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<DIV><FONT face=Arial size=2>Hi Tom,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Thanks for that explanation. Now I understand where
the WGS84/LOCAL info keeps coming from. So probably ArcGIS doesn't like that
dataset, because the SDK (or Gdal) didn't put this fake coordinate system in it.
It could be because of the inverted Y sign, but if I copy the .tfw (of the
original TIF) to a .eww, ArcGIS reads and registers the ECW fine.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Regarding rotated and sheared registrations: ArcGIS
allows you to georeference an ECW, and as long as you use an affine projection,
it will create a .eww file for it (with rotation or shear if applicable). So is
this technically incorrect?</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>regards,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Jacob</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>-----Original Message-----<BR>From: Tom Lynch
[mailto:tflynch@gmail.com] <BR>Sent: 10 June 2005 09:43<BR>To: Delfos,
Jacob<BR>Cc: <A
href="mailto:gdal-dev@remotesensing.org">gdal-dev@remotesensing.org</A><BR>Subject:
Re: [Gdal-dev] ECW registration information not recognised in ESRI</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>On 6/10/05, Delfos, Jacob <<A
href="mailto:jacob.delfos@maunsell.com">jacob.delfos@maunsell.com</A>>
wrote:<BR>> <BR>> <BR>> Hi All, <BR>> <BR>> Have a bit of a
mystery: <BR>> <BR>> I am translating some geotiffs to ecw. These geotiffs
have their geographic<BR>> information internally in the headers, as well as
in a tfw file. They all<BR>> translate fine, and the coordinate information
in the ecw file is recognised<BR>> in ESRI ArcGIS.
<BR><snip></FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>Hi Jacob,</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>There are some subtleties in ECW georeferencing,
which I can explain<BR>for you. I think the issue you're seeing is
probably a minor flaw in<BR>GDAL, though I haven't checked this.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>ER Mapper uses a proprietary georeferencing system
called GDT (which<BR>AFAIK predates the popularisation of EPSG) in which a
dataset is<BR>referenced by a projection string, a datum string, and
registration<BR>information.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>This data is embedded in a binary header of an ECW
file, and includes<BR>x and y origins, x and y cell sizes, and cell size units,
as well as<BR>the projection and datum. For example, "WGS84/GEODETIC" is
one of the<BR>most common datum/projection pairs, corresponding to EPSG code
4326.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>ER Mapper supports importing datasets with EPSG
codes using an<BR>in-built correspondence between EPSG and GDT. Usually
non-geographic<BR>rasters are stored with RAW/RAW as their
projection/datum. However,<BR>if a raster marked RAW/RAW also contains
embedded georeferencing<BR>information (such as you might find in a TFW)
indicating it has a<BR>geographical purpose, it is imported into ER Mapper
software with a<BR>"fake" projection/datum "WGS84/LOCAL" allowing it to be
world-aligned<BR>with datasets in other map layers.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>There are several flaws in the system. One of
them is that the<BR>EPSG<-->GDT correspondence is not 1-1, so converting
between a GeoTIFF<BR>and an ECW file and back again may lose or change the
referencing. <BR>Another is that ECW does not support the storage of rotated or
sheared<BR>registrations (only origin and pixel scale are supported).
Finally,<BR>non-EPSG coordinate system information embedded in a GeoTIFF
will<BR>usually be ignored by ER Mapper software at this point in
time.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>As far as registration is concerned, the ECW JPEG
2000 SDK is fairly<BR>"blind" concerning the binary values stored in the
registration area<BR>of an ECW file, so third party applications may store
invalid data in<BR>this location. ArcGIS is one application that has
problems if (for<BR>example) the sign of the y cellsize is inverted. So
that's probably<BR>the reason you can't view the GDAL-translated file in ArcGIS
with the<BR>ECW plug-in.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>-- <BR>Tom Lynch<BR>Development, ER
Mapper<BR>Phone: +61 8
93882900<BR>Fax: +61 8
93882901<BR>Email: <A
href="mailto:tom.lynch@ermapper.com">tom.lynch@ermapper.com</A><BR>Web:
<A
href="http://www.ermapper.com">http://www.ermapper.com</A><BR>Forums:
<A
href="http://forum.ermapper.com">http://forum.ermapper.com</A><BR></FONT></DIV>
<DIV><FONT face=Arial size=2><A
href="mailto:jacob.delfos@maunsell.com"></A></FONT> </DIV></BODY></HTML>