[Qgis-user] NTV2 Grid Shits

[stevejking]

Last I checked QGIS 2.14 ignored any +geoidgrid=
But it works fine in various stand alone Proj4 tools.

Alternatively, any nadgrids and any geoidgrids files will open in QGIS as
raster grids, so you can use them that way too. For example A gsb file will
be multi band, latitude shift, longitude shift (usually arc seconds of a
degree), and two error values usually set to zero. And for example a GTX
file will be a grid of vertical shifts usually to the WGS84 or GRS80
ellipsoid. Both grid files need to be realised on the CRS native ellipsoid
in latlong coordinates, ie. not necessarily on WGS84 ellipsoid. Note some
nadgrids files are produced in the reverse direction, but proj4 assumes
shifts are to WGS84 from the CRS ellipsoid.

Steve


Siki Zoltan wrote
> Dear Tyler!
> 
> , I don't know if
> QGIS allow +geogrids, too...
> 
> Best regards,
> Zoltan
> 
> On Tue, 24 Oct 2017, Tyler Veinot wrote:
> 
>> Since you guys both know more about PROJ4 than I do I thought I would
>> just
>> tag this question on;
>> Where is QGIS/PROJ4 with support for vertical datums? Is there a way I
>> can
>> attach a vertical datum to a CRS in PROJ4?
>> Tyler
>>
>> On Mon, Oct 23, 2017 at 9:09 AM, Tyler Veinot <

> tylerkveinot@

> >
>> wrote:
>>
>>> Andre;
>>> It seems QGIS does not like my gsb file, I downloaded the NTV2 gsb from
>>> NRCan and pointed to it and it worked. For some reason the one specific
>>> for
>>> PEI does not. However, the grid shift could be the same as far as I
>>> know;
>>> cannot find any information on what mathematical principles are used to
>>> define the shift to correct the coordinates.
>>> Tyler
>>>
>>> On Mon, Oct 23, 2017 at 8:01 AM, André Joost <

> andre.joost@

> > wrote:
>>>
>>>> Am 20.10.2017 um 20:19 schrieb Tyler Veinot:
>>>>
>>>>> Andre;
>>>>> Again, sorry accidently sent the email mid writing. Where was I....
>>>>> Right so I added
>>>>>
>>>>> nadgrids=PE7783V2.gsb a number of ways trying to sort it based on
>>>>> online
>>>>> information and how some other CRS's were setup within QGIS but
>>>>> nothing
>>>>> worked yet.
>>>>>
>>>>
>>>> It might be better to post the full string.
>>>>
>>>>
>>>> When I add the directory to my gsb where do I add it in the line and
>>>> does
>>>>> it need quotes?
>>>>>
>>>>
>>>> If you have blanks in folder names, surely.
>>>>
>>>> HTH,
>>>> André Joost
>>>>
>>>>
>>>>
>>>>>
>>>>> On Fri, Oct 20, 2017 at 2:55 PM, Andre Joost <

> andre+joost@

> >>>>>
>>>>> wrote:
>>>>>
>>>>> Am 20.10.2017 um 14:31 schrieb Tyler Veinot:
>>>>>>
>>>>>> Anyone knowledge in using grid shift files (gsb) with QGIS?
>>>>>>> I have a local CRS NAD27 Double Stereographic Imperial that I need
>>>>>>> to
>>>>>>> transform to NAD83 CSRS 2010 Epoch. I have the CRS properly defined
>>>>>>> using
>>>>>>> the provincial description
>>>>>>>
>>>>>>>
>>>>>> Can you add your custom CRS? Maybe you have +datum=nad27 which uses
>>>>>> the
>>>>>> main nadcon transformation file. use +ellps=clrk66
>>>>>> +nadgrids=/path/to/your/file
>>>>>>
>>>>>>   but QGIS using its Clark 1866 to GRS80
>>>>>>
>>>>>> transformation puts the points a little off from the NTV2 coordinate
>>>>>>> transformation software we have. I have the GSB used (even though it
>>>>>>> says
>>>>>>> ATS77 the provincial geomatics guys tell me it shits NAD27 and ATS77
>>>>>>> into
>>>>>>> CSRS 2010 NAD83). I followed some directions on putting the file in
>>>>>>> a
>>>>>>> specific folder and restarted QGIS set the settings to always ask
>>>>>>> for
>>>>>>> grid
>>>>>>> shift file for transformations yet I do not see the one I put in
>>>>>>> there
>>>>>>> in
>>>>>>> my list to pick from.
>>>>>>>
>>>>>>>
>>>>>> Can you explain where that setting should be?
>>>>>>
>>>>>>
>>>>>> I also tried to install a plugin for NTV2 transformations but cannot
>>>>>>
>>>>>>> activate it. When I go to Activate Additional Providers in my
>>>>>>> Processing
>>>>>>> Toolbox where NTv2 Datum Transformations resides I get the
>>>>>>> "Specified
>>>>>>> Path
>>>>>>> does not exist: C:\PROGA~1\QGIS~1.18\bin" error that I have not yet
>>>>>>> managed
>>>>>>> to resolve. To my understanding has something to do with GRASS
>>>>>>> plugins
>>>>>>> and
>>>>>>> past versions of QGIS.
>>>>>>>
>>>>>>>
>>>>>> You can workaround that GRASS error by creating that folder, or
>>>>>> changing
>>>>>> the folder entry for GRASS 7 to an existing folder.
>>>>>> But to use your grid in the plugin, you would need some python code
>>>>>> around
>>>>>> it, like the other grids have in C:\Users\
> <username>
> \.qgis2\pyt
>>>>>> hon\plugins\ntv2_transformations
>>>>>> Without that processing would not know about your grid.
>>>>>>
>>>>>> HTH,
>>>>>> André Joost
>>>>>>
>>>>>>
>>>>>>
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>>>>>
>>>>
>>>>
>>>
>>
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