# Correct planet radius, for calculating metres per pixel?

I am putting together some code to create tiles from various maps, for various planetary bodies, but I am not sure which value of planet radius I should be using, for indicating the metres/pixel during display?

• When I look at the Open Street Maps entry on 'zoom levels' a value of 6372.7982 km is given.
• Looking at the 'Earth Fact sheet' I see:

• Equatorial radius (km): 6378.137
• Polar radius (km): 6356.752
• Volumetric mean radius (km): 6371.008
• Wikipedia provides a value of 6371 km, which corresponds to the 'Volumetric mean radius' of the NASA fact sheet.

• Google appears to be using the value of 6378.13 km, basing on the circumference indicated in this other Stack Exchange answer.

Ideally I would like to use the data from the 'Planetary Factsheet' (for consistency across planetary bodies), so I am wondering whether I better off using the 'Equatorial radius' or the 'Volumetric mean radius' as reference value? BTW I should note that I will be using this along with metres per pixel equation, specified on the Open Street Map entry, on 'zoom levels':

``````S=C*cos(y)/2^(z+8)
``````
• My guess is that the main impact would be on the error of the reading depending on location. Choosing 6378.1 on Earth would give you an exact value on the equator, but a slightly high reading on other latitudes. Using the volumetric mean radius (close to what OSM is using) would give you an error distribution centered on 0. I wouldn't be surprised if Google used some corrective factors in the displayed value. – Fabian Dubois Feb 3 '17 at 12:06