ECW Projection Translation - linear parameters wrong units

[warmerdam]

From: Hong Xu


Here is the problem. If the projection's linear unit is in feet, the projection parameter (false easting and false northing) will be wrong. Below is the comparison between ArcGIS 9.1 (with ECW plug-in) and ArcGIS 9.2, the result is the ECW images that have same projection with other data (such as shape file) does not line up with other data in 9.2. Since this breaks the existing application, it would be good to have a fix.

 

 

Same ECW file, read in 9.1

> Projection: Lambert_Conformal_Conic

> Parameters:

>   False_Easting: 6561666.666667

>   False_Northing: 1640416.666667

>   Central_Meridian: -116.250000

>   Standard_Parallel_1: 32.783333

>   Standard_Parallel_2: 33.883333

>   Latitude_Of_Origin: 32.166667

> Linear Unit: Foot_US (0.304801)

> Geographic Coordinate System:

> Name: GCS_North_American_1983

>

>

Read in 9.2 it is

> Projection: Lambert_Conformal_Conic

> false_easting: 2000000.002109

> false_northing: 500000.000451

> central_meridian: -116.250000

> standard_parallel_1: 33.883333

> standard_parallel_2: 32.783333

> latitude_of_origin: 32.166667

> Linear Unit: Foot_US (0.304801)

>

> Geographic Coordinate System: GCS_North_American_1983

[warmerdam]

Hong Xu, 

This is a frequent area of problem in different translators.  It would be
helpful if you could download the FWTools tools from:

  http://fwtools.maptools.org/

And run the "gdalinfo" command against the ecw file, including the
"--debug on" commandline switch. 

If this works properly, there should be a line in the output looking 
something like:

ECW: projection=LOCAL, datum=WGS84

Based on that, I should be able to work out some of the details. 

ECW projection names are turned into OGR style WKT strings using the file
ecw_cs.dat, normally distributed in gdal_data.  I suspect I will need to 
regenerate this file with a corrected understanding of the units of 
projection parameters.  

[warmerdam]

Hong gets the following:

ECW: projection=L2CAL6F83, datum=NAD83

[warmerdam]

I have altered lookup.py to *not* try and convert the false easting and northings
to meters.  ecw_cs.dat regenerated.  All committed in CVS.