[daip] forwarded message from Eric Greisen

[Eric Greisen]

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From: Eric Greisen <egreisen at cv3.cv.nrao.edu>
To: Joseph.Lazio at nrl.navy.mil
Subject: Re: [daip] FLATN
Date: Mon, 8 Jul 2002 20:03:06 -0600

Joseph Lazio writes:
 > Somebody asked about DOWEIGHT in FLATN.  In staring at both the help
 > file and the code, it's become considerably less obvious what this
 > adverb does.  In FLATN.FOR 
 > 
 >             CALL OGET (IN(LFIELD), 'DOWEIGHT', TYPE, DIM, RFACT, CDUMMY,
 >      *         IRET)
 > 	    [...]
 >             RFACT = RFACT * 3600.0
 > 
 > That would seem to convert RFACT from degrees into arcseconds, yet the 
 > HELP file indicates that DOWEIGHT should be given in arcseconds.

    No it does not.  It says that the radius is in arc seconds and
DOWEIGHT multiplies that.  As you point out, angle is in degrees so
RFACT has the deg to arc sec built into it (fewer multiplies).

ERic

 > Moreover, later in the code
 > 
 >                            ZREF = SIN (YREF) * SIN (DECREF) + COS (YREF)
 >      *                        * COS (DECREF) * COS (XREF - RAREF)
 >                            ANGLE = ACOS (ZREF) * RAD2DG
 >                            WT = WT * MAX (0.0001D0, 1.0D0-ANGLE*RFACT)
 > 
 > ANGLE would appear to be the distance from the map center as measured
 > in degrees.  In addition to the fact that the units of ANGLE*RFACT
 > don't appear to make sense, 1-ANGLE*RFACT would seem to be negative
 > for most naive choices of RFACT.
 > 
 > What are we missing?
 > 
 > -- Joe
 > 
 > -- 
 > T. Joseph W. Lazio, Ph.D.        voice: +1 202 404 6329
 > Remote Sensing Division          fax:   +1 202 404 8894
 > Naval Research Lab, Code 7213    Joseph.Lazio at nrl.navy.mil
 > Washington, DC 20375-5351 USA    http://rsd-www.nrl.navy.mil/7213/lazio/
 > _______________________________________________
 > Daip mailing list
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 > 

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