Purpose

The purpose of the project is to create and maintain a mapping of an archaeological investigation at Little Millhams, Christchurch Dorset UK; the aim of the archaeological project is to find the site of the so-called Port Mill. The mapping aims to provide a plan of the site, including topographical data; plans of the excavation including plan-view images; associated pit-sections, finds analysis by geographical distribution.

Software Support

The mapping project is based around open-source GIS software known as QGIS. The version in use is QGIS 2.8.1 (Wien).

Methodology

The mapping is carried out on the Ordnance Survey National Grid for the UK, OSGB 1936. The basic outline for the site was traced from a sketch map of the area, geo-referenced to OSGB 1936 using features visible on Google Maps, with the transformation from WGS84 used on Google Earth to OSGB 1936 carried out using plug-ins available within QGIS using the NTv2 transformation of OSTN02 data ( https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/ostn02-ntv2-format.html )

Types of Data

Raster Data

Basemap

The basemap is a .tif image, geo-referenced within QGIS to a Google Earth satellite image to form a geotiff.

Pit Images

The pit images are digital, plan-view, photographs of the individual excavations on site which are scaled and geo-referenced within QGIS to form a geotiff.

Finds Distribution

The distribution of various sorts of finds, such as medieval pottery sherds, are represented by a coloured density map showing number and weight of finds across the site.

Results of geophysical surveys

The results of geophysical surveys, such as ground-penetrating radar (GPR) or resistivity measurement are usually presented as a plan view of the survey grid with the survey results/ geophysical anomalies represented as a colour scale. These are geo-referenced to the plan using site reference marks.

Elevation Representation

A colour-coded raster can be used to represent ground elevation, derived from specific or interpolated spot elevation data.

Hillshade

A hillshade raster can be derived from elevation data and used in conjunction with the elevation representation raster as an aid to visualize the topography. Hillshade is a grey-scale raster aiming to show the shadows cast by the topographical features when illuminated from a specific direction

Vector Data

Location of Excavations

The location of the pits are represented by ESRI shapefiles, showing polygonal shapes (features) representing the position of each trench. Each feature may have attributes associated with it to enable further processing and analysis.

Pottery Distribution

A point-data shapefile is produced from the centroid of each pit associated with the quantity in weight and number of finds associated with the pit, used as a basis for the raster plots of finds density.

Levels and Depths

The site levels are shown as a point-data shapefile derived from a spreadsheet of levels measured at known locations across the site in relation to a number of benchmarks established on site and surveyed in with respect to the Ordnance Survey datum. From the pit sections, the depths of a number of significant layers can be traced across site and these can the related to ground level and hence to the site benchmark.

Contouring

QGIS provides a number of options to produce contouring from the levels data, some more successful than others, and these contours are produced as vector line shapefiles.

Non-geometric Data

QGIS provides the option to join non-geometric data, such as the density of particular categories of finds across the site, to the particular areas/ excavations where the finds were discovered. From this various pictorial raster representations can be derived to allow a visual interpretation of the hot spots for finds discovery.

External Documents

Mapped features can be linked to external documents which are stored with the project. External documents represent text or image data which record aspects of the archaeological project which are not representable by plan-view mapping data. They could comprise , for example, representations of the vertical end-sections of each individual pit. In general, external documents will be in pdf format, this being the most flexible and universal format for representing text or image data.