You will likely want all your downloaded data to be in a common Horizontal Datum. Each GEODAS survey file on the CD/DVD has positions based on one of a set of commonly used Horizontal Datums; on Download these surveys can be converted to a common Horizontal Datum. If you click on the Horizontal Datum Button, you will be able to choose to convert all your survey data to North American Datum 1927 or Regional (see Note 2 below), North American Datum 1983, or Do Not Apply Horizontal Datum Conversions, (leave as they are on the CD/DVD). The default is North American Datum 1983, which corresponds closely to World Geodetic System 1984.
During the Download, the HYD93 Headers file for your Search will be updated to show the new datum in the Horizontal Datum of Records field. In rare cases where datum transforms are not possible, this field will remain unchanged.
Differences in geographic positions for different Horizontal Datums within the Continental U.S. can be from 10 to 100 meters, more than 200 meters in Alaska, Puerto Rico and the Virgin Islands, and in excess of 400 meters in Hawaii. So it is important to download data to a common Horizontal Datum, or plan on adjusting horizontal datum with you own software.
For more information about geodetic datums see the National Geodetic Survey FAQ at www.ngs.noaa.gov/faq.shtml
.Note 1: For most of the surveys in this database the Horizontal Datum in the records (on the CD/DVD) is correctly recorded. However, there are a small number of surveys where this information may be in error. Therefore, we cannot be absolutely sure that the datum shifting is correct in all cases.
Note 2: The process GEODAS uses for Horizontal Datum conversion is the National Geodetic Survey's NADCON (North American Datum Conversion Utility) Software. For the Continental U.S. this software converts to and from North American Datum 1927 (NAD27) and North American Datum 1983 (NAD83). For Alaska and island data (such as Hawaii and Puerto Rico) the shifts are actually to and from Regional Island datums and NAD83. (Note that GEODAS does not perform the High Accuracy Reference Networks (HARNS) conversions which are part of NADCON.)
Note 3: For very old Horizontal Datums (i.e. those not directly covered by NADCON) GEODAS uses a single pair of datum lat/lon shift values for the whole survey, which will transform the postions approximately to North American Datum 1927. (These will be the final positions for users Downloading to NAD27.) The software will then use NADCON to further transform the positions to North American Datum 1983, for users selecting that option.
Note 4: The Marine Trackline Geophysics database does not have this option enabled, because very few of these surveys document a datum in the MGD77 Header.