How to theoretically calculate the value of gravitational acceleration of my town? We normally consider the value of gravitational acceleration $g = 9.8 m/s^2$ while solving the problem. But that is the value of $g$ at poles (if I am not wrong). 
My teacher have given as homework to find the value of $g$ of my town. 
I know it should be approximately 9.8, but I have no idea how to find it. 
 A: Gravitational acceleration at the surface of earth varies with latitude (North-South position). This is due to 1) the outward centrifugal force produced by Earth's rotation, and 2) the equatorial bulge (itself caused by Earth's rotation). Both effects cause the gravitational acceleration to decrease away from the poles. The net effect is a gravitational acceleration of about $9.832$ $m/s^2$ at the poles, and about $9.780$ $m/s^2$ (0.5 % lower) at the equator. 
For any sea level position on Earth (your city, Jamnagar, at 17 m elevation can indeed be considered to be roughly at sea level), we can estimate the gravitational acceleration $g$ at any latitude $\phi$ using Helmert's equation:
$$g(\phi ) =  g_0 (1 + 0.0053024 sin^2 \phi - 0.0000058 sin^2 2\phi) $$
where $g_0 = 9.780327 m/s^2$ denotes the gravitational acceleration at the equator.
Local variations in Earth's topography and geology cause local deviations from the above formula. Such local variations are known as gravitational anomalies. To include these would go way beyond the intentions of the exercise.
A: You can calculate the value of acceleration due to gravity in your town  by finding the latitude of your town. Once you find the latitude use this equation $g'=g-R\omega^2\cos^2\lambda$.
Here $R$ is the radius of the earth, $\omega$ the angular velocity of the earth and $\lambda$ the latitude. This equation comes from the fact that the earth is rotating and a particle  thus will experience a centrifugal force.
Source
A: The value of acceleration due varies with altitude as well. It actually decreases with increase in altitude. 
The following formula approximates the Earth's gravity variation with altitude:
$g_h = g_0\left(\frac{r_\mathrm{e}}{r_\mathrm{e}+h}\right)^2$
where 


*

*$g_h$ is the gravitational acceleration at height $h$ above sea level.

*$r_e$ is the Earth's mean radius.

*$g_0$ is the standard gravitational acceleration.


So all you need to do is find the elevation of your town. But remember that this formula treats the Earth as a perfect sphere with a radially symmetric distribution of mass and you will only get an approximate value. However, if you take into account the fact that the earth is spinning about it's axis then it will depend on the latitude (this is because of the centrifugal force and also because of the "equatorial bulge") but this is usually very small. Moreover, it also depends on a host of other factors like the density of the ground, air density etc. You can see the Wikipedia article for more details.
