[Proj] cs2cs for datum conversion and the "towgs84" parameter

[Gerald I. Evenden]

On Sunday 18 June 2006 12:31 pm, Hyer, Dr. Edward (Post Doc) wrote:
> Hi,
>
> I got involved with this because I received some imagery in plate-carree
> projection which used the SAD69 datum. Interestingly, while ENVI claims to
> support this projection, IDL does not, which is odd since they are supposed
> to be the same product. For my application, I am working with a large list
> of points for which I need imagery values, so IDL was my choice, and I
> found I could do the datum conversion with a shell script using cs2cs.

Within standard proj/libproj system, Plate-Carree is referred to a Equidistant 
Cylindrical or +proj=eqc.  There are many names for this projection with some 
based upon specifying a latitude of true scale.

> Googled, googled, googled, someone must have done this before. And found
> different solutions: 1)  'cs2cs +proj=latlong +datum=WGS84 +to
> +proj=latlong +ellps=GRS67 +towgs84=-57,1,-41,0,0,0,0 +no_defs -f "%.5f"

I do not believe you did a projection in the above, just a geographic 
coordinate to geographic coordinate conversion while shifting datums.  If 
+proj=eqc were selected it is only in spherical form so selecting +ellps=xxx 
only means that the major axis (a) of xxx is used as radius of the sphere.

> -w5 ' 2) People who were dealing with UTM coordinates had a different value
> for +towgs84 : 75,1,44 3) This guy's doctoral dissertation (in Portuguese)
> (http://www.teses.usp.br/teses/disponiveis/3/3138/tde-06122002-115813/publi
>co/03capitulo02.pdf) claims that SAD69 to WGS84 is nothing but a shift of
> -66.87 meters X, +4.37 meters Y, -38.52 meters Z. He also states that this
> is by Presidential Decree. Is this like the "pi=3" thing here in the
> States?

I feel that confusion of the above is related to persistant lack of 
documentation/education that will separate the concepts of projections and 
datum shifts.  But, alas, I am beating an old drum.  Sorry.

> The error with getting the datum conversion wrong is below the resolution
> of any of my data sets, so I don't have easy ways to check my conversion.
> However, since my project here is actually about the consequences of this
> poor spatial resolution, I think I should try and get it right. Can someone
> help?
>
> Obrigado antes,
>
> Dr. Edward J. Hyer
> Post-Doctoral Researcher
> Naval Research Laboratory
> Marine Meteorology Division
> 7 Grace Hopper Avenue, Stop 2
> Monterey, California 93940
> 831-656-4023
> FAX 831-656-4769
> edward.hyer at nrlmry.navy.mil
>
>
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