[OpendTect_Users] Users Digest, Vol 24, Issue 3

[Maurizio Ercoli]

Hi all,

sorry for my very late reply...

first, thank you Bert for your $0.02, it was a precious support!

I finally was able to correctly load coordinates of 2D GPR profiles using
the 'Simple file'
format (ASCII file) suggested by Bert.

I have discovered that the problem wasn't with Odt, but when exporting the
processed data with the processing software (in my case Reflex, after a
coordinates editing), it can't store enough precision in a signed 4 byte
integer the values..it probably makes a rounding of coordinates therefore
within ODT the profile was displayed with a "zig-zag" shape..

So, as Dr. Julien Moreau has already highlighted:

"...you cannot do that so according to your processing software
it will raise an error or not. And you don't want to know how the rounding
is made, it is at bit level so..."

and

"it is not possible to always attain GPR precision with the SEGY format in
some of the orthometric projections....in a signed 4 byte integer "


Finally, the manual of GPR acquisition software reports coordinates are
stored within rev.0 SEGY, but I think it is not completely a standard
because it uses

73-76 4 byte float format X source coordinate
77-80 4 byte float format Y source coordinate

The 4 byte values should not be used because of the too small precision,
and in fact, after better checking the SEGY, it stores also 8 byte float
values 8 (bytes 181-196) but (I think...I'm not so expert on this topic..)
without using a standard 8 byte double format...


I'd like to read some comments about or if some of you had similar problems...I
can upload a raw file if you are curious..

thanks to all for the help and for the attention..

maurizio.


2014-02-13 12:00 GMT+01:00 <users-request at opendtect.org>:

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> Today's Topics:
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>    1. Re: GNSS coordinates, Decimal degree (Bert Bril)
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> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 12 Feb 2014 11:10:37 +0100
> From: Bert Bril <bert at opendtect.org>
> To: users mailing list <users at opendtect.org>
> Subject: Re: [OpendTect_Users] GNSS coordinates, Decimal degree
> Message-ID: <52FB489D.50301 at opendtect.org>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Hi all,
>
>
> I see nobody responding, so here's my $0.02 on it. Maurizio wrote:
>
> > I can export directly UTM coordinates from my acquisition software with
> > a cm/mm resolution, but then I can't import e.g.
> >
> > 0 607778.360 4404971.106    33S    384.77
>
> This is a matter of making a small script to convert to what is
> required. For 2D, you need to make a file that OpendTect can load - with
> trace number and coordinates, like:
>
> 101 607778.3 4404971.1
> 102 607780 4405001.4
>
> Note that you only need to put bend points in the file (OpendTect will
> inter- and extrapolate).
>
> For 3D data, you can only use the 'manipulate' tool to construct your
> coordinates from anything that is available in the headers. Besides, the
> Rev.1 specification says the inline/crossline number is mandatory. So
> you can simply set up the survey's coordinate vs inline/crossline system
> and then import using inline/crossline as the key. OpendTect will
> generate the coordinates
>
>
> > Dr. Julien Moreau highlights the real problem, I think..thanks.
> >
> > A Odt tool (the 2D geometry works in this way?), that allows to directly
> > load the coordinates from the ascii files,
>
> Which is there, for 2D data. You have to un-check the 'X-coordinate
> byte' checkbox, then you get a choice on how the coordinates should be
> defined. If the line's a straight line, then you can simply define that
> without any coordinate file.
>
>
> > could be really useful and it
> > can be a simple solution to that problem (e.g. also using a scalar like
> > 1000). In this way, you could import the coordinates in Odt without
> > directly read into the SEG-Y file...also in the case of numerous and
> > long 2D lines and/or 3D GPR volume.
>
> If you can make any file you want, I'd always go for the 'Simple file'
> format. Make an ASCII file with every trace on one line, like:
>
> 101 607778.3 4404971.1 sample sample ...
> 102 607780 4405001.4 sample sample ...
>
> It's simple to make, and you can QC and edit with any good text editor.
>
>
> /Bert
>
> --
> -- Bert Bril / OpendTect developer at dGB
> -- mailto:Bert.Bril at opendtect.org , http://opendtect.org
>
>
>
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