{"id":5382,"date":"2018-05-30T05:19:03","date_gmt":"2018-05-30T03:19:03","guid":{"rendered":"http:\/\/www.sigterritoires.fr\/?p=5382"},"modified":"2023-04-28T14:56:43","modified_gmt":"2023-04-28T12:56:43","slug":"how-to-find-the-coordinates-system-of-a-layer-in-qgis","status":"publish","type":"post","link":"https:\/\/www.sigterritoires.fr\/index.php\/en\/how-to-find-the-coordinates-system-of-a-layer-in-qgis\/","title":{"rendered":"How to find the coordinates system of a layer in QGis?"},"content":{"rendered":"

Generally, all layers of geographic data have the description of the coordinates system.\u00a0Unfortunately, this is not always the case.\u00a0Usually, this problem increases by not being able to contact the creator of the data.<\/p>\n

Diagnosis of the problem<\/strong><\/p>\n

A- You ask QGis to load a vector layer and you see a window displayed that reads \u201cReference Coordinates System Selector\u00a0\u201c.\u00a0If it’s a layer with a \u201cshape\u201d format, the data must not have a defined coordinates system.\u00a0In the case of shapefiles, the PRJ file is missing.<\/p>\n

B- You ask QGis to load the layer and, without any window opening, you do not see your new data, or they elsewhere from where they should be.
\nBefore believing that there is, actually, a problem with your data, make sure that you have checked the Projection in \u00a0\u00bb\u00a0\u00a0\u00a0Project Properties\u00a0\u00a0\u00a0\u00bb->\u00ab\u00a0\u00a0\u00a0SCR\u00a0\u00a0\u00a0\u00ab\u00a0.
\nIf the box is checked, the data has a defined coordinates system, but it is false.\u00a0The PRJ file exists, but its contents is wrong.<\/p>\n

Search procedure<\/strong><\/p>\n

In this article we try to supply you a guide to determine the projection system of a layer of GIS data, when it is unknown.<\/p>\n

We have limited ourselves to the systems commonly used in France by the different organizations and administrations: Lambert and Lambert93 projections, the UTM projections and the non-projected data (geographical latitude \/ longitude).<\/p>\n

Of course, there are many others, used occasionally.\u00a0But it is impossible to determine them without real detective work.<\/p>\n

We assume that you know how to use QGis and that you have other reference data that will allow you to compare and judge the results obtained.<\/p>\n

You can download the pdf document of this article on the website of\u00a0\u00a0\u00a0NASCA<\/a>\u00a0, by clicking\u00a0\u00a0\u00a0here\u00a0.<\/p>\n

You must find the range of X and Y values <\/strong>for<\/strong> the data.<\/strong><\/p>\n

1- Start QGis with a new empty project<\/strong>
\n2- Add data with the unknown coordinates system.<\/strong>\u00a0The data must not have a defined coordinates system.<\/strong>\u00a0In the case of shapefiles, it must not have PRJ files.<\/strong>\u00a0If there is one, rename it differently.<\/strong>
\n3- In the window \u00ab<\/strong>\u00a0\u00a0<\/strong>\u00a0Reference Coordinate System Selector<\/strong>\u00a0\u00a0<\/strong>\u00a0\u00bb, Select the system WGS84, EPSG: 4326<\/strong><\/p>\n

\"\"<\/p>\n

4- On the loaded layer, open the context menu<\/strong>
\n5- Click Properties to open the Layer Properties dialog box.<\/strong>
\n6- Select the tab \u00ab<\/strong>\u00a0\u00a0<\/strong>\u00a0metadata<\/strong>\u00a0\u00a0<\/strong>\u00a0\u00bb, then<\/strong>\u00a0\u00ab<\/strong>\u00a0\u00a0<\/strong>\u00a0properties<\/strong>\u00a0\u00a0<\/strong>\u00a0\u00bband click on \u00ab<\/strong>\u00a0\u00a0<\/strong>\u00a0hold<\/strong>\u00a0\u00a0<\/strong>\u00a0\u00ab\u00a0.<\/strong><\/p>\n

