<html><body name="Mail Message Editor"><div>Noel, I object to this characterization:</div><div><br></div><div>______</div><div><br></div><span class="Apple-style-span" style="font-family: -webkit-monospace; font-size: 11px; ">Map conversions (geographical<br>to grid) are merely mathematical mappings that are (theoretically) without<br>empirical error. Except - and this is my objection in this thread<br>heretofore - except when the ellipsoid is changed and a datum transformation<br>is implicitly coupled with a map conversion.</span><div><span class="Apple-style-span" style="font-family: -webkit-monospace; font-size: 11px;">______</span></div><div><span class="Apple-style-span" style="font-family: -webkit-monospace; font-size: 11px;"><br></span></div><div><span class="Apple-style-span" style="font-family: -webkit-monospace; font-size: 11px;"><br></span></div><div>As we discussed, the original geographic coordinates are recoverable through a known, rigorous process. There is nothing empirical; no information is lost; and there is no error. There is not even a good reason to state that the datum has changed in the case of Google Maps. They have simply defined a map projection of WGS84 that is not conformal.</div><div><br></div><div><span class="Apple-style-span" style="font-family: -webkit-monospace; font-size: 11px;"><span class="Apple-style-span" style="font-family: Helvetica; font-size: 12px; ">Regards,</span></span></div><div>— daan Strebe</div><div><span class="Apple-style-span" style="font-family: -webkit-monospace; font-size: 11px;"><br></span></div><div><span class="Apple-style-span" style="font-family: -webkit-monospace; font-size: 11px;"><br></span>On Dec 7, 2008, at 8:30:09 AM, "Noel Zinn" <ndzinn@comcast.net> wrote:<br><blockquote style="padding-left: 5px; margin-left: 5px; border-left-width: 2px; border-left-style: solid; border-left-color: blue; color: blue; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div style="width: 100%; "><div id="felix-mail-header-block" style="color: black; background-color: white; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: silver; padding-bottom: 1em; margin-bottom: 1em; width: 100%; "><table border="0" cellpadding="1" cellspacing="1" width="100%"><tbody><tr><td width="70px" style="font-family: 'Lucida Grande'; font-size: 8pt; color: gray; text-align: right; vertical-align: top; font-weight: bold; "><span>From:</span></td><td style="font-family: 'Lucida Grande'; font-size: 8pt; color: black; text-align: left; vertical-align: top; padding-left: 5px; "><span title=""Noel Zinn" <ndzinn@comcast.net>">"Noel Zinn" <ndzinn@comcast.net></span></td></tr><tr><td width="70px" style="font-family: 'Lucida Grande'; font-size: 8pt; color: gray; text-align: right; vertical-align: top; font-weight: bold; "><span>Subject:</span></td><td style="font-family: 'Lucida Grande'; font-size: 8pt; color: black; text-align: left; vertical-align: top; padding-left: 5px; "><span style="font-weight: bold; ">RE: [Proj] "Double Ellipsoid" error, reproduction</span></td></tr><tr><td width="70px" style="font-family: 'Lucida Grande'; font-size: 8pt; color: gray; text-align: right; vertical-align: top; font-weight: bold; "><span>Date:</span></td><td style="font-family: 'Lucida Grande'; font-size: 8pt; color: black; text-align: left; vertical-align: top; padding-left: 5px; "><span>December 7, 2008 8:30:09 AM PST</span></td></tr><tr><td width="70px" style="font-family: 'Lucida Grande'; font-size: 8pt; color: gray; text-align: right; vertical-align: top; font-weight: bold; "><span>To:</span></td><td style="font-family: 'Lucida Grande'; font-size: 8pt; color: black; text-align: left; vertical-align: top; padding-left: 5px; "><span title=""'PROJ.4 and general Projections Discussions'" <proj@lists.maptools.org>">"'PROJ.4 and general Projections Discussions'" <proj@lists.maptools.org></span></td></tr></tbody></table></div><div id="felix-mail-content-block" style="color: black; background-color: white; width: 100%; "><div style="font-family: monospace; color: black; background-color: white; font-size: 8pt; ">Gerald,<br><br>The input and output of datum transformations are geographical coordinates.<br>Some methods of datum transformation (e.g. NADCON, NTv2, multiple regression<br>equations) work in geographicals and ellipsoid parameters are not required.<br>Other methods of datum transformations (e.g. 3 through 7 parameter<br>similarity, Helmert, Bursa-Wolf, 10-parameter Molodensky-Badekas) require a<br>transit through geocentric Cartesian, Earth-Centered Earth-Fixed (ECEF)<br>coordinates for which ellipsoid parameters are required. Datum<br>transformation parameters are always empirically derived and spatially<br>variant and, therefore, always contain error. Map conversions (geographical<br>to grid) are merely mathematical mappings that are (theoretically) without<br>empirical error. Except - and this is my objection in this thread<br>heretofore - except when the ellipsoid is changed and a datum transformation<br>is implicitly coupled with a map conversion.