[Proj] Differences in Output between proj and cs2cs

[Gerald Evenden]

If they haven't messed with "proj" and the way I programmed it, the
"datum=WGS84" parameter on the proj runline is ignored.  Only the
+ellps=WGS84 has any meaning to proj.  It converts the geographic
coordinates to planar coordinates based upon a spherical earth
having a radius the same as the semi-major axis of the WGS84 ellipsoid.
That is, the Equidistant Cylindrical only functions in spherical mode.
If you execute proj with the -V option, proj will list +parameters that
are not used by the projection as well as a moderately complete
diagnostic of the projected numbers.

In the case of the cs2cs program a datum change is taking place because
of the two +datum arguments.  Thus the geographic coordinates are
probably changed before they get to the projection program/procedure.

On Mar 1, 2005, at 12:46 PM, Martin Thierer wrote:

> Hi,
>
> while trying to understand proj (and projections in general) I'm having
> difficulties to understand the differences I get when using the cs2cs 
> program
> to convert from Lat/Lon coordinates to Equirectangular Projection and 
> the
> same (at least: intended to be the same) conversion done with the proj
> program:
>
> 1. With proj:
>
> proj +proj=eqc +datum=WGS84 +lat_ts=48.8 +lon_0=9.48 +ellps=WGS84
> 9.48 48.8
> 0.00    5432391.15
>
> 2. With cs2cs:
>
> cs2cs -v +proj=longlat +datum=WGS84 +to  +proj=eqc +datum=WGS84 
> +lat_ts=48.8
> +lon_0=9.48
> # ---- From Coordinate System ----
> #Lat/long (Geodetic)
> #
> # +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
> # ---- To Coordinate System ----
> #Equidistant Cylindrical (Plate Caree)
> #       Cyl, Sph
> #       lat_ts=
> # +proj=eqc +datum=WGS84 +lat_ts=48.8 +lon_0=9.48 +ellps=WGS84 
> +towgs84=0,0,0
> 9.48 48.8
> 0.00    5411149.43 -12062.35
>
> Why this difference in x the y-coordinate? And why is there a 
> difference in
> the z-coordinate? As the WGS84 ellipsis is used for both source and 
> target
> shouldn't the z-coordinate in the result be 0 as well?
>
> Could someone please help me?
>
> Thank you very much!
>
> Bye,
>
> Martin
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>
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