\"\"<\/p>\n

Line\u00a0\u00a0\u00a0\u00ab\u00a0<\/em>\u00a0\u00a0<\/em>\u00a0xMin, yMin -13146.9,6755689.52: xMax, yMax 169765.86,6890746.18<\/em>\u00a0\u00a0<\/em>\u00a0\u00ab\u00a0<\/em>\u00a0\u00a0\u00a0gives you the\u00a0\u00a0\u00a0\u00ab\u00a0Scope\u201d\u00a0\u00a0\u00a0which we will use later.
\nYou will have the range Y: yMin to yMax (6755689.52, 6890746.18) and,
\nthe range of X: xMin to Xmax (-13146.9, 169765.86)<\/p>\n

The term coordinates system can be applied to data expressed in decimal degrees (geographic coordinates) or a projected coordinates system expressed in meters.
\nIf the coordinates found for the Range are expressed in decimal degrees, they will range from -180 to +180 for longitudes (X range) and between -90 and +90 for latitudes (Y range).\u00a0The data system (SCR) will be searched among Geographic Coordinates Systems.\u00a0It remains to find out the geodetic system (Datum) of the data.\u00a0(See later)<\/p>\n

If the coordinates found for the Extent are of the order of hundreds of thousands or millions, then are meters.\u00a0The data system will be searched among the projected coordinates systems. You still have to find the datum system of the data.<\/p>\n

For the time being it is enough to resume the method described in the article\u00a0\u00a0\u00a0How to find the coordinates system of a layer in ArcGis 10.X?<\/a>\u00a0\u00a0\u00a0using the values \u200b\u200bfound for the\u00a0\u00a0\u00a0\u00ab\u00a0Scope\u201d starting at\u00a0Find the projection system<\/a>\u00a0.<\/p>\n

When you have found the projection system, come back here to finish the job by determining the DATUM of your data.<\/p>\n

How to find the geodetic system (DATUM)<\/strong><\/p>\n

Once you have accomplished the first two stages, you have one last point to determine.\u00a0Any location system necessarily refers to a centre of the Earth.\u00a0Since the Earth is not a perfect sphere, and its centre has to be calculated, there are several ways to calculate it and, consequently, several different \u00ab\u00a0centres\u00a0\u00bb.\u00a0The difference is not huge and until a few decades ago it was a rather theoretical discussion, the resulting positioning difference being, in general, less than 300m.
\nIn theory, a coordinates system is always associated with a geodetic system.
\nThe Lambert 1, 2, 3,4 and 2 extended projections are always associated with the NTF system (New French Triangulation)
\nThe Lambert 93 and CC42 to 50 projections are still associated with the RGF system.
\nSo, if in the previous step you succeded to define one of these projections, the work is finished.
\nFor UTM 30 to 32 projections, theoretically the associated system is the WGS84 system.\u00a0But, can also be associated with the Europe 50 system.
\nIn the case of geographical (non-projected) data, they are also generally associated with the WGS84 system, but, also with the NTF or Europe 50 system.
\nIn the previous step we chose, by default, the WGS84 systems.\u00a0But we must now verify that this choice is the right one.
\nHow to find out?
\n\u2022 It is necessary to have a reference data layer, with the projection system defined correctly and especially, good precision (detail).
\n\u2022 In a new QGis project, load this reference layer.
\n\u2022 Make sure the option \u00ab\u00a0Enable on-the-fly projection\u00a0\u00bb is checked in \u00ab\u00a0Project Properties\u00a0\u00bb
\n\u2022 Load the layer to be determined in QGis, indicating the projection found in the previous step in the \u00ab\u00a0Reference Coordinate System Selector\u00a0\u00bb window.<\/p>\n

If<\/p>\n