On the fly transformation From WGS84 to NAD27 with proj4 code

Question

I want to use this proj4 code:

+proj=utm +zone=19 +ellps=WGS84 +toclrk66=-3,142,183,0,0,0,0 +units=m +no_defs

or this one:

+proj=utm +zone=19 +ellps=WGS84 +tonad27=-3,142,183,0,0,0,0 +units=m +no_defs

to make on the fly transformations from WGS84 to NAD27, using the geocentric tanslation parameters recommended for Dominican Republic. I typed this codes using the "New CRS" capacity in QGIS 2.0 (Configuration>Custom CRS), but it doesn't accept the transformation parameters. After typing the codes, and pressing OK, the CRS created looks like this:

+proj=utm +zone=19 +ellps=WGS84 +units=m +no_defs

QGIS deletes the geocentric traslation parameters. Maybe I'm violating proj4 codes rules, and that's why I'm asking for help.

Answer 1

The layers are in WGS84, and the project CRS is NAD27 UTM 19 North.

I am not 100% sure this will work, but the convention in PROJ.4 for 3 and 7 parameter transformations is to use

+towgs84=-3,142,183,0,0,0,0

In the project properties, CRS tab, PROJ.4 text box, enter this definition:

+proj=utm +zone=19 +datum=NAD27 +ellps=clrk66 +towgs84=-3,142,183,0,0,0,0

If it doesn't appear to be working, try dropping the +datum=NAD27 option.

Answer 2

Proj does not know any +toclrk66 or +tonad27 parameters. It is always necessary to give the datum shift from a projection to WGS84, which is the universal reference ellipsoid and datum. With that proj can make the backward transformation on its own.

For 3-parameter transformations, the +towgs84 is gained from your values by just exchanging the sign, for 7-parameters, the rotation and scale values are a bit more difficult.

Alternatively, +datum=nad27 will look up all north American datum shift values from conus and other grid shift files. This overrides any +towgs84 parameter set, as long as you are inside the area of grid file definition.