A Heritage Assessment Toolkit

Archaeogeomancy: Digital Heritage Specialists – archaeological geomatics – the majick of spatial data in archaeology – archaeological information systems for the digital age:

The latest development project for Archaeogeomancy is a GIS toolkit to support the work of a heritage consultancy team. The toolkit needed to be extensible so as to be able to add new tools as required and easy to deploy across an organisation with multiple users at multiple sites with full version control. It also needed to make complex analytical workflows accessible to users who may not necessarily be expert GIS users.

Output from the Heritage Toolkit; visual and spatial analysis

Output from the Heritage Toolkit; visual and spatial analysis using the OS Terrain 50 Digital Terrain Model

A toolkit

The optimal solution for these requirements: a toolkit implemented as an Add-In for ArcGIS. This solution leveraged the Add-In framework for the existing corporate GIS platform to provide a simple means of installing a toolbar to access a set of bespoke tools.These tools automated data management workflows and standardised analysis with a limited range of options from predefined specifications.

In addition to the spatial analysis tools, a range of tools to assist with gazetteer compilation from the usual range of statutory and non-statutory sources (eg Historic Environment Records, National Heritage Lists, etc) was implemented. These tools use source data to create a standardised gazetteer of Heritage Assets including metadata about sources used. An additional proximity output shows distances between Heritage Assets and the Development Site(s).

The Heritage Assessment Tools Toolbar

The Heritage Assessment Tools Toolbar

The toolkit was implemented using ArcGIS 10.2 as a Python Add-In; the use of an Add-In helps with deployment, version control and updates. The toolkit also makes use of the 3D Analyst extension to provide the core visibility functions. The standard ArcGIS Toolbox help system was used to provide context sensitive help for each tool and any parameters and a full html user guide was incorporated into the Add-In using standard Python webbrowser functionality.

Spatial Analysis; patterns of visibility

The initial brief was to provide tools to assist with producing Heritage Statements and other planning documents particularly those involving elements of Landscape and Visual Impact Assessment. A standard product for such assessments is a Zone of Theoretical Visibility so a tool was implemented to support this using either identified locations (eg cell towers, wind turbines, etc) or development areas (eg housing developments, infrastructure projects, etc). It can also be used to produce a ZTV for one or more Heritage Assets.

This was then taken one step further and a tool was developed to produce outputs to support more detailed visual impact assessment, based on the approach used for the Stonehenge Visitor Centre assessments. This tool uses a probabilistic methodology to give a better indication of likely patterns of visibility by building in error models to a Monte Carlo simulation. This provides a probability of any given lines of sight existing rather than the usual binary yes/no output. The error simulation is based on the stated accuracies of the Digital Terrain Model used; currently the tool supports the use of the OS Terrain 50 DTM (published as Open Data) and the OS Terrain 5 DTM (one of the Ordnance Survey’s commercially available DTMs). Having undertaken the analysis, the toolkit then supports the assignment of results back to features such as Heritage Asset layers to provide metrics for detailed assessment work.

The toolkit also produces a range of standardised derived products including weighted impact scores (a combination of probability and significance of visibility) and vector polygon layers optimised for cartographic use.

Open Data

The Terrain 50 DTM is suitable for most purposes but where finer resolution is required, Terrain 5 tiles can be procured and used instead. Being Open Data, a Terrain 50 geodatabase was shipped with the Toolkit, containing a managed Image Catalog and a Referenced Mosaic Database. This geodatabase provides national coverage in a compact format (around 220mb) and is easily distributed, having no external dependencies. It has all the performance benefits of a tiled raster resource and yet can also be used as a functional surface for geoprocessing and spatial analysis. And the display optimisation in ArcMap means it looks beautiful.

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