Re: distance on the equator by map project

Hi Walter-

You are right of course (I should not post while heavily sedated for bad flu...)

For WGS-84 sphere with radius 6371008.7714, one degree along Equator is not 60 nm:

gmtmath -Q 6371008.7714 2 MUL PI MUL 360 DIV 1852 DIV =
60.0405398134

-p

On Mar 13, 2014, at 11:42 AM, Walter (HF) Smith <[email protected]> wrote:

> Paul,
> 
> No, that is not necessarily wrong.
> 
> The radius used for the sphere has to be chosen to give a 'best fit' (in what sense has to be defined) to distance, or to area, or to volume, or to something else. It is not necessarily chosen to best fit the definition of a nautical mile in the east-west direction on the Equator.
> 
> in fact, due to the flattening of the ellipsoid, choosing the best-fit sphere to fit the Equator would be wrong. It has to best fit somewhere at mid-latitudes.
> 
> Walter
> 
> On Mar 13, 2014, at 5:39 PM, Paul Wessel <[email protected]> wrote:
> 
>> On Mar 13, 2014, at 11:19 AM, Namba Takaya <[email protected]> wrote:
>> 
>>> Dear Sir
>> Careful, the female GMT users would think badly of you.
>> 
>>> 
>>> I tried to calculate the distance on the equator to check the result of mapproject as
>>> 
>>> lon=2
>>> lat=0
>>> lonn=3
>>> latn=0
>>> echo $lon $lat |mapproject -f0x,1y -G$lonn/$latn/n 
>>> 
>>> and the result is
>>> 2       0       60.0405398135.
>>> 
>>> My question is why it is not 60.0000 nautical miles exactly?
>>> 
>> Think that is only true for a sphere.  A nautical mile is defined to be 1852 meters, not 1 minute of arc.
>> 
>> But I see
>> echo 0 0 | mapproject -G1/0/n --PROJ_ELLIPSOID=Sphere
>> 0	0	60.0405398134
>> which seems wrong, so we will have to have a look.
>> -p
>> 
>>> regards,
>>> 
>>> takaya
>>> 
>>> 
>> 
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> 
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