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

[Clifford J Mugnier]

I find it hard to believe that ANY datum shift algorithm or data set is good to centimeters!

If a classical horizontal datum is involved; that’s impossible.  Two inertial datums … maybe.

European Datum 1950?  Not gonna happen.

Clifford J. Mugnier, CP,CMS,FASPRS
Chief of Geodesy,
Center for GeoInformatics (C4G)
266 ERAD (Research)
3335 PFT  (Academic)
Dept. of Civil & Environmental Engineering
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA  70803
Research  225-578-4578
Academic   225-578-8536
Cell          225-328-8975









On 10/13/16, 1:09 PM, "proj-bounces at lists.maptools.org on behalf of Andre Joost" <proj-bounces at lists.maptools.org on behalf of andre+joost at nurfuerspam.de> wrote:

>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
>
>
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