Feature #1212

Add diurnal correction to mgd77list -Am

Added by Mike 11 days ago. Updated 7 days ago.

Status:FeedbackStart date:2018-03-08
Priority:NormalDue date:
Assignee:-% Done:


Target version:-


Currently mgd77list -Am computes mag anomalies as:

m1 return mag anomaly stored in file
m2 return mtfx - igrf (mtf1 by default)
m4 return mtfx - igrf (mtf2 by default)

But currently mgd77list cannot compute mag as

mtfx + diur - igrf

Of course users will want to do this whenever they add in the diurnal correction column, so it should be useful. Hopefully it will also be easy to do.


#1 Updated by Paul 10 days ago

  • Status changed from New to Feedback

Not so sure. The MGD77 docs says regarding the diurnal column:

In tenths of nanoteslas (gammas). If 9-filled (i.e., set to "+9999"), total and residual fields are assumed to be uncorrected; if used, total and residuals are assumed to have been already corrected.

I think you are saying that, after a MGD77 file has been produced (without using any diurnal correction), there becomes available new information in the form of a diurnal correction. You now want to recompute the residual via mftx + diur - igrt. I am not sure if this is a typical situation. If the diurnal column exists then the docs says the correction has already been applied. It sounds like the better solution is to recreate the entire MGD77 file using the updated information to make a revised residual and store the diurnal column as well.

To compute mtfx + diur - igrf with a MGD77 file would be incorrect since the correction has already been applied.

#2 Updated by Mike 7 days ago

Yes, but this reasoning applies to gravity as well because gobs is supposed to be corrected for vessel motion. Yet mgd77list offers two options to recompute faa: faa = gobs + eot - normal_g and faa = gobs + ceot - normal_g. For those building new archive files and for those cases where the older dataset didn't correct gobs for vessel motion, it's useful. The reasoning for doing this with magnetics is more or less the same. I think it's useful for building new archive files and perhaps for cases where you'd add diur columns to mgd77+ files that weren't previously corrected for diurnal variation.

You can reject this feature request. It's not that difficult to do the calculation outside of mgd77list.

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