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Re: [ferret_users] Ferret's definition of the earth radius

Hi Olivier,
Thank you for writing.  This is always a great way to find something out: get Ferret to solve a small, known problem that you set up.

It turns out to our dismay, that the definition 6371 is also used, for the purpose of computing the conversion from one measure of units to another. This discrepancy has been in the Ferret code forever. For consistency, we will change this to 6371 throughout. This value is very near the authalic radius of earth, which is 6,371.0072 km: https://en.wikipedia.org/wiki/Earth_radius#Authalic_radius

On a side topic: Given the limits of the spherical representation of the earth, the precision of the results here are not valid to a large number of decimal places. In any computation like this, it's worth thinking about what resolution can really be expected of a calculation.


On 2/29/2016 8:59 AM, Olivier Marti wrote:

Le 29 févr. 2016 à 16:18, Andy Jacobson - NOAA Affiliate <andy.jacobson@xxxxxxxx> a écrit :


I'm trying to track down a problem conserving tracer mass in the GEOS-Chem model, using an emissions module called HEMCO.  It has been suggested that differences may be due to conflicting definitions of the radius of the earth.  As it turns out, I used Ferret to do my regridding, so I'm trying to discover what value Ferret is using.  I found one definition in FERRET/fmt/cmn/gt_lib.parm (6371.e5 cm).  Is there anywhere else that a value for the radius of the earth is defined in the ferret source code?



Andy Jacobson

NOAA Earth System Research Lab
Global Monitoring Division
325 Broadway R/GMD1
Boulder, Colorado 80305


you can also ask to Ferret !

i.e : 

! What is the Earth radius in Ferret

DEFINE SYMBOL MyPrec $1"360"

DEFINE AXIS/X=0:360/NPOINTS=($MyPrec)/EDGES/UNITS=degrees_east   xax
DEFINE AXIS/Y=-90:90/NPOINTS=($MyPrec)/EDGES/UNITS=degrees_north yax

LET             aire       = 1.0 + 0*(X[GX=xax]+Y[GY=yax])
LET/UNITS="m^2" aire_sum   = `aire[X=@DIN,Y=@DIN]`
LET             pi         = `ACOS (-1.0)`
LET/UNITS="m"   ra         = `(aire_sum/ 4.0 / pi)^0.5`

LIST/NOHEAD/QUIET/FORMAT=("Earth Radius in Ferret :", 1F20.6, " m") ra

Earth Radius in Ferret :      6370499.317638 m

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