[Proj] lonlat default datum confusion

[Frank Warmerdam]

On Thu, Feb 2, 2012 at 12:27 PM, Andrew Simpson
<drew.simpson.home at gmail.com> wrote:
> -- Inverse Proj from UTM
> invproj -v -f "%.5f" +init=epsg:32620  <<EOF
> 588710.63       1851124.52
> EOF
> #Universal Transverse Mercator (UTM)
> #       Cyl, Sph
> #       zone= south
> # +init=epsg:32620 +proj=utm +zone=20 +ellps=WGS84 +datum=WGS84 +units=m
> # +no_defs +towgs84=0,0,0
> #--- following specified but NOT used
> # +ellps=WGS84
> -62.16775       16.74132

Andrew,

Note that the above is effectively producing WGS84 output since the
UTM was WGS84.
But the following transformation does not provide a datum with the
source latlon so
no datum transformation will be applied.  So you have effectively
treated the above
WGS84 latlong as if it were Monstserrat 1958.

The thing to keep in mind is if either the source or destination
coordinate system does not have datum parameters defined
(either +datum, +towgs84 or +nadgrids) then no datum transformation
will be applied and your accuracy may be significantly impacted.

Best regards,

>
> ======================================
> -- lonlat to Monty
> cs2cs -v +proj=lonlat +to +init=epsg:2004  <<EOF
> -62.16775 16.74132
> EOF
> # ---- From Coordinate System ----
> #Lat/long (Geodetic)
> #
> # +proj=lonlat +ellps=WGS84
> # ---- To Coordinate System ----
> #Transverse Mercator
> #       Cyl, Sph&Ell
> # +init=epsg:2004 +proj=tmerc +lat_0=0 +lon_0=-62 +k=0.9995000000000001
> # +x_0=400000 +y_0=0 +ellps=clrk80 +towgs84=174,359,365,0,0,0,0 +units=m
> # +no_defs
> 382121.20       1850599.03 0.00
>
> ======================================
> -- lonlat with a datum to Monty
> cs2cs -v +proj=lonlat +datum=NAD27 +to +init=epsg:2004  <<EOF
> -62.16775 16.74132
> EOF
> # ---- From Coordinate System ----
> #Lat/long (Geodetic)
> #
> # +proj=lonlat +datum=NAD27 +ellps=clrk66
> # +nadgrids=@conus, at alaska, at ntv2_0.gsb, at ntv1_can.dat
> # ---- To Coordinate System ----
> #Transverse Mercator
> #       Cyl, Sph&Ell
> # +init=epsg:2004 +proj=tmerc +lat_0=0 +lon_0=-62 +k=0.9995000000000001
> # +x_0=400000 +y_0=0 +ellps=clrk80 +towgs84=174,359,365,0,0,0,0 +units=m
> # +no_defs
> 381799.69       1850374.10 37.86
>
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Software Developer