SOCIETY EXPLORATION GEOPHYSICISTS (SEG)
GRAVITY(US), 4 KM

Gravity data from the Defense Mapping Agency (DMA) files consisting of about 1 million land values and 800 thousand marine values have been transformed into a digital gridded data set suitable for contouring and data manipulation. The land data consist of Bouguer gravity anomaly values computed using a density of 2.67 g/cm*3. The offshore data consist of free-air gravity anomaly values. All computations were performed using the International Gravity Standardization Net of 1971 (Morelli, 1974) and the 1967 Geodetic Reference System formula for theoretical gravity (International Association of Geodesy, 1971). The compilation of these data was supported by the Society of Exploration Geophysicists (SEG), who also published a map displaying these data.

The initial data set was first screened to eliminate redundant values, and terrain corrected in areas that had substantial relief. The file was then edited to remove erroneous data values. Even though the data have been extensively edited, a few incorrect values unknown to the authors may still be present in the data set. Terrain corrections were computed at radial distances from the station locations ranging from .895 km (Hammer Zone F) to 166.7 km (Hayford Zone 0) using a density of 2.67 g/cm*3. The final corrected data were then transformed from geodetic coordinates to ground kilometers using an Albers equal-Area projection with a central meridian of 96 degrees West and standard parallels of 29.5 degrees and 45.5 degrees.

A gridding program (Webring, 1981) based on a minimum curvature procedure (Briggs, 1974) was used to produce values spaced at 4 km intervals. The program used a search radius of 40 km to determine values at grid positions in areas of sparse data. This distance was sufficient to calculate values at all grid locations inside the boundaries of the data except for one offshore area east of southern Florida. This procedure extrapolated the data out to the search radius around the irregular boundaries of the data, and , therefore, these extrapolated values should be used with caution.

Since a rectangular grid is produced from irregular distributed data, there are many grid locations around the edges of the data beyond the search radius where no values were calculated. These grid locations were arbitrarily assigned a value of 9999.

NOTE: This binary file was converted from the ASCII file on the Geophysics of North America CD-ROM by Ted Habermann. The important information from the header was preserved in its original format (94 chars). The data values were converted to 2 byte binary integers.

HEADER RECORD:

U.S. GRAVITY DATA;4 KM GRID;ALBERS PROJECTION;96 CM MIN-CURV 1300 772-2574 4 2539 4

1. 64 alphanumeric characters of identification. The identification is: U.S. GRAVITY DATA;4 KM GRID;ALBERS PROJECTION;96 CM MIN-CURV recorded in a A64 format.

2. The number of columns of the grid. This value is 1300 and is recorded in I5 format.

3. The number of rows of the grid. This value is 772 and is recorded in I5 format.

4. The x coordinate of the first column. The value is -2574 and is the distance in kilometers west of the central meridian longitude 96 degrees West. It is recorded in I5 format.

5. The distance in kilometers between the columns of the grid. This value is 4 and is recorded in I5 format.

6. The y coordinate of the first row. The value is 2539 and is the distance in kilometers north of the equator. It is recorded in I5 format.

7. The distance in kilometers between the rows of the grid. The value is 4 and is recorded in I5 format.

DATA RECORD FORMAT:

There are 772 data records following the header record. Each record contains 1300 values recorded in I4 format and defines one row of the data matrix.

The following diagram shows the relationship of the grid elements.

Data Records:

The data are binary and are in 2 byte integers. A histogram showing the distribution of these data is in the file \lists\seggrav.lst

Data File:

1300 columns X 772 rows = 1,003,600 data
2 bytes / data point = 2,007,200 bytes
+ 94 byte header = 2,007,294 bytes