[Proj] a simple coordinate conversion problem

[Mikael Rittri]

Oscar, you wrote:

> 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.

Well, I could be wrong, but I think that meteorologists locate their 
sensors, and thus the sensor data, by ordinary WGS84 long/lat (not
sure about the kind of height).  Then, what they say is roughly, 

"Okay, let's all pretend that these long/lat (and height) 
 values refer to a perfect sphere with radius = 6371 km." 

Then, they can use spherical projections (like Web Mercator) and
do whatever computations they want on a sphere. Distances computed
on this sphere will vary by up to +/- 0.5 percent from true distances
on Earth, but I don't think they care. 

The 21 km displacement you mention is what the meteorologists 
would get if they their long/lat values really did refer to a
true sphere of radius 6371 km, georeferenced by GPS for example,
but I don't think that's what they do. 

Reference: http://www.ie.unc.edu/cempd/projects/mims/spatial/grids_ellipsoids_map_proj.html

Regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of OvV_HN
Sent: Tuesday, August 14, 2012 2:04 PM
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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