Maps and Geographic Information Systems (GIS)
GIS has as many definitions as there are disciplines using it. One of these definitions is "An information system that is designed to work with data referenced by spatial or geographic coordinates. In other words, a GIS is both a system with specific capabilities for spatially-referenced data, as well as a set of operations for working [analysis] with the data" (Star and Estes, 1990).
The use of a GIS is extremely valuable in archaeology since it integrates within its database management system: data entry, storage and retrieval, manipulation and analysis, and visualization and reporting. A GIS offers archaeologists with an enhanced analytical framework that links spatial and non-spatial data (for additional details see Ebert 2004).
There are two main types of data in a GIS database: cartographic and non-cartographic or in other words spatial and non-spatial. Cartographic data refers to spatial observations on features, activities or events. It can be in a vector (points , lines, polygons) or a raster (pixels with associated values) format. Non-cartographic data, known as attributes, is the descriptive information about the cartographic features.
Spatial data in a GIS needs to be georeferenced, i.e. its location in geographic space is known. Just like on paper maps, the curved surface of the earth needs to be projected on a 2D flat surface. This process of projection involves the choice or knowledge of the datum which refers to the spheroid that best fits the earth, and a coordinate system which projects the spheroid onto a flat surface. WGS84 is a datum that is commonly used while Universal Transverse Mercator (UTM) is a widely used coordinate system. The choice of a datum and a coordinate system depends on the geographical area, i.e. which projection accurately and precisely represents the area of enquiry.
In archaeological research, spatial information is of critical importance such as topographic and bathymetric data. Nowadays, spatial data can be browsed and downloaded from online resources in raster or vector formats (depending on the type of data).
Newly Released Charts and Datasets
- Map historical weather from ship logs, a crowdsourcing approach.
- Historic maps online, search by area. Also see Where to Look for Historical Maps.
- The GEBCO Gazetteer of undersea feature names covering 3,600 features is now available for viewing and download.
- Free NOAA Nautical Charts
Data Access Sources
- Reverb- Platform for searching and ordering earth science data from NASA data centres.
- USGS EarthExplorer (EE) -Search, browse and download Land Processes Distributed Active Archive Center ( LP DAAC) MODIS and selected ASTER data holdings.
- USGS Global Visualization Viewer ( GloVis) - LP DAAC ASTER and selected MODIS available for browsing and download. Viewer offers enhanced visualization.
- Global Data Explorer - ASTER Global Digital Elevation Model (GDEM) data available. Search, select the area of interest, reformat and download data.
- TerraLook - Aster scenes available to download in Georeferenced JPEG images.
- Mercury - Web-based system for finding and retrieving Biogeochemical, Ecological and land-based data.
- Channel Coast Observatory (CCO) - Topographic, hydrographic, Lidar, photogrammetric, and photographic data available for download for Great Britain.
- EDINA Digimap- Ordnance Survey data, historic maps, marine, environmental and geological data available for browsing and download (UK only).
- NOAA National Geophysical Data Centre (NGDC)- Provides seafloor and lakebed data including geophysics (gravity, magnetics, bathymetry, water column sonar,etc.), and derived data from sediment and rock samples.
- GTOPO30: global digital elevation model (DEM) developed by EROS Data Center. Elevations spaced at 30-arc seconds (c. 1 km). HYDRO1K provides global coverage of streams, drainage basins and ancillary layers derived from GTOPO30. Available in raster and vector formats.
- GMTED2010: latest enhancement on GTOPO30. Delivered at three different resolutions ( c. 1, 0.5 and 0.25 km).
Available from USGS EarthExplorer (EE)
- SRTM: Shuttle Radar Topography Mission. Produced from a joint shuttle mission. C-Band and X-Band topographic radar systems were used in a joint shuttle mission ran by The National Aeronautics and Space Administration (NASA) and the National Geospatial-Intelligence Agency (NGA). C-Band data is available at a resolution of 1 arc-second for the US and 3 arc-second for global coverage ( download from CGIR ). X-Band data processed by the German Aerospace Center (DLR) is produced at 25 x 25 m grid ( download from DLR). Coverage 60° N to 60° S. Vertical accuracy of ±16 m.
- ASTER GDEM: Global digital Elevation Model (Version 2) produced from ASTER images. ASTER, an imaging instrument, was used on satellite Terra which was launched in December 1999. The mission was run by NASA and the Ministry of Economy, Trade and Industry (METI) of Japan. ASTER GDEM V2 has a 30 m resolution and a vertical accuracy of ±20m. Download from NASA Reverb tool.
- GEBCO: Global Bathymetric Chart of the Oceans. It operates under the Intergovernmental Oceanographic Commission (IOC) and the Internation Hydrographic Organization (IHO). Two gridded bathymetric datasets are distributed by the British Oceanographic Data Centre (BODC) and available for download as netCDF files. GEBCO One Minute Grid ( one arc-minute resolution, c. 2 Km), is based on recent version of the bathymetric contours. GEBCO_08 Grid is a 30 arc-second grid. It was generated by combining ship depth sounding with interpolation based on satellite-derived gravity data. Its land data are taken from SRTM30. Both digital terrain models are continuous covering ocean and land.
- ETOPO1: a 1 arc-minute (c. 2 Km) global relief model that integrates topographic and bathymetric datasets produced by the National Oceanic and Atmospheric Administration (NOAA). It is available in Ice Surface and Bedrock versions, available for download from the National Geophysical Data Centre. ETOPO5 and ETOPO2v2 are also available global relief grids but deprecated.
- SRTM30_PLUS: this data consists of global topography in the same format as in SRTM30. It has a 30 arc-second (c. 1 Km) grid resolution. Land data are based on SRTM30 grided DEM data and GTOPO30 data. Ocean data are based on Smith and Sandwell global 1 arc-minute grid. Higher resolution grids were added from the LDEO Ridge Multibeam Synthesis Project, the NGDC Coastal Relief Model, and the JAMSTEC Data Site for Research Cruises. Arctic bathymetry is based on the International Bathymetric Chart of the Oceans (IBCAO). SRTM30_PLUS can be accessed and downloaded from SCRIPPS website.
Tips for Data Acquisition
- Check the data accuracy and the margin of error. Vertical accuracy and inherent errors can vary according to the source of data and the methods used while creating it.
- Make sure the data is georeferenced, and its coordinate system known, otherwise using the data in its correct location in space is not possible.
- Knowledge of the data's vertical datum is required specifically for elevation data.
- Read through the metadata which sums up the information related to the data including its collection, processing, and dissemination.
EBERT, D. 2004. Applications of Archaeological GIS. Canadian Journal of Archaeology 28, No 2, pp. 319-341.
STAR, J. & ESTES, J. E. 1990. Geographic Information Systems: An Introduction, Prentice Hall.