# C025 – Local Gravity Correction

## Description

This calculates the value of gravity based on latitude and elevation above sea level.

## References

The Geodetic Reference System 1967

Woollard G.P. 1979 “The new gravity system – changes in international gravity base values and anomaly values” Geophysics, 44 1352 – 1366

IAG Developed Geodetic Reference System 1980, leading to World Geodetic Reference System 1984 (WGS84)

Kelton calculation reference C025

FLOCALC calculation reference F025

## Options

### Formula

- IGF 1967

Calculation is performed in accordance with the Geodetic Reference System 1967

- Wollard 1979

Calculation is performed in accordance with the formulae given in Woollard G.P. 1979 “The new gravity system – changes in international gravity base values and anomaly values” Geophysics, 44 1352 – 1366.

- IAC 1980/ WGS 1984

Calculation is performed in accordance with the IAG Developed Geodetic Reference System 1980, leading to World Geodetic Reference System 1984 (WGS84)

### Location

- Offshore

Use this option for calculating gravity for an offshore location

- Onshore

This gives the option of making an additional correction for the density of the rock base

## Calculation

All calculations are performed in SI units unless otherwise stated, the formulae use the geographic latitude in radians.

### Calculation of gravity at sea level

#### IGF 1967

**Wollard 1979**

**IAC 1980/ WGS 1984**

Where | ||

λ | = | Geographic latitude in radians |

### Elevation Correction

The effect of elevation allows for the ‘free-air’ gradient of gravity given by Newtons Law of Gravity, and Bouguer slab effect resulting from the layer of rock between sea-level datum and the site.

Where | ||

ρ_{r} |
= | Density of the rock |

e | = | Elevation above sea level |

### Height Correction

A small effect for height above ground level of the site (i.e. bench height) involving the free air gradient alone

Where | ||

h |
= | Height above ground level |

### Bouguer Gravity Anomaly B

The Bouguer gravity anomaly depends upon the distribution of the underlying rocks anc can only be determined by measurement.

### Local Gravity

The value of local gravity can be calculated by