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<p>Hello,</p>
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<p>yes, that is exactly the idea in Proj .. if the datum is not Proj predefined then you should use the towgs84 statement. You can either use the "towgs84=.." statement or "datum=..." and the conversion mechanism is the same, assuming that you use the same parameters in your towgs84 statement as in the library definition. You should also use the correct ellipse always ... and preferably always define it.</p>
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<p>Here are the usual library predefinitions:</p>
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<p>Datum ID - Ellipse - Definition/comments</p>
<p><br />WGS84 - WGS84 - towgs84=0,0,0<br />GGRS87 - GRS80 - towgs84=-199.87,74.79,246.62 / Greek Geodetic Ref Sys 1987<br />NAD83 - GRS80 - towgs84=0,0,0 / North American Datum 1983<br />NAD27 - clrk66 - nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat / NAD 1927<br />potsdam - bessel - towgs84=606.0,23.0,413.0 / Potsdam Rauenberg 1950 DHDN<br />carthage - clark80 - towgs84=-263.0,6.0,431.0 / Carthage 1934 Tunisia<br />hermannskogel - bessel - towgs84=653.0,-212.0,449.0 / Hermannskogel<br />ire65 - mod_airy - towgs84=482.530,-130.596,564.557,-1.042,-0.214,-0.631,8.15 / Ireland 1965</p>
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<p>see also:</p>
<p><a href="https://en.wikipedia.org/wiki/Helmert_transformation">https://en.wikipedia.org/wiki/Helmert_transformation</a></p>
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<p>regards: Janne.</p>
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<p>Dan Crosby kirjoitti 2018-03-20 00:15:</p>
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<p class="MsoNormal"><span style="font-size: 11.0pt; font-family: 'Calibri',sans-serif; color: #1f497d;">Hi all,<!-- o ignored --></span></p>
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<p class="MsoNormal"><span style="font-size: 11.0pt; font-family: 'Calibri',sans-serif; color: #1f497d;"></span><span style="font-size: 11.0pt; font-family: 'Calibri',sans-serif; color: #1f497d;"><!-- o ignored -->.... </span></p>
<p class="MsoNormal"><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 ignored --></span></p>
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<p class="MsoNormal"><span style="font-size: 11.0pt; font-family: 'Calibri',sans-serif; color: #1f497d;">Dan<!-- o ignored --></span></p>
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