Re: Low latitude RTP / RTE transformation

Curious, one other of Markku's codes (to compute the 3 components of the 
magnetic anomaly) has recently landed in gmtgravmag3d (5.2 branch), 
again because I was doing "some experiments".
But the one you pointed has no source code available.

> Hi Joaquim,
>
> That's useful to know. Yes the RTP is problematic at low latitudes, 
> which does make it tricky knowing how to work the data.
>
> There is a free sofware by Markku Pirttijarvi called Fourpot (v1.3) 
> that does both th classic RTP and RTE as well as the various 
> derivatives, but does take some getting used to the inputs etc:
>
> http://www.cc.oulu.fi/~mpi/Softat/Fourpot.html 
> <http://www.cc.oulu.fi/%7Empi/Softat/Fourpot.html>
>
> I guess that as long as we are looking at areas +/-15 deg grdredpol 
> should be able to handle things.
>
> Did come across an article that discusses a modified RTP and RTE 
> method, although it would need someone that understands the 
> methodology to assess the validity of the equations:
> LUO Yao, XUE Dian-Jun, WANG Ming (2010), Reduction to the pole at the 
> geomagnetic equator.
> CHINESE JOURNAL OF GEOPHYSICS Vol.53, No.6, 2010, pp: 1082-1089
>
>
> Cheers
> Lester
>
>
> ------------------------------------------------------------------------
> Date: Fri, 28 Feb 2014 12:18:58 +0000
> From: [email protected]
> Subject: Re: [GMT-HELP] Low latitude RTP / RTE transformation
> To: [email protected]
>
> Lester,
>
> Its is well known that the Reduction-To-Pole operators simply do not 
> work at low latitudes (~10-15 deg) and the problem of an RTE is also 
> harder than the RTP.
> For cases like this, or others like for instances the interpolation 
> with break lines, it is normally necessary that one of the developers 
> has a personal interest in the problem so that the result ends up in 
> GMT. That was what happened with grdredpol and it may happen again, 
> but in an uncertain future.
> However, things could go much faster if someone had open source code 
> to contribute (but not GPL'de one).
>
> Joaquim
>
>     Hello,
>
>     With GMT5 we now have a proper RTP transform tool (GRDREDPOL) for
>     performing both the classic (constant field vector) and
>     differential (variable field vector) for working on large areas.
>     However, RTP at low latitudes is unstable and the results are skewed.
>
>     Would it be feasible to include either a low-latitude correction
>     for the RTP and/or a separate "Reduce-to-Equator RTE" program?
>
>     Cheers
>     Lester
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>
>
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