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

[Clifford J Mugnier]

In general, geodetic relationships, formulae, and parameters as you list
below "by Presidential Decree" are the normal and proper way of providing
order in a legal manner for a nation.  We do the same through the Federal
Register and the various Revised Statues of our 50 states.  The magnitude
of the values are within the expected range that DMA/NIMA/NGA posted in TR
8350.2, although the U.S. Military did not go to that level of precision.
I would expect the centimeter-level of precision to be for an area no
larger than a province/state, if even that large.

Some real property rights are conveyed via descriptions with only
coordinates, particularly rights to explore and develop mineral rights,
including offshore hydrocarbons.  In the U.S. Gulf of Mexico, we do it with
"NADCON" as published in the Federal Register for offshore lease blocks
that sell via sealed bids for hundreds of millions of dollars through the
U.S. Minerals Management Service in Kenner, LA.  Same goes for Trinidad &
Tobago, Angola, Nigeria, Greece, Indonesia, etc.

The concept is well-known; I cannot speak for the specific values of the
parameters listed.  No, it is not the same as Pi=3.0.

De nada,

Clifford J. Mugnier, C.P., C.M.S.
National Director (2006-2008),
Photogrammetric Applications Division
American Society for Photogrammetry and Remote Sensing
and
Chief of Geodesy,
CENTER FOR GEOINFORMATICS
Department of Civil Engineering
CEBA 3223A
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic]
Voice and Facsimile:  (225) 578-4474 [Research]
Honorary Life Member of the
Louisiana Society of Professional Surveyors
======================================================
http://www.asprs.org/resources/GRIDS/
http://appl003.lsu.edu/eng/ceeweb.nsf/$Content/Mugnier
======================================================



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