Usually, all layers of geographic data have the description of the coordinate system. Unfortunately, this is not always the case. In general, the problem greatly increases with the fact that, usually, it is quite (or completely)impossible to contact the producer of the data.
Diagnosis of the problem
A- You ask Arcmap to load the layer and you see a warning message: “ The following added data sources do not have spatial reference information. This data can be displayed in ArcMap, but it cannot be projected. “
If it is a layer of vector data (points, lines, polygons), the data must lack a defined coordinate system. In the case of shapefiles, the PRJ file is missing.
B- You ask Arcmap to load the layer and, without having any warning or error message, you do not see your new data or, else, they are not where they should be.
If it’s a vector data layer (points, lines, polygons), the data has a defined coordinate system, but it’s wrong. The PRJ file exists, but its contents are wrong.
Search procedure
In this article we try to give 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: the Lambert and Lambert93projections, the UTM projections and the non-projected data (geographical latitude / longitude).
Of course, there are many others, used in specific cases. But it is impossible to determine them without real detective work.
The following maps show the different areas affected by each type of projection system.
We assume that you know how to use ArcGis and that you have other reference data that will allow you to compare and judge results.
You can download the pdf document of this article on the NASCA website, by clicking here .
You must find the range of X and Y values in the data.
1- Start ArcMap with a new, empty document
2- Add data with the unknown coordinate system. The data must not have a defined coordinate system. In the case of shapefiles, they must not have PRJ files. If there is one, rename it differently.
3- Right-click on the name of the layer in the table of contents,
4- Click Properties to open the layer Properties dialog box.
5- Select the Source tab, then examine the data scope, in the upper part of the window)
The term coordinate system can be applied to data expressed in decimal degrees (geographic coordinates) or a projected coordinate system expressed in meters.
If the coordinates shown in the range are expressed in decimal degrees, they will range from -180 to +180 for longitudes ( Left and Right values ) and between -90 and +90 for latitudes ( High and Low values ). The data system will be searched in the section “Geographic Coordinate Systems” of ArcGis. It remains to find the geodetic system data (Datum). (see further)
If the coordinates given in the Range are in the order of hundreds of thousands or millions, they are meters. The data system will be searched in the section “Projected coordinate systems” of ArcGis, and it will also remain to find the datum system of the data.
Find the projection system
Here is a logical progression based on the value range “Extended” of your layer. In addition to these values, we will use the data area in relation to three maps Map N ° 1: NTF Lambert zones
Map N ° 2: Lambert 93 areas
Map
N ° 3: UTM time zones
Y coordinates <1,000,000
—- X coordinates between 0 and 1 100 000
—— See on the map N ° 1
——- If your data is in Lambert I zone
——– Data is in NTF Lambert I area, EPSG code 27561.
——- If your data is in Lambert II zone
——– The data is in NTF Lambert II zone, code EPSG 27562. –
——- If your data is in Lambert III zone
——– Data is in NTF Lambert III area, EPSG code 27563.
—— If your data is in Lambert IV zone
——– The data is in NTF Lambert IV zone, EPSG code 27564.
Y coordinates between 1,000,000 and 2,000,000
—– X coordinates between 0 and 1 100 000
—— See on the map N ° 1
——- If your data is in Lambert I zone
——– The data is in NTF Lambert I carto, EPSG code 27571.
—– X coordinates between 1,000,000 and 2,500,000
—— See on the map N ° 2
——- If your data is in zone 1 (green)
——– Data is in Lambert CC42 RGF, EPSG code 3942.
Y coordinates between 2,000,000 and 3,000,000
—– X coordinates between 0 and 1 100 000
—— See on the map N ° 1
——- If your data is in Lambert II zone
——– The data is in NTF Lambert II carto, code EPSG 27572
——- If your data is outside the Lambert II zone
——– The data are in NTF Lambert II extended, EPSG code 27572.
—– X coordinates between 1,000,000 and 2,500,000
—— See on the map N ° 2
——- If your data is in zone 2 (blue)
——– The data is in Lambert CC43 RGF, code EPSG 3943.
Y coordinates between 3,000,000 and 4,000,000
—– X coordinates between 0 and 1 100 000
—— See on the map N ° 1
——- If your data is in Lambert III zone
——– Data is in NTF Lambert III carto, code EPSG 27573.
—– X coordinates between 1,000,000 and 2,500,000
—— See on the map N ° 2
——- If your data is in zone 3 (green)
——– The data is in Lambert CC44 RGF, code EPSG 3944.
Y coordinates between 4,100,000 and 4,300,000
—– X coordinates between 500,000 and 600,000
—— See on the map N ° 1
——- If your data is in Lambert IV zone
——– The data is in NTF Lambert IV carto, code EPSG 27574.
Y coordinates between 4,000,000 and 5,000,000
—– X coordinates between 200,000 and 750,000
—— See on the map N ° 3
——- If your data is in Time Zone 30
——– Data is in WGS84 UTM 30N, EPSG code 32630.
