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<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>Oops, I happened to hit Control + Enter by mistake, which
means “Send”. Sorry. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>The end was supposed to be:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>I wonder if anyone knows of other NTv2 files where
zero shifts are supposed to mean undefined?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-US style='font-size:
10.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>As far as I know, most NTv2 files extrapolate the
datum shifts, so that they can fill a rectangular grid with <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>shift values that are continuous and sensible (for
some definition of sense). <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>But maybe there are other countries that use the
British style? <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>Best regards,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>Mikael Rittri<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>Carmenta<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>Sweden<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>http://www.carmenta.com<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<div>
<div class=MsoNormal align=center style='text-align:center'><font size=3
face="Times New Roman"><span lang=EN-US style='font-size:12.0pt'>
<hr size=2 width="100%" align=center tabindex=-1>
</span></font></div>
<p class=MsoNormal><b><font size=2 face=Tahoma><span lang=EN-US
style='font-size:10.0pt;font-family:Tahoma;font-weight:bold'>From:</span></font></b><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>
Mikael Rittri <br>
<b><span style='font-weight:bold'>Sent:</span></b> Thursday, February 09, 2012
4:46 PM<br>
<b><span style='font-weight:bold'>To:</span></b> 'proj@lists.maptools.org'<br>
<b><span style='font-weight:bold'>Subject:</span></b> How common is it that a
zero shift means undefined in an NTv2 file?</span></font><span lang=EN-US><o:p></o:p></span></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>Hello,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>I have noticed that the grid shift files for <st1:place
w:st="on"><st1:country-region w:st="on">Great Britain</st1:country-region></st1:place>
(that do datum shift from OSGB36 to ETRS89) <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>have meaningful shift values only up to 10 km
offshore. Beyond that limit, OSGB36 is not defined,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>the shift values are zero, and the software is
supposed to signal an error. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>At least, the original British software does so:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
lang=EN style='font-size:12.0pt;color:black'> When
transformation software encounters a zero parameter it will return an
"outside transformation boundary" type error.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
lang=EN style='font-size:12.0pt;color:black'> </span></font><font
color=black face=Arial><span lang=EN style='font-family:Arial;color:black'><a
href="http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/about/surveying_osnet.html#6">http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/about/surveying_osnet.html#6</a><o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
lang=EN style='font-size:12.0pt;color:black'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN style='font-size:12.0pt;
font-family:Arial'>However, </span></font><font face=Arial><span lang=EN-US
style='font-family:Arial'>when using the NTv2 representation of the grid file,
the zero shifts may be accepted as valid:<o:p></o:p></span></font></p>
<p class=paraleft1 style='background:white'><font size=3 color=black
face=Arial><span lang=EN-US style='font-size:12.0pt;line-height:140%;
font-family:Arial;color:black'><br>
</span></font><font color=black face=Arial><span lang=EN style='font-family:
Arial;color:black'> </span></font><font color=black><span
lang=EN style='color:black'>The 10km off shore buffer is retained in the NTv2
version of OSTN02 so users must be aware that<br>
<strong><b><font face="Times New Roman">no transformation
will take place beyond this limit. Unlike in a dedicated implementation of the</font></b></strong><b><span
style='font-weight:bold'><br>
<strong><b><font face="Times New Roman"> original OSTN02
grid, a GIS system using the NTv2 version of OSTN02 is unlikely to warn the</font></b></strong><br>
<strong><b><font face="Times New Roman"> user that
coordinates are outside the 10km boundary and will therefore not be
transformed.</font></b></strong><br>
<strong><b><font face="Times New Roman"> </font></b></strong></span></b></span></font><strong><b><font
color=black face=Arial><span lang=EN style='font-family:Arial;color:black'><a
href="http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/furtherinfo/ostn02_ntv2.html"><span
style='font-weight:normal'>http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/furtherinfo/ostn02_ntv2.html</span></a></span></font></b></strong><strong><b><font
face="Times New Roman"><span lang=EN><o:p></o:p></span></font></b></strong></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>I have noticed that the grid shift files for <st1:place
w:st="on"><st1:country-region w:st="on">Great Britain</st1:country-region></st1:place>
(that do datum shift from OSGB36 to ETRS89) </span></font><span lang=EN-US><o:p></o:p></span></p>
<p class=paraleft1 style='background:white'><font size=3 face=Arial><span
lang=EN-US style='font-size:12.0pt;line-height:140%;font-family:Arial'>have meaningful
shift I wonder if anyone knows of other NTv2 files where zero shifts are
supposed to mean undefined?<br>
As far </span></font><strong><b><font face="Times New Roman"><span
style='font-weight:normal'><o:p></o:p></span></font></b></strong></p>
<p class=paraleft1 style='background:white'><strong><b><font size=3
color=black face=Arial><span lang=EN style='font-size:12.0pt;line-height:140%;
font-family:Arial;color:black'><o:p> </o:p></span></font></b></strong></p>
<p class=paraleft1 style='background:white'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt;line-height:140%'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
lang=EN style='font-size:12.0pt;color:black'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
lang=EN style='font-size:12.0pt;color:black'> </span></font><font
face=Arial><span lang=EN-US style='font-family:Arial'><o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span lang=EN-US style='font-size:
10.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
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