçalves-Seco, L., González-Ferreiro, E., Miranda, D., Crecente, R., Diéguez, U. (forthcoming) "Assessing attributes of high density stands using Airborne Laser Scanner data".

Hesse, R. (2010). "LiDAR-derived Local Relief Models - a new tool for archaeological prospection", 17: 67-72.

Zhang, K., Chen, S., Shyu, M. and Zhang, C., (2003). "A progressive morphological filter for removing nonground measurements from airborne LiDAR data", 4: 872-882.

Acknowledgements

The authors are grateful to the Land Laboratory of the University of Santiago de Compostela for having provided the LiDAR data.