Paper short abstract:

Under the guidance of the Khmer Archaeology Lidar Consortium project, a comprehensive Lidar survey of Koh Ker was undertaken in 2012. New processing methods have been applied to the Lidar dataset based on surface water flow analysis enabling new ways of visual interpretation.

Paper long abstract:

Under the guidance of the Khmer Archaeology Lidar Consortium (KALC) project, a comprehensive lidar survey of greater Angkor was undertaken in 2012. Koh Ker was also included in this survey of the central 67 km2 area.

The lidar dataset contains several data products: terrain (surface) model, colour aerial photography and raw lidar measure data. Our first level approach was to use the terrain model exclusively, leaving the option of utilising aerial photography for interpretation later where necessary. Examining the density of the dataset, the decision was made to use a grid density of 0.2 metres. In technical terms this represents a major oversampling of the area. Inverse distance squared weighting (IDW) was applied to the sparse set of points.

By thinking of an Angkorian landscape in which a network of roads, buildings, hydrologic features and irrigated agricultural areas built up the mesh of a low-density habitation pattern, we realised that our tests should target water flow. Thus instead of using traditional coloured or shaded terrain model visualisations we saw water flow as the key phenomenon. Water flow directions are highly sensitive even to the smallest changes in surface curvature.

The results of flow-direction analysis are not intended for visual interpretation in general. Assisting professionals working on the interpretation, we worked out a method that converted the result of water flow analysis into a pseudo shading image. This change enabled us to visualise the area as a whole without losing the detail of local topographical differences.