From: "Saved by Windows Internet Explorer 7" Subject: Global Volcanism Program | Volcanoes of the World | Global Volcano Lists | Google Earth Placemarks Date: Fri, 28 Mar 2008 10:58:51 -0500 MIME-Version: 1.0 Content-Type: multipart/related; type="text/html"; boundary="----=_NextPart_000_0000_01C890C2.B54D3410" X-MimeOLE: Produced By Microsoft MimeOLE V6.0.6000.16545 This is a multi-part message in MIME format. ------=_NextPart_000_0000_01C890C2.B54D3410 Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.volcano.si.edu/world/globallists.cfm?listpage=googleearth =EF=BB=BF
Smithsonian Global Volcanism Program data for known or = inferred=20 Holocene volcanoes are now available as a Google Earth layer, displaying a = photo (when=20 available), geographic data, and links to more detailed information from = the GVP=20 and international volcano observatories or other websites focusing on = regional=20 volcanoes. The Google Earth software must be downloaded and = installed=20 to use this placemark file.
Users will note a frequent mismatch between plotted = volcano=20 locations and the summits of volcanoes on the satellite imagery forming = the base=20 layer for Google Earth. This can occur for several reasons. In many = cases the=20 volcano is not a single edifice, but a volcanic field with many = individual=20 vents. Because the Smithsonian data registers the center coordinates for = the=20 entire volcanic field, it typically will not correspond to a specific = volcanic=20 vent. In many other cases the latitude/longitude coordinates are listed = only in=20 degree and minutes (DM) rather than degree, minutes, and seconds (DMS). = This can=20 result in apparent plotting discrepancies of up to about a kilometer = =E2=80=94 at the=20 equator one minute is equivalent to 1.9 km. DM locations can be = distinguished in=20 the data by the presence of only two decimal places after the latitude = or=20 longitude degree; DMS data include three decimal places. Another = potential=20 source of error is that DM or DMS data in some cases originated from = older=20 sources that did not have access to accurate modern topographic = maps.
A more fundamental problem originates from the fact that = regional=20 topographic mapping does not utilize a standardized global datum, or a = surface=20 defined as "zero elevation" with respect to local gravity fields. = Consequently,=20 the European Datum, North American Datum, and Tokyo Datum, for example, = do not=20 provide an integrated global standard. Efforts to provide a world = geodetic=20 system to reference elevations and locations to an ellipsoidal model = rather than=20 to the geoid began in the 1950s. The current standard World Geodetic = System,=20 WGS84, was developed in the early 1980s and is now used by the Global=20 Positioning System (GPS). This standard is periodically revised, and a = new WGS=20 standard is being developed. The discrepancies between regional and = global=20 datums can be significant. The Tokyo Datum used for topographic maps in = Japan,=20 for example, results in locations that commonly vary by about 10 to 15 = seconds=20 of latitude and longitude from those using the global standard WGS84. = Thus=20 volcano locations plotted from Japanese topographic maps can be offset = by up to=20 about a half kilometer from the locations of the summits that appear on = the=20 satellite imagery used in Google Earth. Similarly, regional datums = elsewhere can=20 affect plotting of volcano locations.
Global Volcanism Program = =E2=80=94 Department of Mineral = Sciences =E2=80=94 National Museum of Natural History = =E2=80=94 Smithsonian=20 Institution