[Proj] Algorithms & Plate Carrée

[Noel Zinn (cc)]

Thanks for the link, Irwin.  It's telling.  My interpretation is that the GE 
imagery is stored (databased) in Plate Carree (the first image, Simple 
Cylindrical (Plate Carree) Projection), but displayed quite differently (the 
second image, Google Earth Image Base), which appears to me to be an 
orthographic (either spherical or ellipsoidal), but it may not be.

It's not relevant that polar areas are of no interest for your application. 
GE displays polar areas quite nicely, which is prima facie evidence that the 
projection (north, south and in between) is not Plate Carree.  Regardless of 
where you capture your image from GE, it's not Plate Carree.  The 
orthographic perspective is infinite.  GE may present in some other, 
non-infinite, vertical perspective, but who knows?  By the way, the maths 
for vertical perspectives are presented in the EPSG Guidance Note 7 Part 2 
at www.epsg.com.  If it is a non-orthographic vertical perspective, the 
elevation of the viewing point may vary as a function of zoom.  I don't 
think there's an easy answer to your question, Irwin.

Noel

Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)

-----Original Message----- 
From: Irwin Scollar
Sent: Monday, July 23, 2012 11:47 AM
To: proj at lists.maptools.org
Subject: [Proj] Algorithms & Plate Carrée

Noel Zinn wrote:

"Where does Plate Carree figure into this?  Not from Google Earth where the
poles can be displayed, not possible in Plate Carree.  I'd guess that the
"projection" you get in Google Earth is an ellipsoidal orthographic, ..."

That applies at the Poles, but:

The Google Earth help page says:

Google Earth uses Simple Cylindrical projection with a WGS84 datum
for its imagery base. Simple Cylindrical (Plate Carree) Projection
and displays an image of it.  See:

http://support.google.com/earth/bin/answer.py?hl=en&answer=148110

Hopefully, given some of the sources of distortion in a GE image, a
correction solution may be fairly simple within the limits of the
resolution of the imagery..

By small area, I mean something between 1 and 4 km square. Polar
areas are of no interest for my application.  Only those areas with
high resolution imagery are of concern. By this I mean the products
which Google buys in the form of geometrically corrected orthophotos
from national e.g. IGN-France or local mapping services, e.g.
Germany, in those parts of the world where they expect that
advertising revenue will accrue,  or where they get good quality
imagery from their leased GeoEye now merged with Digital Globe which
can be used in much of North Africa and the Middle and Far East.

20-30 cm per pixel is available and in a very few instances, ca.10-15
cm (Netherlands), but 50 cm. per pixel is about average for built-up
areas in Europe and the Middle East.  In some cases, Google has
obtained their data from national or local GeoPortal sites whose
imagery has been placed on the Web under the European Union Inspire 
directive.

It's usually easier to search with Google Earth or even with Bing
Maps which sometimes has better material if Microsoft's own aircraft
has been making the pictures using their Austrian subsidiary's Vexcel
mapping cameras.  GeoPortals do not have uniform user interfaces or
languages and quality varies widely.  Google is the only supplier of
fairly decent data over much of the world, hence my interest.

Irwin

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