One of the first problems that the ArchAtlas team encountered with published data about the position of archaeological sites is the diversity of co-ordinate systems used across different gazetteers and databases. While many wide-ranging atlas gazetteers use the common Geodetic system (latitude/longitude in degrees) to describe site positions, there are a large number which are based upon more detailed, local or regional maps which have specific non-universal co-ordinate systems. Still others use short-hand co-ordinates based upon specific map-series which assume that the reader has the maps to hand (for example in the ASPRO or GACBA databases in Sources). Of course to be useful in a globally-based application such as the ArchAtlas project, these co-ordinates need to converted into a universally recognized system such as the Geodetic, UTM or Geocentric systems (see Grids and Datums from the American Society for Photogramming and Remote Sensing).

Of course, attempting to represent the Earth's surface in a two-dimensional medium, whether on paper or on a computer screen, has long been recognized as problematic by cartographers. There are various issues relating to the visual distortion created by the selection of projection system, and to the representation a three-dimensional world in a two-dimensional co-ordinate system. A large range of solutions (in the form of diverse projection and co-ordinate systems) have been developed to overcome this. Often a map printed using a particular projection system may have more than one co-ordinate or grid system printed over it.

Note on ArchAtlas Conversion

Modern GIS software can recognise many different standard co-ordinate types into 'on-the-fly', and some are programmable so that unknown systems may be added to the co-ordinate database. There is of course an extensive literature on the topic but without detailed knowledge of the mathematics behind these conversions, these guides can be obtuse.

In the ArchAtlas project, extensive use has been made of the excellent and free Geographic Translator 'GEOTRANS' from the American National Geospatial-Intelligence Agency (NGA). This utility can be used to convert multiple co-ordinates using a standard text/data file format which can be created by stripping a CSV (Comma-separated text) file of co-ordinates.

Useful links

Guides to Grids and Datums used across the world published by the American Society for Photogramming and Remote Sensing
http://www.asprs.org/resources/grids/

Overview of Geodesy and Geographic Reference systems from the Harvard Design School
http://www.gsd.harvard.edu/geo/manual/projections/fundamentals/

A useful guide to projection systems
http://erg.usgs.gov/isb/pubs/MapProjections/projections.html

Example co-ordinate systems and Map Projection overviews at
http://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.html
http://www.colorado.edu/geography/gcraft/notes/mapproj/mapproj_f.html

A similar introduction to co-ordinate systems
http://www.ncgia.ucsb.edu/education/curricula/giscc/units/u013/u013.html

Convertion formulas for a variety of projections
http://www.posc.org/Epicentre.2_2/DataModel/ExamplesofUsage/eu_cs34.html

Some notes on British Military grids (sadly not comprehensive)
http://www.echodelta.net/mbs/eng-welcome.php

For a detailed description of the terminology (metadata) to be used to describe geospatial data
http://www.fgdc.gov/metadata/csdgm/

More information about GROTRANS
http://earth-info.nga.mil/GandG/geotrans/