Gravity Data Handler ( Spanish version)


Program objectives

The Gravity Data Handler is a computer program developed to calculate free-air and Bouguer gravity anomalies and perform the conversion between different type of gravity data.

Program description

The FORTRAN program GRS80.exe is a Gravity Data Handler developed to perform the following computations:

1. Convert data from IAG 1930 to GRS80.
2. Convert data from GRS67 to GRS80.
3. Apply atmospheric correction.
4. Run options 1 and 3.
5. Run options 2 and 3.
6. Computation of free-air anomalies.
7. Computation of Bouguer anomalies.
8. Convert free-air anomalies to Bouguer anomalies.
9. Convert Bouguer anomalies to free-air anomalies.

This menu is displayed when the program is executed. We can select any option of this menu giving the corresponding number, after the following request:

Select the option (1,2,3,4,5,6,7,8 or 9):

OPTION 0: change defaults. If the value 0 is given, the program allows the change of the values of the crust density (rho) and the sea-water density (rhow), to be used in the computation of Bouguer anomalies. The values used by default are rho = 2.67 g/cm3 and rhow = 1.03 g/cm3 (Torge, 1989). These are the values used by the program in the computation of the Bouguer anomalies, if the option 0 is not selected. If the option 0 is selected, the new values of rho and rhow are requested by the text:

Give the NEW values for rho and rhow:

OPTION 1: convert data from IAG 1930 to GRS80. If the value 1 is given, the gravity anomalies enclosed in the input-data file, given in the IAG 1930 reference system, are converted in gravity anomalies given in the GRS80 reference system. These new gravity anomalies are saved in the output-data file. For it, the program asks the data file names with the requests:

Input-data file name:
Output-data file name:

These data files must be in the format: longitude (east degrees), latitude (north degrees) and gravity anomaly (mgal). The files must have a record like this by each gravity point. The gravity anomalies are converted from IAG 1930 to GRS80, by means of the formula (Torge, 1989):
where

in the above-presented formulas the notation is

OPTION 2: convert data from GRS67 to GRS80. If the value 2 is given, the gravity anomalies enclosed in the input-data file, given in the GRS67 reference system, are converted in gravity anomalies given in the GRS80 reference system. These new gravity anomalies are saved in the output-data file. For it, the program asks the data file names with the requests:

Input-data file name:
Output-data file name:

These data files must be in the format: longitude (east degrees), latitude (north degrees) and gravity anomaly (mgal). The files must have a record like this by each gravity point. The gravity anomalies are converted from GRS67 to GRS80, by means of the formula (Torge, 1989):
where

OPTION 3: apply atmospheric correction. If the value 3 is given, the atmospheric correction is applied to the gravity anomalies enclosed in the input-data file. These new gravity anomalies are saved in the output-data file. For it, the program asks the data file names with the requests:

Input-data file name:
Output-data file name:

The input-data file must be in the format: longitude (east degrees), latitude (north degrees), gravity anomaly (mgal) and height (m). The input-data file must have a record like this by each gravity point. The gravity anomalies are corrected by means of the formula (Kuroishi, 1995):
where AC is the atmospheric correction (mgal) and H is the orthometric height (m).

OPTION 6: computation of free-air anomalies. If the value 6 is given, the free-air gravity anomalies are computed using the gravity data enclosed in the input-data file. These gravity anomalies are saved in the output-data file. For it, the program asks the data file names with the requests:

Input-data file name:
Output-data file name:

The input-data file must be in the format: longitude (east degrees), latitude (north degrees), gravity value (mgal) and height (m). The input-data file must have a record like this by each gravity point. The gravity anomalies are computed by means of the formula (Heiskanen and Moritz, 1967):
where F is the free-air correction (mgal), which is computed by


in the above-presented formulas h is the ellipsoidal height (m) and H is the orthometric height (m). The normal gravity (mgal) and the constants: a, m, f; are computed in the GRS80 system. The values of (a, m, f) are (Leick, 1995):

a = 6378137 m
m = 0.00344978600308
f =1/298.257222101

OPTION 7: computation of Bouguer anomalies. If the value 7 is given, the Bouguer gravity anomalies are computed using the gravity data enclosed in the input-data file. These gravity anomalies are saved in the output-data file. For it, the program asks the data file names with the requests:

Input-data file name:
Output-data file name:

The input-data file must be in the format: longitude (east degrees), latitude (north degrees), gravity value (mgal) and height (m). The input-data file must have a record like this by each gravity point. The gravity anomalies in land are computed by means of the formula (Heiskanen and Moritz, 1967):
and for marine gravity data the used formula is (Torge, 1989):
where F is the free-air correction (computed like in the option 6), H is the orthometric height or the water depth (m) and k is Newton’s gravitational constant. The values of the crust density and the sea-water density, used by default, are 2.67 and 1.03 g/cm3. These defaults can be changed with the option 0 of this program, as above described.

OPTION 8: convert free-air anomalies to Bouguer anomalies. If the value 8 is given, the Bouguer gravity anomalies are computed using the free-air anomalies enclosed in the input-data file. These Bouguer gravity anomalies are saved in the output-data file. For it, the program asks the data file names with the requests:

Input-data file name:
Output-data file name:

The input-data file must be in the format: longitude (east degrees), latitude (north degrees), free-air anomaly (mgal) and height (m). The input-data file must have a record like this by each gravity point.

OPTION 9: convert Bouguer anomalies to free-air anomalies. If the value 9 is given, the free-air anomalies are computed using the Bouguer anomalies enclosed in the input-data file. These free-air anomalies are saved in the output-data file. For it, the program asks the data file names with the requests:

Input-data file name:
Output-data file name:

The input-data file must be in the format: longitude (east degrees), latitude (north degrees), Bouguer anomaly (mgal) and height (m). The input-data file must have a record like this by each gravity point.

References

Heiskanen W. A. and Moritz H., 1967. Physical geodesy. W. H. Freeman, San Francisco.

Kuroishi I., 1995. Precise gravimetric determination of geoid in the vicinity of Japan. Bull. Geographical Surv. Inst., 41, 1-94.

Leick A., 1995. GPS satellite surveying. John Wiley, New York.

Torge W., 1989. Gravimetry. W. de Gruyter, Berlin.