[Proj] puzzled with +towgs84=... being ignored

[J. Luis]

OK, thanks though before this I would be ready to swear that I had used 
in past only by specifying the +towgs84 in one side only (and get the 
correct conversion)

Regarding Just's remark on the on the +towgs84 parameters, well I was 
trying to help someone who sent them to me but I'm cc-ing him this mail 
so he can correct. Thanks

Joaquim

> Hi,
>
> I agree with Andre. One remark. Looking at the parameters, I presume you
> try to transform to Dutch, EPSG:28992. Your +towgs84 parms seem have an
> "unusual" value, maybe outdated, or perhaps on purpose. The others are ok.
>
> Your value:
> +towgs84=593.0297,26.0038,478.7534,0.40685733032239757,-0.3507326765425626,1.8703473836067959,4.0812
>
>
> Proj 4.8.0 value:
> +towgs84=565.417,50.3319,465.552,-0.398957,0.343988,-1.8774,4.0725 (see
> <28992>  entry in http://trac.osgeo.org/proj/browser/trunk/proj/nad/epsg)
>
> I've seen slight differences in, and even omission of, +towgs84 for
> EPSG:28992 here and there, as there has been some confusion over the
> years, but the Proj 4.8.0  value is now settled on (e.g. found in
> PostGIS 2.x as well).
>
> best,
>
> Just van den Broecke
> The Netherlands
>
> On 10-01-14 05:55, Andre Joost wrote:
>> Am 10.01.2014 01:28, schrieb J. Luis:
>>> I'm very lost with this results where setting the +towgs84 is ignored
>>> unless I set "+datum=WGS84"
>>> Why so? and why is the source "+ellps=WGS84" ignored as well?
>>>
>> This is the usual behaviour: +towgs84 is only used when a datum shift is
>> defined on both sides (from and to), either by specifying
>>
>> +datum=...
>> +nadgrids=...
>> +towgs84=...
>>
>> HTH,
>> Andre Joost
>>
>>
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>
>
>
>
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