[Proj] "Double ellipsoid" case?
Christopher Barker
Chris.Barker at noaa.gov
Thu Dec 4 12:50:33 EST 2008
Noel Zinn wrote:
> My apologies for being obtuse. My 1.0067 scale and your 0.5 percent are
> about the same (and it’s a function of latitude), but how differently we
> react to knowledge of this fact. The angular distortion and the scale
> change are related, of course. 100 meters times the tangent of 0.4
> degrees is about 70 centimeters. That’s 7m per km and 70m per 10km.
>
> For me (meters on the ground) it’s a big deal, while you just shrug it
> off.
whether errors of that magnitude are a "big deal" or not completely
depends on the application.
We're talking about google maps (and OSM, and ??) here: it's a web
mapping application -- it shows roads, and gas stations, and whatever
other features people have added to it.
No one is putting a dividers and protractor on their computer screen to
measure things (at least I hope not!). At most they point on the map
with mouse and want to know what's there (or where they clicked). I've
haven't seen even a ruler tool on it (though it could be done, of
course). Somewhere in this thread, I think someone calculated that the
errors could add up to a pixel or two -- who click on a screen with an
accuracy of one pixel?
So: what possible application do you have in mind where 0.5 percent
error could matter?
one more note: if there was a ruler tool, it could (and should) convert
pixel coords to lat-long, and do the distance calculation with that, anyway.
-Chris
--
Christopher Barker, Ph.D.
Oceanographer
Emergency Response Division
NOAA/NOS/OR&R (206) 526-6959 voice
7600 Sand Point Way NE (206) 526-6329 fax
Seattle, WA 98115 (206) 526-6317 main reception
Chris.Barker at noaa.gov
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