[Proj] Re: GGRS87 TO WGS84 PROBLEM(?)

[Eric Miller]

I don't think there is any problem with PROJ4 or ESRI's products.  I
compared the output of both and they agree exactly to at least 6 decimal
places.  I think you probably did something wrong initially and didn't
realize it.

METHOD:

1. Create a dBase table with fields X and Y with the following values:
X, Y
100000,4000000
400000,4200000
700000,4400000
1000000,4600000

2.  Open the table with ArcMap.
3.  Define an "X Y" event theme from the table selecting the "Greek
Grid" as the coordinate system.
4.  "Export" the event theme as a shapefile using the event themes
defined coordinate system.
5.  From ArcToolbox, "project" the shapefile from "Greek Grid" to WGS84
lat/long.  Select the GGRS_1987_To_WGS_1984 datum transformation (the
only choice).
6.  Add the projected shapefile to ArcMap.
7.  Open the shapefile's attribute table.
8.  Add the double precision fields "longitude" and "latitude"
9.  Calculate the values of the longitude and latitude fields using the
follow "advanced" method:
(a) longitude
Pre-Logic VBA Script Code
------------------------------
Dim pPoint as IPoint
Set pPoint = [Shape]
-----------------------------
longitude = pPoint.X
(b) latitude
Pre-Logic VBA Script Code
------------------------------
Dim pPoint as IPoint
Set pPoint = [Shape]
-----------------------------
latitude = pPoint.Y

10.  The attribute table of the projected shapefile should now look
like:
x	y	longitude	latitude
100000	4000000	19.561249	36.065025
400000	4200000	22.863594	37.944697
700000	4400000	26.335437	39.728994
1000000	4600000	29.981731	41.398427

11.  Compare to cs2cs
$> cs2cs -f "%.6f" +proj=tmerc +lon_0=24 +k=.9996 +x_0=500000
+towgs84=-199.87,74.79,246.62 +ellps=GRS80 +to +proj=latlong
+datum=WGS84
100000 4000000
19.561249       36.065025 13.179308
400000 4200000
22.863594       37.944697 29.319636
700000 4400000
26.335437       39.728994 45.376183
1000000 4600000
29.981731       41.398427 61.249505

Case closed.
-- 

Eric G. Miller
Staff Programmer
CA Dept. of Fish & Game


>>> On 12/14/2006 at 4:23 AM, "Andreas T" <haristeas at gmail.com> wrote:
> Hi all again,
> 
> As exposed on my previous mail about the transformation FROM GGRS87
TO WGS84
> we have to deal with a real problem of PROJ. Furthermore, in addition
to the
> tests exposed on my first mail, I inform you that the conversion is
also
> problematic even if we use degrees as initial values (using the same
datum
> of course --GGRS87--).
> I want to ask if the transformation FROM GGRS87 TO WGS84 is isolated
or if
> we have to deal with a more general problem with Transverse Mercator
> systems.
> If not then is there a possible solution for this particular problem
other
> than writing the transformations for this particular system my self
from
> scratch?
> 
> Can any of the more experienced users of me of PROJ, please, answer
to these
> questions? I suppose that the problem exposed is valuable for the
project
> itself.
> 
> Thanks on advance
> 
> Andreas Trantidis
> e-labs
> map-enhanced Web applications