Preserving the impossible: conservation of soft-sediment hominin footprint sites and strategies for three-dimensional digital data capture.

Human footprints provide some of the most publically emotive and tangible evidence of our ancestors. To the scientific community they provide evidence of stature, presence, behaviour and in the case of early hominins potential evidence with respect to the evolution of gait. While rare in the geological record the number of footprint sites has increased in recent years along with the analytical tools available for their study. Many of these sites are at risk from rapid erosion, including the Ileret footprints in northern Kenya which are second only in age to those at Laetoli (Tanzania). Unlithified, soft-sediment footprint sites such these pose a significant geoconservation challenge. In the first part of this paper conservation and preservation options are explored leading to the conclusion that to 'record and digitally rescue' provides the only viable approach. Key to such strategies is the increasing availability of three-dimensional data capture either via optical laser scanning and/or digital photogrammetry. Within the discipline there is a developing schism between those that favour one approach over the other and a requirement from geoconservationists and the scientific community for some form of objective appraisal of these alternatives is necessary. Consequently in the second part of this paper we evaluate these alternative approaches and the role they can play in a 'record and digitally rescue' conservation strategy. Using modern footprint data, digital models created via optical laser scanning are compared to those generated by state-of-the-art photogrammetry. Both methods give comparable although subtly different results. This data is evaluated alongside a review of field deployment issues to provide guidance to the community with respect to the factors which need to be considered in digital conservation of human/hominin footprints

Human Footprints: Fossilised Locomotion?

XII, 216 p. 92 illus., 28 illus. in color.online

For the eight beach prints the two models/shells for the scanner and one for the photo-model were co-registered in Rapidform 2006 and the maximum model thickness or deviation was calculated and attached as vertex colour map to the combined model.

<p>Warm colours indicate maximum thickness or deviation.</p

Contour maps for the eight beach prints generated from either the scan or photo-model.

<p>Contour interval is 1 mm. Note that the left prints have been inverted to be consistent with the right ones a necessary step in the application of pSPM to the two print populations.</p

A summary of the relative merits in terms of field deployment of optical laser scanning versus photogrammetry.

<p>A summary of the relative merits in terms of field deployment of optical laser scanning versus photogrammetry.</p

Results showing statistical comparison of photogrammetry and optical laser scanning methods applied to beach prints. A.

<p>Vectored deviation maps for selected prints. Blue colours indicate situations where the scanned images underlie the photo-model and the red colours where the photo-model is slightly elevated. The speckled red reflects the fact that the photo-model resolves individual sand-grains whereas the scan does not. <b>B.</b> Mean images for all eight prints one for the scanned images and one for the photo-model showing the subtle differences in print typology that result from the different data capture techniques. Note the colour map is revised here, warm colour indicate areas of maximum depth. <b>C.</b> The left hand images is the Statistical Parametric Map (SPM) of t-values produced by a pixel-wise comparison of the two means – photo-model versus scan; warm colours show maximum positive deviation, cool colours negative deviation. The right hand images shows the results when a threshold of T<0.1 is applied with probability values. Given the very low threshold value applied here it is safe to say that there is little statistical difference between the two means. What differences are visible at this low threshold value occur around the longitudinal medial arch and in a proximal position to the toe pads.</p

Illustration of a landmark analysis conducted from the scanned and photo-models.

<p>Note the almost identical overlap between the 95% probability ellipses.</p

Examples of soft-sediment footprint sites.

<p><b>A.</b> Footprint site south of Walvis Bay, Namibia. Migration of active dunes across silt surfaces reveals a range of Holocene footprints. <b>B.</b> FwJj14E footprint site close to the village of Ileret in northern Kenya. The prints occur at multiple levels within the eroding silt bluff. Note the rock armour introduced below the lower surface to combat seasonal storm run-off and erosion. <b>C.</b> GaJi10 footprint site south of Koobi Fora in northern Kenya. The site is located adjacent to the bed of a seasonally active river and as shown the footprint rich beds dip into the slope away from the thalweg of the channel.</p