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About the Data


Gravity data at NGDC include land and marine surveys, grids, models, and geoids. In general, data record parameters include Latitude, Longitude, Observed Gravity and Elevation, Bouguer Gravity Anomaly (land), and Free-air Anomaly (ocean). In all cases, each data set contains associated documentation, including source contributor.

Instead of following the traditional approach of making all data sets fit into a fixed common format, NGDC developed a standard format description for the land gravity data, preserving fields unique to a specific data collection and allowing access to the data without concern for formats. The marine gravity data are stored in the MGD77 format, although a shortened version of this was used for the 1999 CD-ROM product. Terminology has been standardized within a terms dictionary facilitating comparison of different data sets.

To support end-user validation of the data, minimum and maximum values and histogram distributions have been prepared for each parameter within each data set format. In addition, point distribution plots have been prepared for many data set and are included as .PCX and .GIF images.


Land Gravity Data

U.S. Terrain Corrected Free Air Anomalies

Approximately 70 percent of the data are observed values--regional station data collections (separated primarily by contributors) and absolute gravity measurements. Grids and other derived summary data sets represent another 30 percent of the data. The image at left is terrain corrected free-air anomoly data for the U.S. from 1996.

File extensions:

  • .bin = binary data
  • .ast = ASCII data
  • .fmt = ASCII format description
  • .hdr = ASCII header/field description
  • .asc = ASCII other information
U.S. station data
Absolute Measurements
Gridded Data (Regional and Global)
Regional Surveys (U.S. and International)
Correlative Data -- Geoids (GEOID96, Canada, and Global)
Global Topography

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Marine Gravity Data

Free-air gravity anomaly data Freeair.gif is a composite plot of all free-air gravity anomaly data included on the Marine Gravity CD-ROM. This image is best used as a general reference to data density and location. NOTE: all data points are not represented due to overplotting data values on individual pixels.

Gravity data from 1,555 ocean surveys are contained on the 1999 CD-ROM, collected from ships operated between the years 1961 and 1996. The Marine gravity surveys are provided in a modified MGD77 format containing survey location, observed gravity and free air-anomaly. This ASCII header file is in MGD77 Header format. These marine gravity data are a subset of NGDC's Marine Trackline Geophysics database.

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