g is variable

Introduction

What is the gravitational field strength on Earth? About 9.81 N/kg, obviously. That is correct, but only in certain select locations, which makes sense as g is the average gravitational field strength on Earth.

What is g?

What g means is that the force exerted by a 1kg object on the ground is about 9.81N - this is the basic form of the equation W=mg, and you can obviously plug in various masses to get various weights. 

Who defines g?

The General Conference on Weights and Measures in 1901 saw many things established - notably the establishment of the kilogram as the base unit of mass, but also the standard acceleration due to gravity, which was at 980.665 cm/s^2, or 9.80665 m/s^2. Apply Newton's Second Law to this (F=ma) with a 1kg object, and the force is 9.80665 N. This is equivalent to its weight, so g is 9.80665 N/kg. This also makes sense when you realise that the acceleration of a free falling object is somewhat equivalent to g (as evidenced here).

Why call this blogpost "g is variable" then if it's been defined by an international body?

The Earth is not perfectly round - rather, it's an oblate spheroid, with loads of bumps and peaks and troughs making its surface not exactly smooth. It thus makes sense that g wouldn't be the same value everywhere - if you're further away from the Earth's centre, gravity would be slightly weaker - so g is of a lower value if you're standing on Mount Everest compared to if you're lying down by the Tottenham Hotspur Stadium.

g would also be of a lower value should you be standing on the Equator - this is due to centrifugal forces acting on the Earth which cause this damned planet to rotate. The centrifugal forces acting on the poles are zero, so you weigh more on the poles than you do on the Equator, where the forces are at their most accentuated (because the Earth spins the fastest here). So g is much lower if you're on a mountain along the Equator, such as Chimborazo, which is also the furthest point from the Earth's centre, or on Huascaran, which has the lowest g value on Earth, with an acceleration at free fall of about 9.764 m/s^2. The highest g value unsurprisingly can thus be found along the poles where you're closer to the centre of the Earth, or by the Arctic Circle at 9.834 m/s^2. Notice these aren't too different to the actual established value, and that's because Huascaran doesn't have that high a relief compared to sea level (its elevation is about 6,768 km) - you'd need to go way higher than that to have a greater impact.