[Gdal-dev] problems with datum transform AGD66 to GDA94

[Andrew Williams]

Mike,
Based on your DSE link, one of the things you may be missing is that GDAit is using a local grid distortion model as well as doing a point to point conversion from AMG to MGA. The local distortion model uses a grid of observed values to determine additional local shifts. GDAit is considered the most correct conversion for AMG to MGA and depending on location this can be a metre in magnitude.

-----Original Message-----
From: gdal-dev-bounces at lists.maptools.org [mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of Michael Sumner
Sent: Wednesday, 7 November 2007 7:57 AM
To: gdal-dev at lists.maptools.org
Subject: [Gdal-dev] problems with datum transform AGD66 to GDA94

Hello,

The use of ellps=aust_SA does not seem to correspond to AGD66
(Australian Geodetic Datum 1966)
as suggested in epsg:4202.

I want to convert +longlat +ellps=aust_SA to +longlat +ellps=WGS84  (or
longlat GDA94)

I thought "GDA94" was close enough to WGS84 to be fine (within a metre
or so).

Also, the use of
+proj=longlat +a=6378160 +rf=298.250000000055
+towgs84=-29.193,41.212,130.73,-0.246,-0.374,-0.329,-2.955 +no_defs
as suggested by
http://lists.maptools.org/pipermail/gdal-dev/2006-February/008090.html
seems *closer*, but no where on a par with the example data here

http://www.dse.vic.gov.au/land/lcnlc2.nsf/LinkView/9B33746B3D77040F4A256A250008E70A5B923E73477D6D65CA256E5F0013D73B

I wonder is there something stupid I'm missing?

The examples below convert longlat AGD66 to MGA94, simply because I find
it easier to compare the results by eye. My original goal was simply to
convert longlat AGD66 to longlat GDA94/WGS84, with similar problems.

I'm using FWTools 1.4.2 .
The following examples attempts to convert the AGD66 lonlat to MGA94
(using first line from example file "transform66_99.csv" on that site).
I'd appreciate any feedback about either what might be wrong or what I
am missing.

Here I give the GDAit benchmark, for AGD66 to MGA94 (55), then some
variants with the resulting Northin/Easting from converting a shapefile
using aust_SA, the full parameter list, OGC WKT, and Manifold GIS.

Manifold's is the closest, but also the easiest to not get wrong - so
I'm hoping to be pointed in the right direction for using ogr2ogr for
datum conversion.

Cheers, Mike.

##########################
##-- GDAit Benchmark
longlat AGD 66
144.423551833 -37.952535778

MGA(55) GDA94
273741.3314    5796489.766
##--
#########################

#########################
##-- aust_SA
ogr2ogr -s_srs "+proj=longlat +ellps=aust_SA" -t_srs "+proj=utm +zone=55
+south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs" MGA94.shp
AGD66.shp
POINT (273630.24303696747 5796320.4235594533)
##--
#########################

#########################
##-- expanded PROJ
ogr2ogr -s_srs "+proj=longlat +a=6378160 +rf=298.250000000055
+towgs84=-29.193,41.212,130.73,-0.246,-0.374,-0.329,-2.955 +no_defs"
-t_srs "+proj=utm +zone=55 +south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0
+units=m +no_defs" MGA94.shp AGD66.shp
POINT (273602.06779312098 5796438.2737073004)
##--
#########################

#########################
##-- With PRJ (AGD66.prj)
AGD66.prj =
GEOGCS["GCS_Australian_1966",DATUM["D_Australian_1966",SPHEROID["Australian",6378160.000000,298.250000]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]
ogr2ogr  -s_srs "AGD66.prj" -t_srs "+proj=utm +zone=55 +south
+ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs" MGA94.shp AGD66.shp
POINT (273630.24303696747 5796320.4235594533)
##--
#########################

#########################
##-- With Manifold GIS:
273741.3462 5796489.7426
##--
##########################








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