[Proj] lonlat default datum confusion

[Jose Gonçalves]

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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