[GRASS-dev] Re: datumtransform.table question

[Paul Kelly]

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.

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.

Paul

>
> nad83    "nadgrids=FL"           "Florida" "Transforms 'Old NAD83' to 'HPGN
> NAD83'"
> nad83    "nadgrids=MD"           "Maryland" "Transforms 'Old NAD83' to 'HPGN
> NAD83'"
> nad83    "nadgrids=TN"           "Tennessee" "Transforms 'Old NAD83' to
> 'HPGN NAD83'"
> nad83    "nadgrids=WI"           "Wisconsin" "Transforms 'Old NAD83' to
> 'HPGN NAD83'"
> nad83    "nadgrids=WO"           "Washington - Oregon" "Transforms 'Old
> NAD83' to 'HPGN NAD83'"
>
> Transform #1 seems to be a kind of generic one for NAD83, but is quite
> useful programmatically. I've got error checking such that, if there is a
> match for datum transforms between datum.table and datumtransform.table, you
> need to pick a transform before continuing--the same as with g.setproj. This
> prevents users from picking a datum without transforms when one is required.
>
> I can drop this check, but it could be confusing to users to be presented
> with a list of transforms, none of which fit their situation. It would be
> better to have a generic transform, like #1, available so that there is
> always something to pick if transforms show up.
>
> Since datumtransform.table is created out of the same data used by
> g.setproj, I'm wondering what happened to transform #1 in the example above.
> Could this also be a problem with other datum/transform combinations?
>
> 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
>
>
>
>