[GRASS-dev] Re: datumtransform.table question

[Michael Barton]

On 7/4/07 3:21 PM, "Paul Kelly" <paul-grass at stjohnspoint.co.uk> wrote:

> On Wed, 4 Jul 2007, Michael Barton wrote:
> 
>> Glynn and Paul,
>> 
>> I'm hoping you can help me clear up an issue with datum transforms.
>> 
>> I've got the location wizard in wxgrass creating PROJ.4 strings and ready to
>> test it making new locations that way. I've run into an odd problem. I went
>> to test it using my home state of Arizona, wanting to see the PROJ.4 string
>> for UTM, zone 12, with a NAD83 datum.
>> 
>> The problem is in the datum transform table.
>> 
>> If I use the old g.setproj and pick UTM zone 12, I am prompted to enter a
>> datum transform. There are 6 choices, listed here...
>> 
>> Enter 'list' to see the list of available Parameter sets
>> Enter the corresponding number, or <RETURN> to cancel request
>>> list
>> 
>> Number  Details
>> ---
>> 1       Used in whole nad83 region
>>        (PROJ.4 Params towgs84=0.000,0.000,0.000)
>>        Default 3-Parameter Transformation (May not be optimum for older
>> datums;
>> use this only if no more appropriate options are available.)
>> ---
>> 2       Used in Florida
>>        (PROJ.4 Params nadgrids=FL)
>>        Transforms 'Old NAD83' to 'HPGN NAD83'
>> ---
>> 3       Used in Maryland
>>        (PROJ.4 Params nadgrids=MD)
>>        Transforms 'Old NAD83' to 'HPGN NAD83'
>> ---
>> 4       Used in Tennessee
>>        (PROJ.4 Params nadgrids=TN)
>>        Transforms 'Old NAD83' to 'HPGN NAD83'
>> ---
>> 5       Used in Wisconsin
>>        (PROJ.4 Params nadgrids=WI)
>>        Transforms 'Old NAD83' to 'HPGN NAD83'
>> ---
>> 6       Used in Washington - Oregon
>>        (PROJ.4 Params nadgrids=WO)
>>        Transforms 'Old NAD83' to 'HPGN NAD83'
>> ---
>> 
>> When I pick #1 for Arizona, this create a location with PROJ.4 settings as
>> follows (retrieved with g.proj -j)...
>> 
>> +proj=utm
>> +zone=12
>> +a=6378137
>> +rf=298.257222101
>> +no_defs
>> +towgs84=0.000,0.000,0.000
>> +to_meter=1.0
>> 
>> 
>> However, the datumtransform.table file lacks transform #1. Here are all the
>> NAD83 entries. You can see, that they include transforms #2-6, but that #1
>> is missing.
> 
> In datum.table, the values specified by dx= dy= and dz= , if all less than
> 99999 (see notes at the top of this file), will be offered to the user as
> parameter set number 1 in the form towgs84=dx,dy,dz.

OK. I understand.

> 
> Sorry for the obscurity of this; it's just an historical artefact of the
> first (generally unsuccessful) attempt to add datum transformation support
> to GRASS in the 1990s and something that had to be kept for backwards
> compatibility when the capability to have multiple parameter sets per
> datum was added.

Is there any way that this can be built explicitly into datumtransform.table
when it is created? That is, can this virtual transform 1 be added? It is
doable, but not easy, to parse this out after datum.table and
datumtransform.table have been created and I'm trying to read them into the
location wizard.

Michael

__________________________________________
Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton