gravity - No uncertainty for standard gravitational acceleration?

The other day I asked about the uncertainty of light, and this issue triggered me to start looking into other physical constants and try to understand why other constants have no uncertainty.

One of those constants is the standard gravitational acceleration, which is the acceleration due to Earth's gravity ($\approx 9.8\: \mathrm{m/s^2}$)... which is also a physical quantity for which I could easily suggest a lot of reasons why it should have uncertainties, like:

• The meter itself has uncertainty,


• The radius of the Earth is different everywhere

Why doesn't $g$ have uncertainty?

I would like to point out there are other constants with no uncertainty (like permittivity of free space) that I don't understand. Isn't there a collective reference to explain those conventions?

EDIT:

Maybe I wasn't clear. Check the latest review from Physical Review D, and you'll see that the uncertainty is given as zero at page 108. And also from CODATA NIST:

Answer

The typical gravitational acceleration on the surface of the Earth, $g \approx 9.8\: \mathrm{m/s^2}$, has uncertainty. That's one of the reasons why the $\approx$ symbol is used.

The Earth's gravitational field varies a lot due to oceans, the thickness of the crust, mountains, non-uniform density in the crust and mantel, etc.

A pair of satellites was launched for the Gravity Recovery and Climate Experiment (GRACE) and based on that data a map was made showing the variation:

From Wikipedia:

Apparent gravity on the earth's surface varies by around $0.7\%$, from $9.7639\: \mathrm{m/s^2}$ on the Nevado Huascarán mountain in Peru to $9.8337\: \mathrm{m/s^2}$ at the surface of the Arctic Ocean.

As others have mentioned, the constant of gravitational acceleration, $g_0$ that is defined exactly as $9.80665\: \mathrm{m/s^2}$ is used for the standardization of weight like the pound against units of mass like the kilogram.