<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<div class="moz-cite-prefix">
<p>Hello Dan<br>
</p>
<p>Le 19/03/2018 à 23:15, Dan Crosby a écrit :</p>
</div>
<blockquote type="cite"
cite="mid:fc7ea30c-e4c8-4356-9cce-738b3ee82bcf@lincolnagritech.co.nz">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="Generator" content="Microsoft Word 15 (filtered
medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:Wingdings;
panose-1:5 0 0 0 0 0 0 0 0 0;}
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:DengXian;
panose-1:3 0 5 9 0 0 0 0 0 0;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:"\@DengXian";
panose-1:3 0 5 9 0 0 0 0 0 0;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
p.msonormal0, li.msonormal0, div.msonormal0
{mso-style-name:msonormal;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
span.EmailStyle18
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:#1F497D;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 72.0pt 72.0pt 72.0pt;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
<div class="WordSection1">
<div>
<div>
<p><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Am
I correct in thinking that, provided the relevant
ellipse is defined in Proj, I can perform any datum
shift with the appropriate +towgs84 entry?<o:p></o:p></span></p>
</div>
</div>
</div>
</blockquote>
<p>It depends on the desired accuracy; transformations done with <tt>+towgs84</tt>
are only approximations. For example the accuracy is about 20
metres for NAD27 to WGS84 datum shift in Canada. For determining
the operation accuracy (if the library does not provide this
information by itself), one can consult <a class="moz-txt-link-freetext" href="http://epsg-registry.org/">http://epsg-registry.org/</a>
(e.g. enter "NAD27 to WGS 84" in the name field).</p>
<p>For better accuracy, datum shift grids are required. With those
grids, an accuracy closer to 1 metre can be achieved. Note however
that many datum shift grids are defined between two national datum
(for example from NAD27 to NAD83), not necessarily to WGS 84. The
national datum are often related to a continental plate while WGS
84 is related to Earth center; those two kinds of datum do not
move in the same way. But a transformation to WGS 84 may
nevertheless be performed on the assumption that one of the
national datum is close enough to WGS 84. One may need to chain
two grids however (e.g. NAD27 to NAD83 followed by NAD83 to
NAD83(HARN) on the assumption that the later is equivalent to WGS
84 within the accuracy of the transformation).</p>
<p>Note that not all datum shifts can be relative to WGS 84. For
example in Martinique (for transformation from 1938 system), any
datum shifts going through WGS 84 will put errors; this
transformation can not be described accurately by <tt>+towgs84</tt>.
Note also that there is no longer a single WGS 84 system, but
currently 6 versions of WGS 84 with a difference up to about 1~2
metres between them, which make the use of <tt>+towgs84</tt>
ambiguous.</p>
<p>Whether you need to care about the above or not depend on the
desired accuracy. An issue is that current Proj.4 version gives no
hint about the accuracy of its operation, but I believe that
providing this kind of information is on the roadmap for future
versions.</p>
<p> Martin</p>
<p><br>
</p>
<span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"></span>
</body>
</html>