[Proj] mercator projection to spherical ellipsoid from WGS84:
question
Oscar van Vlijmen
ovv at hetnet.nl
Tue Mar 27 14:35:45 EST 2007
> From: "Scott Ellington"
> Date: Tue, 27 Mar 2007 11:10:04 -0400
> Subject: [Proj] mercator projection to spherical ellipsoid from WGS84:
> question
> $ proj +proj=merc +ellps=sphere +a=6378137 +b=6378137 +lat_ts=0.0
> +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +no_defs
> -84 30
> -9350837.23 3503549.84 <---- This is correct
>
> $ cs2cs +proj=latlong +datum=WGS84 +to +proj=merc +ellps=sphere
> +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0
> +units=m +towgs84=0,0,0 +no_defs
> -84 30
> -9350837.23 3482183.15 -5312.58 <--- Not correct. Y value is
> shifted downward
>
> Yet, if I tell proj that my input data is already based on a spherical
> ellipsoid, it works:
>
> $ cs2cs +proj=latlong +ellps=sphere +a=6378137 +b=6378137 +lat_ts=0.0
> +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +no_defs +to +proj=merc
> +ellps=sphere +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
> +y_0=0 +k=1.0 +units=m +no_defs
> -84 30
> -9350837.23 3503549.84 0.00 <-- correct
>
> ...... My question is this: Why is
> it necessary to do this? Is there another way to write this cs2cs
> reprojection which will allow me to correctly specify my input
> projection?
It is possible to do mighty strange things with cs2cs, possibly even without
knowing it.
I can reproduce your "incorrect" numbers with the following procedure.
Take lon=-84, lat=30 deg. Put them on the WGS84 ellipsoid and convert to
geocentric coordinates with an ellipsoidal procedure.
Now set a sphere with a = 6378137 m and convert the geocentric coordinates
back to lat, lon on this sphere with a spherical procedure.
Then use these lat, lon to project with a spherical mercator projection.
I got indeed:
easting, northing, height = -9350837.227, 3482183.153, -5312.580
Remedy: do not use +towgs84=0,0,0 in your spherical case.
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