puzzled with +towgs84=... being ignored

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puzzled with +towgs84=... being ignored

J Luis
Hi,

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?

cs2cs +proj=latlong +to +proj=sterea +lat_0=52.15616055555555
+lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000
+ellps=bessel
+towgs84=593.0297,26.0038,478.7534,0.40685733032239757,-0.3507326765425626,1.8703473836067959,4.0812
+units=m +no_defs
4.5 62
108156.39       1561986.31 0.00

cs2cs +proj=latlong +to +proj=sterea +lat_0=52.15616055555555
+lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000
+ellps=bessel +units=m +no_defs
4.5 62
108156.39       1561986.31 0.00

********** Now explicitly set the source ellipsoid (but nothing changes)
**********
cs2cs +proj=latlong +ellps=WGS84 +to +proj=sterea
+lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079
+x_0=155000 +y_0=463000 +ellps=bessel
+towgs84=593.0297,26.0038,478.7534,0.40685733032239757,-0.3507326765425626,1.8703473836067959,4.0812
+units=m +no_defs
4.5 62
108156.39       1561986.31 0.00

That is in ALL above cases the 'towgs84' seams to have been ignored.
However, if I use "+datum=WGS84" than the result changes

cs2cs +proj=latlong +datum=WGS84 +to +proj=sterea
+lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079
+x_0=155000 +y_0=463000 +ellps=bessel
+towgs84=593.0297,26.0038,478.7534,0.40685733032239757,-0.3507326765425626,1.8703473836067959,4.0812
+units=m +no_defs
4.5 62
108163.14       1562220.06 -39.17

Joaquim
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Re: puzzled with +towgs84=... being ignored

Andre Joost
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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Re: puzzled with +towgs84=... being ignored

Just van den Broecke
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"unusual" value, maybe outdated, or perhaps on purpose.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
>
>
> _______________________________________________
> Proj mailing list
> [hidden email]
> http://lists.maptools.org/mailman/listinfo/proj
>





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Re: 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
>>
>>
>> _______________________________________________
>> Proj mailing list
>> [hidden email]
>> http://lists.maptools.org/mailman/listinfo/proj
>>
>
>
>
>
> _______________________________________________
> Proj mailing list
> [hidden email]
> http://lists.maptools.org/mailman/listinfo/proj

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Re: puzzled with +towgs84=... being ignored

Andre Joost
In reply to this post by Just van den Broecke
Am 10.01.2014 11:54, schrieb Just van den Broecke:

> 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)
>

The 565.417,... is the correct one. The other parameter set is for
rotation around Amersfoort instead of the ellipsoid center as used by Proj.

See http://trac.osgeo.org/geotiff/ticket/22 and the attached excel sheet
supplied by Jan Hartmann evaluating the different parameter sets.

Greetings,
André Joost


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Re: puzzled with +towgs84=... being ignored

peifer
In reply to this post by J Luis
On 2014-01-10 12:58, J. Luis wrote:
> 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)
>

The default datum application behavior changed with the 4.6.0 release..,
  see the FAQ at
http://trac.osgeo.org/proj/wiki/FAQ#WhydoIgetdifferentresultswith4.5.0and4.6.0

Hermann


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