# C025 – Local Gravity Correction

## Description

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

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 ρr = 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 Back