# Molehill archaeology on the ground and from the air: quantified assessment of field survey results using UAV and airbone data

Sandra Chabert, Élise Fovet, Franck Vautier, Marion Dacko, Bertrand Dousteyssier

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Sandra Chabert, Élise Fovet, Franck Vautier, Marion Dacko, Bertrand Dousteyssier. Molehill archaeology on the ground and from the air: quantified assessment of field survey results using UAV and aircarbon data. Aerial Archaeology Research Group Conference, Sep 2019, Constanța, Romania. hal-05368748

HAL Id: hal-05368748

https://hal.science/hal-05368748v1

Submitted on 17 Nov 2025

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# MOLEHILL ARCHAEOLOGY ON THE GROUND AND FROM THE AIR: quantified assessment of field survey results using UAV and airborne data

Sandra CHABERT, Élise FOVET, Franck VAUTIER, Marion DACKO, Bertrand DOUSTEYSSIER

IntelEspace - MSH Clermont-Ferrand (CNRS / Univ. Clermont Auvergne)

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B. Dousteissier, 2019

A systematic pedestrian survey was carried out in 2019 on the Gallic oppidum of Gergovie (Massif central, France). The goal was to document the ancient occupation of this basaltic plateau, which has only been sparsely excavated.

The plateau is now covered in meadows separated by low walls, stone banks and field hedges. Survey was done by examining molehills, sheep tracks and a number of cairns and stone banks. The majority of material was collected on molehills; such method poses problems of representativity due to uneven distribution of visible soil patches. Therefore, aerial survey was carried out to map surface soil visibility.

**Automatic detection of bare soil patches** was performed using supervised image classification on very high resolution multispectral data acquired by UAV. Such quantification helps to assess potential distortions in pedestrian survey results due to patchy surface visibility in present day meadows.

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Molehills on UAV image

Automatic detection result

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## Methodological issues

Calculating a proper weighted density of surface artefacts requires short time laps between fieldwork and drone flights, which has not always been possible, despite our efforts: consequently, some survey units “caught” number of new molehills, which leads to an underestimation of the weighted density of surface artefacts (1).

In addition, rare patches of bare soil located in the undergrowth could not have been mapped from UAV data, leading to a slight overestimation of weighted density in some survey units (2).

## Historical stone clearance and terracing

Aerial lidar data was used to assess the possible influence of historical agricultural interventions on the distribution of surface artefacts. Archaeological layers may have been disturbed by historical stone clearance and terracing; therefore, these historical landscape improvements were also studied for the entirety of the plateau.

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Cast shadows (IntelEspace - MSH Clermont, 2017)
raw lidar data : CRAIG - FIT Conseil 2016

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Pictures Fabryc, CD63 - 2019

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**Semi-automatic detection of cairns and stone banks** was performed using object-based image classification on several lidar DEM derivatives (Positive and Negative Openness, LRM and Slope).

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artefacts per m² of
bare soil
(min 0 - max 14.31)

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**Weighted density of surface artefacts** was produced by calibrating quantities of archaeological material with the amount of visible land surface for each survey unit. This map produces a better outcome than the “raw” density one.

This weighted model may be related to the ancient layout of the oppidum. However, we also need to evaluate other potential sources of bias, namely subsequent agricultural practices which could have disturbed archaeological layers.

## Results and discussion

This study of the spatial distribution of surface scatters on the oppidum using aerial archaeology techniques allowed us to tackle several problems of spatial representativity:

- Detection of bare soil patches (chiefly molehills) enabled us to produce a weighted density of surface artefacts, revealing a **more coherent pattern of archaeological evidence**.

- The analysis of cumulative volume of cleared stone made it possible to assess the impact of historical agricultural interventions: artefact densities (all periods combined) do **not seem to be directly related to possible disturbance of buried archaeological layers by historical landscape improvements**. Future mapping of densities by chronological phase or type of material (flints vs ceramic sherds) might slightly modify this first comparison.

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cumulative volume
of cleared stones (m³)
(min 0 - max 1,088.67)

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**Cumulative volume of cleared stone:** an indicator of the intensity of historical agricultural interventions. No obvious correlation can be found between intensity of landscape improvement and the artefact density.

MSH CLERMONT-FERRAND

maison des sciences
de l'homme

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UNIVERSITE
Clermont
Auvergne

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PUY-DE-DÔME
LE DÉPARTEMENT

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GERGOVIE
SITE ARVERNE

This study is part of a post-doctoral research carried out by S. Charbert, funded by the Puy-de-Dôme county council, «Contexte archéologique des sites de Gergovie, Corent, Gondole (Puy-de-Dôme, France), de la Protohistoire récente à l'Antiquité tardive (Ile av. - Ve s. ap. J.-C.)».