{"id":142,"date":"2016-12-21T16:00:29","date_gmt":"2016-12-21T15:00:29","guid":{"rendered":"https:\/\/www.picageotag.com\/en\/?page_id=142"},"modified":"2017-05-15T14:00:36","modified_gmt":"2017-05-15T12:00:36","slug":"altitudes","status":"publish","type":"page","link":"https:\/\/www.picageotag.com\/en\/altitudes\/","title":{"rendered":"Altitudes"},"content":{"rendered":"<p><img loading=\"lazy\" decoding=\"async\" class=\"alignright size-full wp-image-146\" src=\"\/wp-content\/uploads\/gravite.png\" alt=\"gravite\" width=\"256\" height=\"256\" \/>By construction, GPS devices records the altitudes relatively to the reference ellipsoid WGS-84. But, since the earth is not homogeneous, there are differences in gravity from one place to another. As a result, the ellipsoid does not match with the sea level. The deviations can reach \u00b180m! In fact the earth is all battered.<\/p>\n<p>Different models more or less accurate or usefull can translate from the ellipsoid to the sea level. PicaGeoTag uses a model of the<a href=\"https:\/\/www.nga.mil\">National Geospatial-Intelligence Agency<\/a> to convert altitudes.<\/p>\n<p>There are essentially two GPS file formats:<\/p>\n<ul>\n<li>The type files <strong>gpx<\/strong> Which explicitly specifies that the altitudes are relative to the ellipsoid.<\/li>\n<li>The type files <strong>kml<\/strong> and <strong>kmz<\/strong> used by Google Earth have several references of which the level of the sea, but <span style=\"color: #ff0000;\"><strong>not<\/strong> WGS-84<\/span>!<\/li>\n<\/ul>\n<p>Unfortunately not everyone follows the same rules:<\/p>\n<ul>\n<li>Some GPS units give the altitudes relative to sea level without knowing which correction model is used.<\/li>\n<li>Some GPS applications generate gpx files that use sea level as a reference.<\/li>\n<li>Conversely, some GPS applications generate kml\/kmz files with altitudes relative to the ellipsoid.<\/li>\n<\/ul>\n<p>In short it is the jungle! So if the altitudes are important to you (do not forget that the GPS has an accuracy of \u00b1 5m), check the data obtained by your devices when you are in places where altitude is known.<\/p>\n<p>For recording in EXIF images, the specification is completely blurred on the reference to be used. If your device has a GPS, check the reference used with the manufacturer if available. PicaGeoTag gives you the choice. The tag <strong>GPSMapDatum<\/strong> is used to records the reference used.<\/p>\n<p>You can <a href=\"\/en\/configuration\">setup<\/a> PicaGeoTag for all these situations.<\/p>\n\n<!-- WP QUADS v. 3.0.5  Shortcode Ad -->\n<div class=\"quads-location quads-ad\" id=\"quads-ad\" style=\"float:none;margin:19px 0 19px 0;text-align:center;\">\n\n <!-- WP QUADS - Quick AdSense Reloaded v.3.0.5 Content AdSense async --> \n\n\n<script >\nvar quads_screen_width = document.body.clientWidth;\nif ( quads_screen_width >= 1140 ) {document.write('<ins class=\"adsbygoogle\" style=\"display:inline-block;width:300px;height:250px;\" data-ad-client=\"ca-pub-8279079058220261\" data-ad-slot=\"4449556074\" ><\/ins>');\r\n            (adsbygoogle = window.adsbygoogle || []).push({});\r\n            }if ( quads_screen_width >= 1024  && quads_screen_width < 1140 ) {document.write('<ins class=\"adsbygoogle\" style=\"display:inline-block;width:300px;height:250px;\" data-ad-client=\"ca-pub-8279079058220261\" data-ad-slot=\"4449556074\" ><\/ins>');\r\n            (adsbygoogle = window.adsbygoogle || []).push({});\r\n            }if ( quads_screen_width >= 768  && quads_screen_width < 1024 ) {document.write('<ins class=\"adsbygoogle\" style=\"display:inline-block;width:300px;height:250px;\" data-ad-client=\"ca-pub-8279079058220261\" data-ad-slot=\"4449556074\" ><\/ins>');\r\n            (adsbygoogle = window.adsbygoogle || []).push({});\r\n            }if ( quads_screen_width < 768 ) {document.write('<ins class=\"adsbygoogle\" style=\"display:inline-block;width:300px;height:250px;\" data-ad-client=\"ca-pub-8279079058220261\" data-ad-slot=\"4449556074\" ><\/ins>');\r\n            (adsbygoogle = window.adsbygoogle || []).push({});\r\n            }\n<\/script>\n\n <!-- end WP QUADS --> \n\n<\/div>\n\n","protected":false},"excerpt":{"rendered":"<p>By construction, GPS devices records the altitudes relatively to the reference ellipsoid WGS-84. But, since the earth is not homogeneous, there are differences in gravity from one place to another. As a result, the ellipsoid does not match with the sea level. The deviations can reach \u00b180m! In fact the [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-142","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.picageotag.com\/en\/wp-json\/wp\/v2\/pages\/142","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.picageotag.com\/en\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.picageotag.com\/en\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.picageotag.com\/en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.picageotag.com\/en\/wp-json\/wp\/v2\/comments?post=142"}],"version-history":[{"count":3,"href":"https:\/\/www.picageotag.com\/en\/wp-json\/wp\/v2\/pages\/142\/revisions"}],"predecessor-version":[{"id":226,"href":"https:\/\/www.picageotag.com\/en\/wp-json\/wp\/v2\/pages\/142\/revisions\/226"}],"wp:attachment":[{"href":"https:\/\/www.picageotag.com\/en\/wp-json\/wp\/v2\/media?parent=142"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}