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<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>The Therion book states<o:p></o:p></span></font></p>
<p class=MsoNormal style='text-autospace:none'><font size=2 face=Arial><span
lang=EN-NZ style='font-size:10.0pt;font-family:Arial'>…</span></font><font
face=CMR12><span style='font-family:CMR12'>The declination is positive when the
magnetic north is east of true north</span></font><font size=2 face=Arial><span
lang=EN-NZ style='font-size:10.0pt;font-family:Arial'>…<o:p></o:p></span></font></p>
<p class=MsoNormal style='text-autospace:none'><font size=2 face=Arial><span
lang=EN-NZ style='font-size:10.0pt;font-family:Arial'>and<o:p></o:p></span></font></p>
<p class=MsoNormal style='text-autospace:none'><font size=3 color=black
face=CMSY10><span lang=EN-NZ style='font-size:12.0pt;font-family:CMSY10;
color:black'> </span></font><font color=blue face=CMTT12><span
style='font-family:CMTT12;color:blue'>north <true/grid> </span></font><font
color=black face=CMMI10><span style='font-family:CMMI10;color:black'>. </span></font><font
color=black face=CMR12><span style='font-family:CMR12;color:black'>specify
default orientation of the map. By default, true (astronomical) north is used.
It is ignored when used with local coordinate system.</span></font><font
size=2 color=black face=CMSY10><span style='font-size:10.0pt;font-family:CMSY10;
color:black'><o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>I’m confused. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>Presumably almost all cave surveys start out aligned
to magnetic north because most of us use compasses that adopt the local
magnetic north as their datum.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>Then, often, we either align the survey to true
(astronomical) north manually by specifying a declination, <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>
Or align the survey to grid north by specifying a co-ordinate system (that
calculates declination automatically)<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>The grid north is usually not the same as the true
(astronomical) north, but for the purposes of cave maps and Therion I would
assume it is usually considered to be so. (Not strictly correct, but I
imagine the error would not be detectable on any printed cave map in most
co-ordinate systems)<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>So what is intended by Therion? Presumably some
distinction between magnetic and grid north?<b><span style='font-weight:bold'><o:p></o:p></span></b></span></font></p>
<p class=MsoNormal><b><font size=2 face=Arial><span lang=EN-NZ
style='font-size:10.0pt;font-family:Arial;font-weight:bold'><o:p> </o:p></span></font></b></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>I have experimented and found that ‘north
grid’ ‘north true’ appear to produce identical outputs
regardless of whether a ‘co-ordinate system’ or a
‘declination’ or a ‘rotation’ is specified.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>Is there a mistake or am I not understanding
something?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>Regards<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-NZ style='font-size:
10.0pt;font-family:Arial'>Bruce<o:p></o:p></span></font></p>
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