Usually, all layers of geographic data have the

description of the coordinate system. Unfortunately,

this is not always the case. Generally, the problem increases by not being able

to contact the producer of the data.

**Problem diagnosis **

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.

be projected.

*“*

If it is a vector data layer (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 receiving

any warning or error message, you do not see your new data, or it is in a

different place from where it should be.

If it’s a vector data layer (points, lines, polygons),

the data has a defined coordinate system, but it’s false. The PRJ file exists, but its contents are wrong.

**Search procedure**

In this article we try to supply you a guide to

determine the projection system of a GIS data layer, when it is unknown.

We have limited ourselves to the most commonly used systems in France by the

various organizations and administrations: Lambert and Lambert93 projections,

UTM projections and non-projected data (geographical latitude / longitude).

Of course, there are many others, used punctually.

But it is impossible to determine them without a truly

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 assess the 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 the data with the unknown coordinate system.

The data must not have a defined coordinate system.

In the case of shapefiles, it 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 and examine the data range.

(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 “ ArcGis Geographic

Coordinate Systems”. It remains to find the data geodetic system (Datum). (See further)

If the coordinates shown in the Range are of the

order of hundreds of thousands or millions, they are meters. The data system will be searched in the section “ ArcGis

Projected coordinate systems “, and it will also remain to find the data geodesic system (Datum).

**How to find the
projection system**

Here you will find a logical progression based on

the value range of your layer. In addition to

these values, we will use the data area against 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 map N ° 1

——- If your data is in Lambert I zone

——– Data is in NTF Lambert I zone, EPSG code

27561.

——- If your data is in Lambert II zone

——– Data is in NTF Lambert II area, EPSG code

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

——– 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 map N ° 1

——- If your data is in Lambert I zone

——– Data is in NTF Lambert I carto, EPSG code

27571.

—– *X coordinates between 1,000,000 and 2,500,000*

—— See map N ° 2

——- If your data is in zone 1 (green)

——– Data is in Lambert CC42 RGF, code EPSG 3942.

**Y coordinates between 2,000,000 and 3,000,000**

—– *X coordinates between 0 and 1 100 000*

—— See 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

——– Data is in NTF Lambert II extended, EPSG code

27572.

—– *X coordinates between 1,000,000 and 2,500,000*

—— See map N ° 2

——- If your data is in zone 2 (blue)

——– 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 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 map N ° 2

——- If your data is in zone 3 (green)

——– 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 map N ° 1

——- If your data is in Lambert IV zone

——– 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 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

——– Data is in WGS84 UTM 32N, EPSG code 32632.

—– *X coordinates between 1,000,000 and 2,500,000*

—— See map N ° 2

——- If your data is in zone 4 (blue)

——– 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 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

——– Data is in WGS84 UTM 32N, EPSG code 32632.

—– *X coordinates between 1,000,000 and 2,500,000*

—— See map N ° 2

——- If your data is in zone 5 (green)

——– Data is 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 map N ° 2

——- If your data **IS** in zone 6 (blue)

——- There are two possibilities, you will have to

test them against a

——- known layer to determine which 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 is in Lambert RGF 93, code EPSG 21546.

—– *X coordinates between 1,250,000 and 2,500,000*

—— See map N ° 2

——- If your data is in zone 6 (blue)

——– 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 map N ° 2

——- If your data is in zone 7 (green)

——– Data is 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 map N ° 2

——- If your data is in zone 8 (blue)

——– Data is 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 map N ° 2

——- If your data is in zone 9 (green)

——– Data is in Lambert CC50 RGF, code EPSG 3950.

If you follow this 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

**How to find the
geodetic system (DATUM)**

Once you have gone through the first two stages, you

have one last point to determine. Any location

system necessarily refers to a center of the Earth. Since the Earth is not a perfect sphere, and that it is

necessary to calculate its center, there are several ways to calculate it and, therefore,

several different ” centers

“. The

difference is not huge and until a few decades ago it was a rather theoretical

discussion, the resulting positioning difference being generally less than

300m.

Technically, 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 (Nouvelle triangulation française)

The Lambert 93 and CC42 to 50 projections are still

associated with the RGF system.

So, if in the previous step you have come to define one

of these projections, the work is finished.

For UTM projections 30 to 32, in theory, the

associated system is the WGS84 systems. But they

can also be associated with the Europe 50 system.

In the case of geographical (non-projected) data, it

is, also generally, associated with the WGS84 system, but can also be found

associated with the NTF or Europe 50 system.

How to find out?

It is necessary to have a data reference layer, with

the projection system defined correctly and above all, having a good accuracy

(detail).

In a new project in ArcMap, load this reference layer.

Open the properties of the layer-> 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, set the unknown layer coordinate system

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. It must be in Europe 50 or

NTF.

On

the other hand your reference data is in WGS84, if no message appears.

If your entities are shifted downwards (SW) by about

230m, your layer is in NTF.

If your entities are shifted upwards (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,**

Your

reference layer is not in WGS84. It is necessary to configure the transformations, if not,

even if everything seems correct, in fact it will be wrong.

Click on the transformations button, open the window «

using » and

select **NTF_Paris_To_WGS_1984** if you are in NTF

or **ED_1950_To_WGS_1984_1** if you are in Europe 50

If the data appears in the right place and there is

no slight shift (100-300m), you are done. The

definition you adopted (WGS84) is the right one.

If you are in NTF and the data is shifted by about

230m, the unknown layer is also in NTF.

If you are in NTF and the data is shifted about 80m,

the unknown layer is in Europe 50.

If you are in Europe 50 and the data is shifted by

about 130m, the unknown layer is also in Europe 50.

If you are in Europe 50 and the data is shifted by

about 80m, the unknown layer is in NTF.