[Proj] lonlat default datum confusion

[Andrew Simpson]

Thank you for the responses Frank and Jose.

So when I do the UTM to lonlat with no datum, the conversion the results
are in WGS84.

When I go from lonlat to epsg:2004, cs2cs assumes there is no datum
conversion needed, so I get the wrong results.

But if I specify a source datum of NAD27 or WGS84 on the latlon, the
towgs84 in the target epsg:2004 kicks in?

Not sure why I can put either datum in and get the same result?

Thanks again,
Drew

On Thu, Feb 2, 2012 at 6:22 PM, Jose Gonçalves <jagoncal at gmail.com> wrote:

> Andrew
>
> Since you don't specify an ellipsoid in the source system, WGS84 is
> assumed:
>
>
> cs2cs -v +proj=lonlat +to +init=epsg:2004
>
> This is not enough because you must specify some datum control, such as
> +towgs84=0,0,0 (notice that +datum=WGS84 is equivalent to +ellps=WGS84
> +towgs84=0,0,0). You get the correct result with:
>
> cs2cs +proj=lonlat +towgs84=0,0,0 +to +init=epsg:2004
> -62.16775 16.74132
> 381799.69       1850374.10 37.86
>
>
> Regards
>
> Jose Gonçalves
>
>
>
>
> 2012/2/2 Andrew Simpson <drew.simpson.home at gmail.com>
>
>> Greetings,
>>
>> I am having trouble understanding how lonlat is handled in cs2cs and proj
>>
>> If I cs2cs straight from "WGS 84 / UTM zone 20N" to "Montserrat 1958 /
>> BWI" I get one result.
>>
>> If I cs2cs to lonlat in between I get a different result.
>>
>> If I provide a datum on the conversion from lonlat to "Montserrat 1958 /
>> BWI" such as WGS84, I get the original result, even if the datum is not
>> applicable as in NAD27.
>>
>> Is the datum enabling a portion of code that wouldn't otherwise be used,
>> and then just being discarded?
>>
>> What EPSG code would lonlat default to, or does it default to the +to
>> coordinate system?
>>
>> Thanks for any thoughts
>>
>> Rel. 4.7.1, 23 September 2009 on Linux 2.6.26-2-686 #1 SMP Thu Sep 16
>> 19:35:51 UTC 2010 i686 GNU/Linux
>>
>>
>> ====================================
>> --  All the way from UTM to Monty
>> cs2cs -v +init=epsg:32620 +to +init=epsg:2004 <<EOF
>> 588710.63       1851124.52
>> EOF
>> # ---- From Coordinate System ----
>> #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
>> # ---- 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
>> =====================================
>>
>> -- 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
>>
>> ======================================
>> -- 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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>
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