Contenu <\/p>\nToggle<\/span>
The Earth is a curved surface, but maps are generally represented on a flat plane. To transform a spherical surface into a flat map, we use a map projection<\/strong>.<\/p>\n\n\n\n
Earth (curved surface)\n       🌍\n        \u2502\n        \u2502 geodetic system\n        \u2502 (for example WGS84)\n        \u25bc\nMap projection\n(mathematical transformation)\n        \u2502\n        \u25bc\nFlat map\nused in QGIS\n<\/code><\/pre>\n\n\n\nA projection transforms the surface of the Earth into a flat map.<\/p>\n\n\n\n
A Coordinate Reference System (CRS)<\/strong> defines:<\/p>\n\n\n\n
\nhow the Earth is represented<\/li>\n\n\n\n
the coordinate system used<\/li>\n\n\n\n
the map projection.<\/li>\n<\/ul>\n\n\n\nEach CRS is usually identified by an EPSG code<\/strong>.<\/p>\n\n\n\n
Examples:<\/p>\n\n\n\n
CRS<\/th> Description<\/th><\/tr><\/thead>
EPSG:4326<\/a><\/td> WGS84 \u2013 latitude \/ longitude<\/td><\/tr>
EPSG:3857<\/td> Web Mercator projection used in web maps<\/td><\/tr>
EPSG:2154<\/td> Lambert 93 (France)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\n<\/span>2. Two Important Projections in QGIS<\/span><\/h2>\n\n\n\nTwo coordinate systems are very common.<\/p>\n\n\n\n
<\/span>WGS84 (EPSG:4326)<\/span><\/h3>\n\n\n\nThis system is used by:<\/p>\n\n\n\n
\nGPS devices<\/li>\n\n\n\n
many geographic databases<\/li>\n\n\n\n
latitude \/ longitude coordinates.<\/li>\n<\/ul>\n\n\n\nExample:<\/p>\n\n\n\n
latitude : -20.3484\nlongitude : 57.5522\n<\/code><\/pre>\n\n\n\n
\n\n\n\n<\/span>Web Mercator (EPSG:3857)<\/span><\/h3>\n\n\n\nThis projection is used by most web maps:<\/p>\n\n\n\n
\nGoogle Maps<\/li>\n\n\n\n
OpenStreetMap<\/li>\n\n\n\n
Leaflet<\/li>\n\n\n\n
OpenLayers.<\/li>\n<\/ul>\n\n\n\nCoordinates are expressed in meters<\/strong>.<\/p>\n\n\n\n
\n\n\n\n<\/span>Example: EPSG:4326 vs EPSG:3857<\/span><\/h2>\n\n\n\nLet\u2019s take a point with the following coordinates:<\/p>\n\n\n\n
Latitude : -20.3484\nLongitude : 57.5522\n<\/code><\/pre>\n\n\n\nThese coordinates are expressed in:<\/p>\n\n\n\n
EPSG:4326 (WGS84)<\/strong><\/p>\n\n\n\n
This system is used by:<\/p>\n\n\n\n
\nGPS devices<\/li>\n\n\n\n
many geographic databases<\/li>\n\n\n\n
latitude \/ longitude coordinates.<\/li>\n<\/ul>\n\n\n\nIf we project this point into:<\/p>\n\n\n\n
EPSG:3857 (Web Mercator)<\/strong><\/p>\n\n\n\n
the coordinates might become:<\/p>\n\n\n\n
X : 6404700\nY : -2309000\n<\/code><\/pre>\n\n\n\nThe coordinates are now expressed in meters<\/strong>.<\/p>\n\n\n\n
This system is used by:<\/p>\n\n\n\n
\nOpenStreetMap<\/li>\n\n\n\n
Google Maps<\/li>\n\n\n\n
web mapping libraries such as Leaflet and OpenLayers.<\/li>\n<\/ul>\n\n\n\n
\n\n\n\n<\/span>3. The Project CRS in QGIS<\/span><\/h2>\n\n\n\nIn QGIS, every project has a project CRS<\/strong>.<\/p>\n\n\n\n
This CRS defines how layers are displayed on the map.<\/p>\n\n\n\n
To view the project CRS:<\/p>\n\n\n\n
\ncheck the status bar at the bottom of QGIS<\/li>\n\n\n\n
or go to:<\/li>\n<\/ul>\n\n\n\nProject \u2192 Properties \u2192 CRS<\/p>\n\n\n\n
<\/a><\/figure>\n\n\n\nQGIS can reproject layers on the fly<\/strong> so they display correctly.<\/p>\n\n\n\n
\n\n\n\n<\/span>4. Reprojecting a Layer<\/span><\/h2>\n\n\n\nSometimes it is necessary to reproject a layer<\/strong> into another CRS.<\/p>\n\n\n\n
In QGIS:<\/p>\n\n\n\n
\nRight-click on the layer<\/li>\n\n\n\n
Select Export \u2192 Save Features As\u2026<\/strong><\/li>\n\n\n\n
Choose a new CRS.<\/li>\n<\/ol>\n\n\n\nQGIS will create a new reprojected layer<\/strong>.<\/p>\n\n\n\n
\n\n\n\n<\/span>5. FAQ \u2013 Common Projection Problems<\/span><\/h2>\n\n\n\n<\/span>Layers do not align<\/span><\/h2>\n\n\n\nThis usually means that:<\/p>\n\n\n\n
