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

[Hyer, Dr. Edward (Post Doc)]

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.

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" -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/publico/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?

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