pssac produces different polarity by Y-size in J-option

[lowbontimp]

I have updated gmt6 recently and found a very useful module pssac (gmt sac as a modern name). It reads sac files directly and make a record section while pswiggle needs an additional conversion from the sac file to xy-file. But, I like to notice that a feature of pssac can mislead gmt users. pssac represents the seismograms with different polarity when Y-size in J-option is positive and negative.

For example, "-J10c/5c" and "-J10c/-5c" make two reversed plots. This behavior is not natural because one expects that the upward direction of the paper is positive, in my opinion.

Let us make a sac file.

sac <<END
funcgen seismogram
write seismogram.sac
quit
END

Captured image of the seismogram.sac

gmt pssac seismogram.sac -R10/12/-1/1 -JX10c/10c -P -M1 -Ba -Xc -Yc > a.ps

Normal polarity (a.ps)

gmt pssac seismogram.sac -R10/12/-1/1 -JX10c/-10c -P -M1 -Ba -Xc -Yc > b.ps

Reversed polarity with the negative Y-size in J-option (b.ps)

The similar module pswiggle does not behave like this way.

Suggestion
The polarity of the seismograms should be fixed as positive upward in the paper.

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[seisman]

Thanks for reporting the issue.

In pssac, the Y-axis can have different meanings, depending on if -E option is used or not.

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In your case, -E option is not used, so the Y-axis means waveform amplitudes. When you give a negative Y-size (e.g., -JX10c/-10c), I think the waveform polarity should be reversed becase now upward means negative amplitude. So I think in your case, it's not a bug. Do you agree?

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When -E option is used, the Y-axis means different profiles, for example -Ek means a distance profile. Using your data as an example:

gmt pssac seismogram.sac -R10/12/350/400 -JX10c/10c -Ba -M3 -Ek > positive.ps
gmt pssac seismogram.sac -R10/12/350/400 -JX10c/-10c -Ba -M3 -Ek > negative.ps

positive.ps	negative.ps

I agree this is a bug and should be fixed.

[lowbontimp]

Ah, I have missed -E option. Thanks for your comments.