[Proj] a simple coordinate conversion problem

[Noel Zinn (cc)]

I'd argue a "transform from spherical to ellipsoidal coordinates is not 
necessary for some kinds of data, like meteorological data" because a 
spherical earth is a fiction.  A spherical model may be used for databases 
or processing simplicity (ignorantly, IMHO), but it's impossible that 
meteorological (or any other) data finding its way into the database is 
based upon a sphere.  Where would that spherical data come from?  Not GPS. 
So, the way to undo this ignorance is to treat (so-called) spherical data as 
ellipsoidal and not apply any datum shift.  Two wrongs in this case make a 
right (well, almost, because the audit trail suffers).  -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: OvV_HN
Sent: Tuesday, August 14, 2012 7:03 AM
To: PROJ.4 and general Projections Discussions
Subject: [Proj] a simple coordinate conversion problem

In reply to:

[Proj] a simple coordinate conversion problem
Martin Ivanov martin.ivanov at ifg.uni-tuebingen.de
Fri Aug 10 07:48:32 EST 2012

Dear users of PROJ,

I have a data set on a perfect sphere with radius 6371 km. The "projection"
is latitude-longitude, so the data practically are unprojected.

I need to transform the coordinates of the points in my data set from the
sphere
to WGS84. For that purpose I need the +towgs84 parameters for my sphere,
which I
do not know. Does someone have a clue how to get these parameters?

Any suggestions will be appreciated.

Best regards,

Martin



Reply:

A decent datum transformation program should be able to transform from
ellipsoid-ellipsoid, ellipsoid-sphere, sphere-ellipsoid, sphere-sphere. I
don't know if PROJ.4 can do this, but it should not be too difficult to
implement this in the code.

A transform from spherical to ellipsoidal coordinates is not necessary for
some kinds of data, like meteorological data?

Hmm..... let's take a test point, say lat=50d; lon=10d; h=120m; on the
Normal Sphere with a (rounded) radius of 6371000 meters. If I transform this
to the WGS84 ellipsoid with translations 0, rotations 0, scaling factor 0,
then I get the coordinates:
lat'=50.1892244; lon'=10; h'=5558.1 m.
I should say that about 5.4 km height difference and around 21 km difference
in northerly direction (with respect to the WGS84 ellipsoid) is quite
significant.
Possibly a bit too much to ignore, even for meteo applications.


Oscar van Vlijmen





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