<br><br>So, the answers to your questions are (1) true and (2) false (in some<br>cases).<br><br>You are absolutely correct that there is no LAW about what a cartographer<br>does, limited by his or her own imagination, but there is a physical reality<br>for those who have an interest in conforming to it as closely as possible.<br>The best fit between physical reality and geographical coordinates is<br>defined by the ellipsoid in which the datum's least-squares adjustment of<br>survey data (collected in the real world, not in pixel space) was done.<br>Change the ellipsoid and the quality of the fit deteriorates (even in a<br>datum transformation and especially in a map conversion). I already<br>demonstrated in this thread that Google Maps Projection grid coordinates<br>bear an undocumented relationship with physical reality for the casual user.<br><br><br>Regards,<br>Noel<br><br>-----Original Message-----<br>From: proj-bounces@lists.maptools.org<br>[mailto:proj-bounces@lists.maptools.org] On Behalf Of Gerald I. Evenden<br>Sent: Sunday, December 07, 2008 9:25 AM<br>To: PROJ.4 and general Projections Discussions<br>Subject: Re: [Proj] "Double Ellipsoid" error, reproduction<br><br>I find this thread so incredible confusing I have tried to stay out of it<span class="Apple-converted-space"> </span><br>entirely but I have one, maybe two questions:<br><br>I thought the basic detail operations of datum conversion were done in<span class="Apple-converted-space"> </span><br>geographic data space (latitude-longitude) or perhaps x-y-z. That is, you<span class="Apple-converted-space"> </span><br>have to have the data in geographic space to do the datum conversion<span class="Apple-converted-space"> </span><br>calculation. True or False?<br><br>Also, ellipsoid factors are not part of the datum conversion as long as the<span class="Apple-converted-space"> </span><br>data is in geographic coordinates: True or False?<br><br>If the above is true, what does the ellipsoid values have to do with<br>anything?<span class="Apple-converted-space"> </span><br>That is a rhetorical question and the answer is obviously: nothing.<br><br>The question of the ellipsoid parameters only comes up when dealing with<br>data<span class="Apple-converted-space"> </span><br>in Cartesian coordinates which need to be transformed back to geographic<span class="Apple-converted-space"> </span><br>coordinates for datum transformation. The ellipsoid values are only a<br>factor<span class="Apple-converted-space"> </span><br>in de-projecting the points.<br><br>The ellipsoid chosen for the Cartesian projection is probably a capricious<br>and<span class="Apple-converted-space"> </span><br>arbitrary choice of the people who created the projected data in the first<span class="Apple-converted-space"> </span><br>place. I know of no LAW that requires someone to choose a particular<span class="Apple-converted-space"> </span><br>ellipsoid purely because of the datum of the data. The plotter may not have<br><br>any idea as to the datum of the data.<br><br>If the issue is simply two completely separate operations--- datum<br>conversion<span class="Apple-converted-space"> </span><br>and ellipsoid-projection--- then what is all the discussion about???<br><br>What I am arguing is that the use of the ellipsoid parameters and datum<span class="Apple-converted-space"> </span><br>conversion are two completely separate issues. Thus the problem discussed<br>in<span class="Apple-converted-space"> </span><br>this thread is merely an issue finding the correct ellipsoid to get data<br>back<span class="Apple-converted-space"> </span><br>into geographic coordinates, doing a datum conversion, and selecting an<span class="Apple-converted-space"> </span><br>arbitrary a new ellipsoid for the target display map.<br><br>--<span class="Apple-converted-space"> </span><br>The whole religious complexion of the modern world is due<br>to the absence from Jerusalem of a lunatic asylum.<br>-- Havelock Ellis (1859-1939) British psychologist<br>_______________________________________________<br>Proj mailing list<br>Proj@lists.maptools.org<br>http://lists.maptools.org/mailman/listinfo/proj<br><br>_______________________________________________<br>Proj mailing list<br>Proj@lists.maptools.org<br>http://lists.maptools.org/mailman/listinfo/proj<br><br></div></div></div></span></blockquote><br><div><br></div></div><div class="aol_ad_footer" id="u00C3B47B047D467CBF306F416954D5EA"></div></body></html>