[Proj] Differences between UTM Zone 30 (ED 50) projections

[Andre Joost]

Am 13.10.2016 um 10:07 schrieb Jelmer Baas:
> Hello,
>
> I currently have a MapInfo file in UTM Zone 30 (ED 50) (EPSG: 23030)
> projection, which, after reprojection via OGR2OGR to EPSG:900913,
> moves a few meters East when compared to OSM.

Looks like a datum shift issue, but kkep in mind that OSM is not 
necessarily "exact".


>
> Oddly enough, the EPSG file defines 23030 as +proj=utm +zone=30
> +ellps=intl +towgs84=-87,-98,-121,0,0,0,0 +units=m +no_defs  <>

... using a mean shift value for whole Europe.

>
> Yet, spatialreference.org
> (http://spatialreference.org/ref/epsg/ed50-utm-zone-30n/ ) defines it
> as +proj=utm +zone=30 +ellps=intl +units=m +no_defs

... omitting the datum shift, which is surely wrong.

>
> For completeness, MapInfo PRO defines this projection system as: UTM
> Zone 30 (ED 50)\p23030", 8, 28, 7, -3, 0, 0.9996, 500000, 0

I'm not sure if this includes an explicit datum shift.


> From a developer at MapInfo I got a tip to try to include an accurate
> datum transform file, like NTV2_0.GSB, but CS2CS doesn't want to use
> this: This file doesn't even exist: cs2cs +nadgrids=blah.gsb
> +proj=utm +zone=30 +ellps=intl +towgs84=-87,-98,-121,0,0,0,0 +units=m
> +to +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
> +y_0=0 +k=1.0 +units=m cs2cs +proj=utm +zone=30 +ellps=intl
> +towgs84=-87,-98,-121,0,0,0,0 +units=m +to +proj=merc +a=6378137
> +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m
> +nadgrids=blah.gsb [no error, Process Monitor shows cs2cs not even
> trying to open the file]

What datum shift grid did you use? the files I know are mainly from 
national coordinate systems to WGS84.



>  Can anyone tell me why there's a difference between the MapInfo PRO
> projection and the Proj.4 projection? And, hopefully, also a solution
> to correct this? Please note that I don't know whether MapInfo or
> Proj is at fault, I justk now that my data is drawn from MapInfo,
> where it's accurate compared to other data.

The EPSG registry lists 42 valid datum shift parameter sets from ED50 to 
WGS84, each with a different area of use. As written above, the values 
in EPSG:23030 are a mean value, which can not provide centimeter accuracy.

HTH,
André Joost