[Gdal-dev] SAR Ceos border pixels

Gillian Walter gillian.walter at atlantis-scientific.com
Tue Aug 24 14:20:34 EDT 2004


I don't know if anyone on this list knows anything about the history of 
the SAR ceos format and its variants, but just in case, here goes (I'd 
be grateful for even partial answers to any of the following questions):

I have been looking at the SAR ceos format driver after running into 
some problems with Alaska SAR Facility (ASF) data, and am a bit confused 
about how gdal is interpreting some of the fields.  I have been using 
the CEOS-SAR-CCT spec (March 1989) and ASF slightly non-ceos format spec 
(see the reference documents under 
http://www.asf.alaska.edu/sitemap.html) as reference.  In particular, 
I'm looking at the following fields of the file descriptor record of the 
imagery options file:

37 (bytes 237-244): Number of lines per data set (one channel) in the 
file (excluding border lines)
38 (bytes 245-248): Number of left border pixels per line
39 (bytes 249-256): Total number of data groups (or pixels) per line per 
SAR channel
40 (bytes 257-260): Number of right border pixels per line
41 (bytes 261-264): Number of top border lines
42 (bytes 265-268): Number of bottom border lines

My questions are:

1) What is the definition of a "border pixel" (padding, invalid data, 
etc.), and is the interpretation consistent between the different 
flavours of SAR ceos (I've looked at a few specs and haven't found any 
info on what the border pixels/lines are meant to be, except in the ASF 
MapProjectDataRecord spec which mentions "black edges", which I take to 
mean zero padding)?

2) Should border pixels be read in?  Gdal seems to use fields 37 and 39 
to determine rasterYsize and rasterXsize, which I think means that all 
pixels (including border pixels) get read in, and some lines (either 
only the valid lines, or the top border lines + (number of valid lines - 
top border lines) valid lines depending on what line offset is used for 
the first line) get read in.  Gdal reads in the top, bottom, right, left 
border pixel values, but doesn't seem to use them anywhere as far as I 
can tell.

3) When do GCP pixel/line coordinates include the border pixel/lines?  
The ASF spec indicated that GCPs in fields 68-75 of the Map projection 
record include the black edges (which I'm assuming are the border 
pixels), but that the corners in fields 18-25 of the Facility data 
record do not include black fill.  I couldn't find any mention of the 
border pixels in the GCP sections of the SAR CCT spec.

4) Are border pixels/lines often non-zero in actual data?

Thanks in advance for any feedback,


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