\nthe layers use different CRS<\/li>\n\n\n\n
or one CRS is incorrectly defined.<\/li>\n<\/ul>\n\n\n\n
\n\n\n\n<\/span>Data appears in the wrong location<\/span><\/h3>\n\n\n\nFor example:<\/p>\n\n\n\n
\npoints appear off the coast of Africa<\/li>\n\n\n\n
data is shifted by thousands of kilometers.<\/li>\n<\/ul>\n\n\n\nIn this case:<\/p>\n\n\n\n
\ncheck the layer CRS<\/li>\n\n\n\n
check the project CRS.<\/li>\n<\/ul>\n\n\n\n
\n\n\n\n<\/span>The CRS is not defined<\/span><\/h3>\n\n\n\nSome datasets do not contain information about their projection.<\/p>\n\n\n\n
In this case, use:<\/p>\n\n\n\n
Set Layer CRS<\/strong><\/p>\n\n\n\n
Important: this does not reproject the data<\/strong>, it only tells QGIS which CRS to use.<\/p>\n\n\n\n
\n\n\n\n<\/span>6. Defining a CRS vs Reprojecting a Layer<\/span><\/h2>\n\n\n\nThis distinction is important.<\/p>\n\n\n\n
<\/span>Defining a CRS<\/span><\/h3>\n\n\n\n\ntells QGIS which coordinate system is used<\/li>\n\n\n\n
does not modify the coordinates.<\/li>\n<\/ul>\n\n\n\n<\/span>Reprojecting a layer<\/span><\/h3>\n\n\n\n\nactually transforms the coordinates into another CRS.<\/li>\n<\/ul>\n\n\n\n
\n\n\n\n<\/span>Conclusion<\/span><\/h2>\n\n\n\nProjections are a fundamental element in geographic information systems. In QGIS, understanding the difference between defining a CRS, reprojecting a layer, and changing the project CRS<\/strong> helps avoid most display issues.<\/p>\n\n\n\n
Once these concepts are understood, it becomes much easier to work with geographic data coming from different sources.<\/p>\n\n\n\n
\n\n\n\n<\/p>\n","protected":false},"excerpt":{"rendered":"
Projections and coordinate reference systems (CRS) are essential concepts when working with geographic data in QGIS. Incorrect handling of projections can cause significant misalignment between layers. When working with geographic data in QGIS, projections and coordinate…<\/p>\n","protected":false},"author":1,"featured_media":16581,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"give_campaign_id":0,"_bbp_topic_count":0,"_bbp_reply_count":0,"_bbp_total_topic_count":0,"_bbp_total_reply_count":0,"_bbp_voice_count":0,"_bbp_anonymous_reply_count":0,"_bbp_topic_count_hidden":0,"_bbp_reply_count_hidden":0,"_bbp_forum_subforum_count":0,"sfsi_plus_gutenberg_text_before_share":"","sfsi_plus_gutenberg_show_text_before_share":"","sfsi_plus_gutenberg_icon_type":"","sfsi_plus_gutenberg_icon_alignemt":"","sfsi_plus_gutenburg_max_per_row":"","_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_post_was_ever_published":false},"categories":[1377,4566],"tags":[4587,4589,1919,4585,1386],"class_list":["post-16618","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-qgis-en","category-tutorial","tag-crs-en","tag-epsg-en","tag-gis","tag-map-projections","tag-qgis-en"],"aioseo_notices":[],"jetpack_featured_media_url":"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2026\/03\/projection_cartographique_qgis.jpg?fit=1024%2C814&ssl=1","jetpack_shortlink":"https:\/\/wp.me\/p6XU0A-4k2","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/posts\/16618","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/comments?post=16618"}],"version-history":[{"count":0,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/posts\/16618\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/media\/16581"}],"wp:attachment":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/media?parent=16618"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/categories?post=16618"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/tags?post=16618"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}