Smithsonian Global Volcanism Program data for known or inferred Holocene volcanoes are now available as a Google Earth layer, displaying a photo (when available), geographic data, and links to more detailed information from the GVP and international volcano observatories or other websites focusing on regional volcanoes. The Google Earth software must be downloaded and installed to use this placemark file.
Users may note location differences between plotted volcano locations and the summits of volcanoes on the satellite imagery forming the base layer for Google Earth. This can occur for several reasons. In many cases the volcano is not a single edifice, but a volcanic field with many individual vents. Because the Smithsonian data registers the center coordinates for the entire volcanic field, it typically will not correspond to a specific volcanic vent. In many other cases the data source for the latitude/longitude coordinates are listed only in degrees and minutes (DM) rather than degree, minutes, and seconds (DMS). This can result in apparent plotting discrepancies of up to about a kilometer — at the equator one minute is equivalent to about 1.9 km. DM locations can be distinguished in the data by the presence of only two decimal places after the latitude or longitude degree; DMS data include three decimal places. Another potential source of error is that DM or DMS data in some cases originated from older sources that did not have access to accurate modern surveys or topographic maps.
A more fundamental problem originates from the fact that regional topographic mapping does not utilize a standardized global datum, or a surface defined as "zero elevation" with respect to local gravity fields. Consequently, the European Datum, North American Datum, and Tokyo Datum, for example, do not provide an integrated global standard. Efforts to provide a world geodetic system to reference elevations and locations to an ellipsoidal model rather than to the geoid began in the 1950s. The current standard World Geodetic System, WGS84, was developed in the early 1980s and is now used by the Global Positioning System (GPS). This standard is periodically revised, and a new WGS standard is being developed. The discrepancies between regional and global datums can result in plotting of volcano locations up to about a half kilometer or more from the locations of the summits that appear on the satellite imagery used in Google Earth.