Earth is not a uniform spherical object. It also has geological anomalies. Thus, gravitational field on Earth is not uniform. NASA's GRACE satellites orbiting around the Earth have been measuring Earth's gravity for several years. The following image is based on data obtained by the two GRACE satellites.

Recently, ESA launched a satellite capable of measuring minute differences in Earth's gravity.

Gravitational Acceleration, g

The average gravitational acceleration on Earth is 9.8 ms^-2 or 980,000 mGal^2,4.

I managed to obtain some data from the GRACE Gravity Model web site². I then did a polynomial curve fitting for latitudes between 5.74°N to 10.25°N.
The longitudinal variation in gravitational acceleration in and around Sri Lanka is negligible (Extent: 79.25 °E to 82.75°E). However, there are some anomalies along the latitudes.


glat = a lat4 + b lat3 + c lat2 + d lat4 + e

with
a = -0.0001328643
b = - 0.0006915536
c = 1.5802225412
d = - 0.0361715810
e = 978032.818

For example in Unawatuna g=978,089.167 mGal
If you weighed 50kg say in Europe where g=980,000 mGal
when you come to Sri Lanka you'd feel like you weigh 49.9 kg :)

Gal

Gal is a unit of measuring gravitational acceleration, named after Gallio Galilee
1 Gal = 0.01 ms^-2
980 Gal = 9.8 ms^-2
1 mGal = 1 mili Gal = 1000 Gal

Anomalies in Earth's Gravitational Field

In school we learned that g=9.8 ms^-2 and we assumed that it was constant throughout the Earth but it is not. The most obvious fact is - Earth is not perfectly spherical. The diameter across the equator (12,756.2 km) is slightly larger (by 42.6 km) than the diameter across the poles (12,713.6 km). Thus Earth is a oblate ellipsoid as opposed to being perfectly spherical.

Another reason is - Earth is not homogeneous in terms of geophysical properties and they vary from place to place.

The actual Earth consists of three major zones concentric about its center (where all its mass is said to concentrate for purposes of calculating its gravity): a Core (solid Inner; a thick liquid Outer Core); a Mantle (somewhat variable in mass distribution and density); and a Lithosphere (made up part of the Upper Mantle and two types of crust - oceanic (more uniform in composition) and continental (variable in thickness and notably heterogeneous). For several reasons, the strength of gravity at the Earth's real surface (the topographic surface) varies from place to place. These include the influence of different rock types (and densities), hot spot sources and mantle convection, and structural/topographic irregularities.¹

GRACE Satellites

The two GRACE satellites are traveling in space, both 500 kilometers above the Earth. As the front satellite approaches an area of higher gravity, it will be pulled toward the area of higher gravity and speed up. This increases the distance between the two satellites. As the satellites straddle the area of higher gravity, the front satellite will slow down and the trailing satellite will speed up. As the trailing satellite passes the area of higher gravity, it will slow down and the lead satellite will not be affected. As the satellites move around the Earth, the speeding up and slowing down of the satellites will allow scientists to measure the distance between the two satellites, and, therefore, map the Earth's gravity field.³

GOCE Satellite

This afternoon (17 March 2009), the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite developed by the European Space Agency (ESA) was lofted into a near-Sun-synchronous, low Earth orbit by a Rockot launcher lifting off from the Plesetsk cosmodrome in northern Russia.

With this launch, a new chapter in the history of Earth observation in Europe has begun. GOCE is the first of a new family of ESA satellites designed to study our planet and its environment in order to enhance our knowledge and understanding of Earth-system processes and their evolution, to enable us to address the challenges of global climate change. In particular, GOCE will measure the minute differences in the Earth's gravity field around the globe.

...

For 24 months, GOCE will collect three-dimensional gravity data all over the globe. The raw data will be processed on the ground to produce the most accurate map of the Earth’s gravitational field to date and to refine the geoid: the actual reference shape of our planet. Precise knowledge of the geoid, which can be considered as the surface of an ideal global ocean at rest, will play a very important role in further study of our planet, its oceans and atmosphere. It will serve as the reference model for our measurement and modelling of sea-level change, ocean circulation and polar ice cap dynamics.

The main payload instrument is a state-of-the-art Electrostatic Gravity Gradiometer incorporating six highly sensitive accelerometers, mounted in pairs along three perpendicular axes on an ultra-stable carbon-carbon structure. The mission will measure not gravity itself but the tiny differences in gravity between the accelerometer pairs 50 cm apart.

The data collected by GOCE will yield accuracy of 1 to 2 cm in the geoid altitude and 1 mGal for the detection of gravity-field anomalies (mountains, for instance, usually cause local gravitational variations ranging from tens of milligals to approximately one hundred). The spatial resolution will be improved from several hundreds or thousands of kilometres on previous missions to 100 km with GOCE.

The mapping of the Earth’s gravity field with such precision will benefit all branches of Earth science.

For geodesy, it will provide a unified reference model for height measurements worldwide, eliminating discontinuities between height systems for the various landmasses, countries and continents. This will enable better surveying of sea-level change, allowing scope to revisit a 200 year-plus history of recorded sea levels around the globe.

Further information:

Franco Bonacina,
ESA Spokesman and Head of Media Relations Office
Communication and Knowledge Department
Tel: + 33 1 5369 7299
Fax: + 33 1 6369 7690
Email: franco.bonacina@esa.int

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References

1. Remote Sensing Tutorial - NASA/Goddard Space Flight Center
2. GRACE Gravity Model
3. GRACE Gravity Measurement
4. Units Associated with Gravitational Acceleration
5. ESA launches first Earth Explorer mission GOCE