Re: Low latitude RTP / RTE transformation

There does seem to be a few papers out there discussing the low latitude correction in RTP, and all seem to revolve around different methods of suppressing or reducing the effects of the amplification in RTP filter factor. So there seems to be scope to expand the grdredpol functionality if the validity of the techniques can be demonstrated: really only seen simple "real-world" and synthetics.

Perhaps Markku would be interested in sharing his code or his ideas on this issue? Worth asking I guess.

Plenty of food for thought!

Lester


Date: Fri, 28 Feb 2014 15:17:27 +0000
From: [email protected]
Subject: Re: [GMT-HELP] Low latitude RTP / RTE transformation
To: [email protected]


  
    
  
  
    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

         

        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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