[OSRS-PROJ] Puzzled dabbler -- when do I need /nad?

[Frank Warmerdam]

Tom Messenger wrote:
> As a dabbler who is only using the shpcs2cs function to convert a NAD27 
> Lambert Conformal Conic shapefile to NAD83 latitude/longitude, I'm 
> unsure if I need the grid shift files in subdirectory \nad.  My example 
> is a shapefile representing the geological formations on a geologic map 
> of Arizona.
>  
> My command line is
>  
> shpcs2cs AZ_GEOL.shp AZ_GEOLll83.shp -v +proj=lcc +datum=NAD27 
> +lon_0=-96 +lat_1=33 +lat_2=45 +lat_0=23 +x_0=5000000 +y_0=0 +to 
> +proj=latlong +datum=NAD83
>  
> and it seems to make no difference to the output whether conus.lla is in 
> the \bin directory or not.  With the low resolution of this particular 
> file, it is hard to check whether the result is really NAD83 or not.  
> Can you tell me what transformation cs2cs actually applies in this 
> case?  (I have seen suggestions in the documentation that the program 
> doesn't complain if files are missing, it just skips steps that require 
> them.)  Am I missing a step?  I'm not going to be using proj often 
> enough to become an expert.

Tom,

I'm not sure where shpcs2cs comes from.  I presume this is a version
of shpproj reimplemented to use the mechanisms of cs2cs?  I seem to
recall someone having contributed that but I don't know the details.

The conus.lla file is raw ascii grid shift file.  It isn't used directly.
First it has to be processed into a binary grid shift file with the nad2bin
program.  I would guess it is being ignored.

If you just want to convert shapefiles, I would suggest using the
prepackaged OpenEV_FW 1.6.3 distribution which includes the ogr2ogr program
which can accomplish the same thing.  The distribution also includes the
binary form of the grid shift files.

OpenEV_FW 1.6.3 can be found at:

   ftp://ftp.remotesensing.org/gdal/openev/OpenEV_FW_163.zip

A command something like this would accomplish approximately what you were
trying to do:

   ogr2ogr AZ_GEOLll83.shp AZ_GEOL.shp \
     -s_srs '+proj=lcc +datum=NAD27 +lon_0=-96 +lat_1=33 +lat_2=45 +lat_0=23 +x_0=5000000 +y_0=0' \
     -t_srs '+proj=latlong +datum=NAD83'

Of course it might be sufficient for you to run nad2bin on conus.lla to
produce conus and then ensure that the PROJ_LIB environment variable points
to the directory with the conus file.

Good luck,
-- 
---------------------------------------+--------------------------------------
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

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