[gdal-dev] Seg-Y : CDP-X/Y and Group-X/Y

[Even Rouault]

Le lundi 19 mars 2012 03:23:58, Glen Atkins a écrit :
> Hi there,
> 
> When getting the properties from the Trace Header of the Seg-Y file the CDP
> X (181-184) and CDP Y (185-188) are not included. This first appeared to
> be because the CDP X and CDP Y were being used to set the OGRPoint X and Y
> but these values are coming from GROUP_ X (81-84) and GROUP_Y (85-88). Is
> there a reason why the CDP_X and CDP_Y aren't included?

Well, I don't recall all the details, but when I tried on various sample files 
available around the web, I found that some used the old seg-y format that 
only used bytes 1-180 (*) (the other ones being vendor specific), so I prefered 
to stick to those ones. And I'm not sure how you can distinguish those files 
that implement the old spec from files that implement the new spec. 
Ultimatilely , if there's no automated way of figuring that out, this could be 
done with the help of a configuration option where the user could tell : use 
CDP instead of GROUP.

Could you open a ticket in GDAL trac on that topic with a sample file (you can 
likely cut it to the first hundreds of kilobytes with an hexadecimal editor so 
it is reasonable small to attach) where CDP X/Y are available ? 

Best regards,

Even

(*) From http://www.seg.org/documents/10161/77915/seg_y_rev1.pdf :

"""
7. Data Traces
7.1. Trace Header
The SEG Y trace header contains trace
attributes, which are defined as two-byte or
four-byte, two's complement integers. The
values in bytes 1-180 were defined in the
1975 standard and these entries remain
unchanged, although clarification and
extensions may be supplied where
appropriate. Bytes 181-240 were for
optional information in the 1975 standard
and this has been the main area of conflict
between different flavors of SEG Y. This
revision defines standard locations in bytes
181-240 for certain values that are needed
in modern data processing. 
"""


> 
> Thanks,
> Glen
> 
> 
> 
> [cid:mm_corp_emailsig_perth.gif at 99c73ceedff042e6be6ce85322d223a5]