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PODifferent calculations between ST_Transform and CoordTransform that involves presition
 

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  1. PODifferent calculations between ST_Transform and CoordTransform that involves presition
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    <p>I am working with GeoDjango and Postgis, and doing transformation on a point from 4326 to 24877, when I have checked the result, it is not the same</p> <p><strong>My values are:</strong></p> <pre><code>Latitude -16.42238172128686 Longitude -71.47541752550751 </code></pre> <p><img src="https://i.stack.imgur.com/b1j1b.jpg" alt="Initial position"></p> <p><strong>Using long lat as x, y coord</strong></p> <pre><code>In [63]: pnt = Point(-71.47541752550751, -16.42238172128686, srid=4326) In [64]: pnt.transform(CoordTransform(SpatialReference(4326), SpatialReference(24877))) In [65]: pnt.x, pnt.y, pnt.srid Out[65]: (1521142.247877425, 8160547.8770980425, 24877) </code></pre> <p><strong>Inverse</strong></p> <pre><code>In [66]: pnt = Point( 1521142.247877425,8160547.8770980425, srid=24877 ) In [67]: pnt.transform(CoordTransform(SpatialReference(24877), SpatialReference(4326))) In [68]: pnt.x, pnt.y, pnt.srid Out[68]: (-71.47541753138003, -16.42238165040434, 4326) </code></pre> <p><img src="https://i.stack.imgur.com/HYdjc.jpg" alt="Position after calculus"></p> <p><strong>SQL on Postgres</strong></p> <pre><code>SELECT ST_AsText(ST_Transform(ST_SetSRID(ST_MakePoint(-71.47541752550751, -16.42238172128686), 4326),24877)) POINT(1520903.86571082 8160169.90886929) </code></pre> <p><strong>Taking data from postgis results</strong></p> <pre><code>In [69]: pgs = Point(1520903.86571082, 8160169.90886929, srid=24877) In [70]: pgs.transform(CoordTransform(SpatialReference(24877), SpatialReference(4326))) In [71]: pgs.x, pnt.y, pgs.srid Out[71]: (-71.47745375612124, -16.42238165040434, 4326) </code></pre> <p><img src="https://i.stack.imgur.com/2UVIb.jpg" alt="Image on sql results"></p> <p>As you could see there is a difference between geodjango and postgres, on at least 300 meters, and that means different places</p> <p>After reading and asking on some forums and chats, I have found that this is related to presition, I was asking more about it on internet, as a newbie on this topic, this is getting some heard to fin. </p> <p>I am looking for information that give me enough knowledge for continue working on it, for example:</p> <ol> <li>The right way on do that</li> <li>The reason on this difference</li> <li>how many meters are the difference between systems or the right way on calculate that</li> <li>how to validate this positions</li> <li>what is the best way on work this information</li> </ol>
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    1. COThe correct axis order is longitude/latitude, i.e: x=longitude, y=latitude. Furthermore, which SRID? `28477` or `24877`? What does `SELECT postgis_full_version()` return?
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    2. COThanks for teh answer, this was a type writing error, the srid that I am looking for is 24877
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