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<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:country-region
w:st="on"><st1:place w:st="on">Great Britain</st1:place></st1:country-region> (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<br>
original OSTN02 grid, a GIS system using the NTv2 version of
OSTN02 is unlikely to warn the<br>
user that coordinates are outside the 10km boundary and will
therefore not be transformed.<br>
</font></b></strong></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=Arial><span lang=EN style='font-family:Arial'><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) <o:p></o:p></span></font></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 <strong><b><font face=Arial><span style='font-family:Arial;font-weight:
normal'><o:p></o:p></span></font></b></strong></span></font></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'><b><font size=3 color=black
face=Arial><span lang=EN style='font-size:12.0pt;line-height:140%;font-family:
Arial;color:black;font-weight:bold'><o:p> </o:p></span></font></b></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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