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

Tom Messenger messengertj at citlink.net
Thu Mar 11 22:18:15 EST 2004

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.
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