WGS84

Definitions...

WGS84: World Geodetic System 1984 - A worldwide datum.

Datum: An Earth-shape (ellipsoid) reference model upon which points on the Earth's surface are calculated. For instance, NAD27 is common for UTM 1:24000 DRGs and 30m and 10m DEMs, though corresponding DOQs may be in a different datum, such as NAD83. Draping data from one datum, such as images or DLGs, onto DEMs in another datum, will result in noticeable misalignments.

NAD27: North American Datum 1927, CONUS - Also called NAS-C (or sometimes just NAS), this datum is being superceded by NAD83 and WGS84, but is still very commonly found used for USGS 1:24000 DRGs and DEMs.

UTM: Universal Transverse Mercator - A rectangular metric mapping coordinate system used instead of latitude and longitude.
UTM grid zone map: www.dmap.co.uk/utmworld.htm.

DRG: Digital Raster Graphic - A scanned or digitally-created topographic ("topo") map just like you may have taken hiking. These usually come as indexed-color GeoTIFF files, sometimes in a .tar.gz file that includes an accompanying .tfw world file.

DEM: digital elevation model - This term is used very generally to describe a great variety of terrain files and file formats. There are also a number of mutually incompatible file formats that use the .dem file name extension.

DOQ: digital orthophoto quadrangle - Aerial photography of the Earth's surface, usually provided in GeoTIFF form, but also seen in other formats. A DOQ for a 7.5-min., 1:24000-scale quad is huge, thus the common use of DOQQs--digital orthophoto quarter quadrangles.

NAD83: North American Datum 1983, CONUS - Also called NAR-C.

DLG: Digital Line Graph - A USGS format, with several variations, for descriptions of individual data types such as transportation (e.g., roads), hydrography (streams, lake shores), and contours.

GeoTIFF: A special expanded version of the TIFF6 standard used for various purposes, especially for DRGs, that provides for embedding topographic information. Embedded geo-referencing removes the need for a companion world file, though one may be provided for compatibility.

USGS: United States Geological Survey (http://www.usgs.gov)

Source: http://www.3dartist.com/WP/dempy/help/glossary.htm