Re: Convert from ED50 to WGS84

Hey Paul,

Thank you for your answer. I am still struggling with this.

First of all, when converting my lat/lons with "mapproject -T59...." and
then again with "mapproject -T59/220.." I get different results. Shouldnt
these commands give the same output? Please compare the two attached
figures. They are very simplistic, but they illustrate my point. The blue
point should fall on the same spot (I thought), but they differ a great
deal (note scale).

Some background: The black dot is lat/lon-point in ED50 - nice to have as a
reference. I need to convert this to WGS84. The yellow, purple and red dot
are conversions of the lat/lon-pair (black dot) to WGS84 done in other
software. They all differ to some degree (which kind of scares me), but at
least they cluster up in a nice way. The GMT-converted point is the blue
one. It falls about 67 m south of the cluster. The latitude isnt that much
off. Coincidence?

The point is located offshore Norway, so I've also tried using several of
the datums that Paul mentioned where Norway is listed (GMT Datum ID #59
gives the 'best fit'). The different parameters of the datums in all cases
should not give this much offset I would have thought; the x, y and z
differs by only a couple of meters! How can this move my point almost 70
meters?

Really appeciate any input.


2014-03-13 22:02 GMT+01:00 Paul Wessel <[email protected]>:

> You should not have to deal with any projection. Barring bugs it should
> just be
>
> mapproject olddatumfile -TFROM/- > newdatumfile
>
> where FROM is the datum ID number from mapproject -Qd.  There are many
> European 1950 listed depending on region, see IDs 54-69.  If you have lon,
> lat, z data then use -Th.
>
> -p
>
> On Mar 12, 2014, at 11:02 PM, Andreas <[email protected]> wrote:
>
> Hei Maarten,
>
> So you have to do this conversion through a projection? Why is it not
> possible to do this purely in the geo domain?
>
> Thank you for your answer.
>
> Andreas
>
> On 13 Mar 2014, at 07:48, Maarten Vanneste <[email protected]>
> wrote:
>
> Hei,
>
> Assume that your coordinates would fall into UTM zone 31, CM3°E, and
> within the geographical area
> RR=-R0/6/60/62
> The conversion would be something like this:
>
> mapproject -C -F -Ju33/1:1 $RR yourdata.ed50 -T65/220 | mapproject -C -F
> -I -Ju33/1:1 -T220/220 > yourdata.wgs84
>
> You may have to play around with the D_FORMAT parameter, not to lose
> resolution.
>
> Maarten
>
>
> With best regards / Vennlig hilsen
> Maarten
>
> Dr. Maarten Vanneste
> Offshore Geotechnics Division
> NGI, Oslo - Norway
> www.ngi.no
>
> See also: International Centre for Geohazards
> www.geohazards.no
>
>
> On Wed, Mar 12, 2014 at 6:53 PM, Andreas B <[email protected]> wrote:
>
>> Hey all,
>>
>> How can I convert a set of lat/lon points in ED50 (International-1924) to
>> WGS84? I guess mapproject is the right tool, but I'm very unsure about the
>> results I'm getting.
>>
>> Best regards,
>> ANdreas
>> To unsubscribe, send the message "signoff gmt-help" to
>> [email protected]
>
>
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