[Gdal-dev] ECW registration information not recognised in ESRI

Tom Lynch tflynch at gmail.com
Thu Jun 9 21:42:33 EDT 2005

On 6/10/05, Delfos, Jacob <jacob.delfos at maunsell.com> wrote:
> Hi All, 
> Have a bit of a mystery: 
> I am translating some geotiffs to ecw. These geotiffs have their geographic
> information internally in the headers, as well as in a tfw file. They all
> translate fine, and the coordinate information in the ecw file is recognised
> in ESRI ArcGIS. 

Hi Jacob,

There are some subtleties in ECW georeferencing, which I can explain
for you.  I think the issue you're seeing is probably a minor flaw in
GDAL, though I haven't checked this.

ER Mapper uses a proprietary georeferencing system called GDT (which
AFAIK predates the popularisation of EPSG) in which a dataset is
referenced by a projection string, a datum string, and registration

This data is embedded in a binary header of an ECW file, and includes
x and y origins, x and y cell sizes, and cell size units, as well as
the projection and datum.  For example, "WGS84/GEODETIC" is one of the
most common datum/projection pairs, corresponding to EPSG code 4326.

ER Mapper supports importing datasets with EPSG codes using an
in-built correspondence between EPSG and GDT.  Usually non-geographic
rasters are stored with RAW/RAW as their projection/datum.  However,
if a raster marked RAW/RAW also contains embedded georeferencing
information (such as you might find in a TFW) indicating it has a
geographical purpose, it is imported into ER Mapper software with a
"fake" projection/datum "WGS84/LOCAL" allowing it to be world-aligned
with datasets in other map layers.

There are several flaws in the system.  One of them is that the
EPSG<-->GDT correspondence is not 1-1, so converting between a GeoTIFF
and an ECW file and back again may lose or change the referencing. 
Another is that ECW does not support the storage of rotated or sheared
registrations (only origin and pixel scale are supported).  Finally,
non-EPSG coordinate system information embedded in a GeoTIFF will
usually be ignored by ER Mapper software at this point in time.

As far as registration is concerned, the ECW JPEG 2000 SDK is fairly
"blind" concerning the binary values stored in the registration area
of an ECW file, so third party applications may store invalid data in
this location.  ArcGIS is one application that has problems if (for
example) the sign of the y cellsize is inverted.  So that's probably
the reason you can't view the GDAL-translated file in ArcGIS with the
ECW plug-in.

Tom Lynch
Development, ER Mapper
Phone:      +61 8 93882900
Fax:        +61 8 93882901
Email:      tom.lynch at ermapper.com
Web:        http://www.ermapper.com
Forums:     http://forum.ermapper.com

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