[Proj] Using a Molodensky transformation instead of NADCON

[Frank Warmerdam]

Tim Loesch wrote:
> Hey folks,
>  
> Is there a way to use a Molodensky transformation with Proj.4 when 
> changing from NAD83 to NAD27?

Tim,

Yes, something like this should do it for UTM 11 for instance:

cs2cs +proj=utm +zone=11 +ellps=clrk66 +towgs84=5,10,15 +to +proj=utm +zone=11 +datum=NAD83

The key is to only provide the ellipsoid and +towgs84 parameters for the
NAD27 side.  If you say +datum=NAD27 it will try and use the grid shift
files associated with that declaration.

Of course, this is a technical answer on how to do it.  Cliff is completely
right to caution you about the accuracy dangers of doing using Molodensky
transformations instead of the grid shift files.

Best regards,

-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent