Converting real world locations and features into spatial data (and consequently maps) depends on:
- assumptions or models of the size and shape of the earth (spheroid),
- with some that apply to specific regions (datum);
- systems of measurement (coordinate systems);
- and mathematical processes for converting three dimensional space into two dimensions ( projections).
Occasionally spatial data files, including some in the CUGIR collection, may be provided without sufficient coordinate system and projection information predefined to enable proper registration within a viewer or GIS. For example, the CUGIR DRG files, although containing partial georeferencing information within the header of the GeoTIFF file and the associated world file (.tfw), still need to have their projection explicitly defined before using in a GIS. The summary information and metadata associated with a file will indicate the spatial reference information needed to properly define a files coordinate system, projection, and units of measurement.
Projection information may be appended to the existing data files, or added through new supplementary files (e.g. *.prj for shapefiles and *.aux for rasters). The process for doing this will vary depending on the data format and software being used. Please consult the appropriate manuals and help files for assistance. It is important that users confirm that projection information has been properly defined before using data, as some software products may assign an erroneous default projection in its absence. Many times problems with data layers not overlaying properly are related to these projection issues . More information on projections, registration and GIS is available at Wikipedia. The Cartographical Map Projections page also has very good information on map projections.