[Proj] HARN nadgrid files

[Richard Greenwood]

I meant to reply to list.

On Thu, Dec 22, 2016 at 2:21 PM, Richard Greenwood <
richard.greenwood at gmail.com> wrote:

>
>
> On Thu, Dec 22, 2016 at 2:00 PM, Even Rouault <even.rouault at spatialys.com>
> wrote:
>
>> On jeudi 22 décembre 2016 13:40:23 CET Richard Greenwood wrote:
>>
>> > I've been following a few of the recent threads on this list regarding
>>
>> > enhancements to Proj's datum transformations. Pretty exciting stuff!
>>
>> >
>>
>> > One group of transformations that seems to have been ignored for a
>> number
>>
>> > of years is applying the HARN hpgn grids in the Proj epsg file. For
>> example
>>
>> > the Proj epsg entry for epsg:3758 uses +towgs84=0,0,0,0,0,0,0 but I
>> believe
>>
>> > that is should use +nadgrids=wyhpgn.gsb, or at least that's what I
>> change
>>
>> > it to, which produces results that match NADCON.
>>
>> >
>>
>> > Is anyone aware of a reason that nadgrids are not referenced in the epsg
>>
>> > file? Is it maybe just an oversight, something that nobody ever got
>> around
>>
>> > to? Or because distributing nadgrids would become burdensome?
>>
>>
>>
>> Richard,
>>
>>
>>
>> I don't think we would want to force people using grids to be able to use
>> EPSG codes (because of the required space or licensing terms),
>>
>
> Space might be an argument, but I don't think licensing is. The grids are
> produced by the USGS and USGS data is freely available (one thing we can
> still take pride in post-election).
>
>
>> but we could generate +nadgrids=@bla syntax to make them optional. What
>> could be useful is to have both +nadgrids= and +towgs84 with the priority
>> given to the grids when they are present
>>
>
> That would be a very nice solution.
>
>
>> but this isn't implemented right now in proj.4 (if you have both
>> +nadgrids and +towgs84, I believe +nadgrids is always used, even if the
>> grids are missing on the system)
>>
>>
>>
>> As far as adding the grid info, in the current workflow, this would be a
>> matter of doing that in the https://svn.osgeo.org/metacrs/
>> geotiff/trunk/libgeotiff/csv/build_pcs.py script, presumably adding a
>> new column in the gcs.csv file, and some logic/dictionary to match the
>> datum name with a grid name, and then updating GDAL to use that new
>> information when reading the .csv files.
>>
>
> I took a look at that workflow a few years ago before the modernization
> and backed off from it, but I'm interested in looking at it again. Not
> being very familiar with build_pcs.py or the epsg database, this is a
> pretty naive question, but why aren't the nadgrids being reference already?
> Is the issue with the build_pcs.py script or with the epsg database?
>
> --
> Richard W. Greenwood, PLS
> www.greenwoodmap.com
>



-- 
Richard W. Greenwood, PLS
www.greenwoodmap.com
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