Re: distance on the equator by map project
"Walter (HF) Smith" <[email protected]>
| Newsgroups | gmane.comp.gis.gmt.user |
|---|---|
| Message-ID | <[email protected]> |
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 >> >> > > To unsubscribe, send the message "signoff gmt-help" to [email protected] To unsubscribe, send the message "signoff gmt-help" to [email protected]