[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.


Christopher Barker, Ph.D.

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
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Chris.Barker at noaa.gov

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