——- If your data is in Time Zone 31
——– Data is in WGS84 UTM 31N, EPSG code 32631.
——- If your data is in Time Zone 32
——– The data is in WGS84 UTM 32N, EPSG code 32632.
—– X coordinates between 1,000,000 and 2,500,000
—— See on the map N ° 2
——- If your data is in zone 4 (blue)
——– The data is in Lambert CC45 RGF, code EPSG 3945.
Y coordinates between 5,000,000 and 6,000,000
—– X coordinates between 200,000 and 750,000
—— See on the map N ° 3
——- If your data is in Time Zone 30
——– Data are in WGS84 UTM 30N, EPSG code 32630.
——- If your data is in Time Zone 31
——– Data are in WGS84 UTM 31N, EPSG code 32631.
——- If your data is in Time Zone 32
——– The data is in WGS84 UTM 32N, EPSG code 32632.
—– X coordinates between 1,000,000 and 2,500,000
—— See on the map N ° 2
——- If your data is in zone 5 (green)
——– Data are in Lambert CC46 RGF, EPSG code 3946.
Y coordinates between 6,000,000 and 7,000,000
—– X coordinates between 0 and 1 250 000
—— See on the map N ° 2
——- If your data ARE in zone 6 (blue)
——- There are two possibilities, you will have to test them against a
——- known layer to determine which one is the right one.
——– Data may be in Lambert RGF 93, code EPSG 21546.
——–Or
——– Data may be in Lambert CC47 RGF, EPSG code 3947.
——- If your data is NOT in zone 6 (blue)
——– Data are in Lambert RGF 93, code EPSG 21546.
—– X coordinates between 1,250,000 and 2,500,000
—— See on the map N ° 2
——- If your data is in zone 6 (blue)
——– The data is in Lambert CC47 RGF, code EPSG 3947.
Y coordinates between 7,000,000 and 8,000,000
—– X coordinates between 1,000,000 and 2,500,000
—— See on the map N ° 2
——- If your data is in zone 7 (green)
——– Data are in Lambert CC48 RGF, code EPSG 3948.
Y coordinates between 8,000,000 and 9,000,000
—– X coordinates between 1,000,000 and 2,500,000
—— See on the map N ° 2
——- If your data is in zone 8 (blue)
——– Data are in Lambert CC49 RGF, code EPSG 3949.
Y coordinates between 9,000,000 and 10,000,000
—– X coordinates between 1,000,000 and 2,500,000
—— See on the map N ° 2
——- If your data is in zone 9 (green)
——– The data are in Lambert CC50 RGF, code EPSG 3950.
If you follow the procedure to find the coordinate system for QGis, it’s time to go back to the article How to find the coordinate system of a layer in QGis?by clicking here
Find the geodetic system (DATUM)
Once you have overcome the first two stages, you have one last point to determine. Any location system necessarily refers to a centre of the Earth. As the Earth is not a perfect sphere, and it is necessary to calculate its centre,there were several ways of calculating 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 difference in positioning being generally less than 300m.
In theory, a coordinate system is always associated with a geodetic system.
The Lambert 1,2,3,4 and extended 2 projections are always associated with the NTF system (New French triangulation)
The Lambert 93 and CC42 to 50 projections are, always, associated with the RGF system.
So, if in the previous step you managed to define one of these projections, the work is finished.
For the UTM 30 to 32 projections, theoretically the associated system is the WGS84 system. But they 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 they can also be found associated with the NTF or Europe 50 system.
How to find out?
It is necessary to have a reference data layer, with the projection system correctly defined and above all, having a high precision (detail).
In a new project in ArcMap, load this reference layer.
Open the layer properties -> Source and note the geodetic system (DATUM) of this layer: you will find it at the very bottom of the Data Source window.
In ArcCatalog, define the coordinate system of the unknown layer as determined in the previous steps, using the most common geodetic system: WGS84
Load the layer in ArcMap.
If you have no message,
-and the data appears in the right place and there is no slight shift (100-300m), you’re done. The definition you have adopted is the right one.
-and 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. They must be in Europe 50 or NTF. On the other hand, your reference data is inWGS84, if this is not the case, there would be a message.
If your entities are shifted downwards (SW) by about 230m, your layer is in NTF.
If your entities are shifted upward (NE) by about 130m, then your layer is in Europe 50.
Remove the ArcMap layer. In ArcCatalog go to Layer Properties and XY Coordinate System, double click on the coordinate system. The system edit window opens. Click edit and go to ” Geographic Coordinate Systems »->« Europe and click on either NTF (Paris) or Europe Datum 1950.
If you have a warning message,
This is because your reference layer is not in WGS84. It is necessary to configure the transformations; otherwise, even if everything seems correct, in fact, it will be false.
Click on the transformations button, open the window «Using» and select
NTF_Paris_To_WGS_1984 if you are in NTF
