Generally, all layers of geographic data have the description of the coordinates system. Unfortunately, this is not always the case. Usually, this problem increases by not being able to contact the creator of the data.
Diagnosis of the problem
A- You ask QGis to load a vector layer and you see a window displayed that reads “Reference Coordinates System Selector “. If it’s a layer with a “shape” format, the data must not have a defined coordinates system. In the case of shapefiles, the PRJ file is missing.
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.
Before believing that there is, actually, a problem with your data, make sure that you have checked the Projection in ” Project Properties »->« SCR “.
If the box is checked, the data has a defined coordinates system, but it is false. The PRJ file exists, but its contents is wrong.
Search procedure
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.
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).
Of course, there are many others, used occasionally. But it is impossible to determine them without real detective work.
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.
You can download the pdf document of this article on the website of NASCA , by clicking here .
You must find the range of X and Y values for the data.
1- Start QGis with a new empty project
2- Add data with the unknown coordinates system. The data must not have a defined coordinates system. In the case of shapefiles, it must not have PRJ files. If there is one, rename it differently.
3- In the window « Reference Coordinate System Selector », Select the system WGS84, EPSG: 4326
4- On the loaded layer, open the context menu
5- Click Properties to open the Layer Properties dialog box.
6- Select the tab « metadata », then « properties »and click on « hold “.
Line “ xMin, yMin -13146.9,6755689.52: xMax, yMax 169765.86,6890746.18 “ gives you the “Scope” which we will use later.
You will have the range Y: yMin to yMax (6755689.52, 6890746.18) and,
the range of X: xMin to Xmax (-13146.9, 169765.86)
The term coordinates system can be applied to data expressed in decimal degrees (geographic coordinates) or a projected coordinates system expressed in meters.
If 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). The data system (SCR) will be searched among Geographic Coordinates Systems. It remains to find out the geodetic system (Datum) of the data. (See later)
If the coordinates found for the Extent are of the order of hundreds of thousands or millions, then are meters. The data system will be searched among the projected coordinates systems. You still have to find the datum system of the data.
For the time being it is enough to resume the method described in the article How to find the coordinates system of a layer in ArcGis 10.X? using the values found for the “Scope” starting at Find the projection system .
When you have found the projection system, come back here to finish the job by determining the DATUM of your data.
How to find the geodetic system (DATUM)
Once you have accomplished the first two stages, you have one last point to determine. Any location system necessarily refers to a centre of the Earth. Since 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 “centres”. The 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.
In theory, a coordinates system is always associated with a geodetic system.
The Lambert 1, 2, 3,4 and 2 extended projections are always associated with the NTF system (New French Triangulation)
The Lambert 93 and CC42 to 50 projections are still associated with the RGF system.
So, if in the previous step you succeded to define one of these projections, the work is finished.
For UTM 30 to 32 projections, theoretically the associated system is the WGS84 system. But, can also be associated with the Europe 50 system.
In 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.
In the previous step we chose, by default, the WGS84 systems. But we must now verify that this choice is the right one.
How to find out?
• It is necessary to have a reference data layer, with the projection system defined correctly and especially, good precision (detail).
• In a new QGis project, load this reference layer.
• Make sure the option “Enable on-the-fly projection” is checked in “Project Properties”
• Load the layer to be determined in QGis, indicating the projection found in the previous step in the “Reference Coordinate System Selector” window.
If
- the data appears in the right place and there is no slight shift (100-300m), you have finished.The definition you have adopted is the right one.
• you have an offset of all your entities, of the order of 100 to 300m, your data is not in WGS84. The coordinate system (UTM or geographic) is good, but your data is not in WGS84. It must be in Europe 50 or NTF.
In order to discover which is the good Datum, you have to change the SRC of the layer you have loaded:
1-Open the context menu by right clicking on the layer,
2-click Set the SCR of a layer,
If the projection found in the previous step is
UTM spindle 30N: enter in “Filter” the IGNF code: UTM30
UTM spindle 31N: enter in “Filter” the IGNF code: UTM31
UTM spindle 32N: enter in “Filter” the IGNF code: UTM33
3-In the list of global SCRs, select this projection and click on OK
If you no longer notice a general data shift, you have found the correct coordinates system. If this is not the case, you are in front of a layer that falls outside the scope of this article.
If the system found in the previous step is a Geographic Coordinates System (X range between -180 and 180 and Y from -90 to 90)
1-Open the context menu by right clicking on the layer,
2-click Set the SCR of a layer,
3-Go into ED50 filter and select the SRC EPSG: 4230
If now you no longer observe a general data shift, you have found the correct coordinates system (EPSG: 4230): your data is in geographical Europe 50.
If it’s not the case,
1-Open the context menu by right clicking on the layer,
2-click Set the SCR of a layer,
3-Go into NTF filter and select the SRC EPSG: 4275
If now you no longer observe a general data offset, you have found the correct coordinates system (EPSG: 4275): your data is in NTF geographic.
If this is not the case, you are facing a layer that is outside the scope of this article
