[Proj] Re: "Double ellipsoid" case?

strebe at aol.com strebe at aol.com
Thu Dec 4 15:07:57 EST 2008



Thank you for summarizing. That largely concludes the discussion. I'll just clarify two matters:

>I regret that GMP is the (free) de facto standard (as you call it)
Actually, it is the practice of using geodetic data from the ellipsoid to project from a sphere that I meant to say is the de facto standard. It always has been. In practice, almost anyone who creates a small-scale map on the Mercator equatorial projection is doing exactly what the Google Maps projection does: culling their data from ellipsoidal datums but treating it as a spherical datum. That is because there is no data available for spherical datums and because the accuracy in play would not benefit from datum conversions. Google's novelty is extending that practice "all the way down" to large-scale mapping.

>Regardless of its asserted engineering optimality (I'd appreciate a reference)

I have no reference; hence my "it sure seems like it". I would be happy to defend that if anyone is interested.

Thanks, Noel, for an engaging discussion and for cogently presenting the geodesist's viewpoint.

— daan Strebe


-----Original Message-----
From: Noel Zinn <ndzinn at comcast.net>
To: 'PROJ.4 and general Projections Discussions' <proj at lists.maptools.org>
Sent: Thu, 4 Dec 2008 11:21 am
Subject: RE: [Proj] "Double ellipsoid" case?


Yes, it does depend on the application.  I typed the following on my PDA
this morning as a response to daan, but it addresses your question about my


First, let me say that although I disagree with your perspective on the
Google Maps Projection (GMP), I do understand it and accept its validity.
There are other, equally valid perspectives, however.  I work in an industry
(oil) with lots of bright people in their fields who need geodetics and
cartography to do their jobs, but who do not share your knowledge or that of
Mikael in this area.  Blunders that can cost lots of money and can risk
lives are best avoided by standardized, well-documented procedures in
geodetics and cartography.  GMP is neither standard (ellipsoid switch
between the datum and the projection) nor well documented (or this thread
would not be so long - we've both learned something).  I regret that GMP is
the (free) de facto standard (as you call it) because (1) that encourages a
relaxation of prudent geodetic and cartographic practice in general and (2)
some creative GMP user will move a GMP map from the monitor to a paper chart
to the field where a regrettable mistake can happen. Regardless of its
asserted engineering optimality (I'd appreciate a reference) Google could
have made a better choice professionally (WGS84 ellipsoidal Mercator).  You
see it differently.  That's fine.  We just disagree. 

BTW, you won't find "Google Sphere" on the web.  It's just my way of
conceptualizing GMP.


-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Christopher Barker
Sent: Thursday, December 04, 2008 11:51=2
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] "Double ellipsoid" case?

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 
 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
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Chris.Barker at noaa.gov
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