# IGNF register and shift grid NTF-RGF93

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This page describes how to well use the projections defined by the French company IGN in GRASS.

The French company IGN manages the projections used in France and provides the IGNF register, which defined them in details. The IGN also provides a shift grid for the transformations between the French geodetic systems NTF and RGF93. The French projections defined by the EPSG codes are not exactly the same than those defined by the IGN. The difference varies between 2 cm and 5 m, with a mean value of 1,3 m.

## Problem description

The application Circé is used as a reference concerning conversions between the French projection systems. We will use it to show the problem previously introduced.

The Linux users can install and use Circé with Wine.

1. Define two GRASS locations with the EPSG code 2154 (for the projection RGF Lambert-93) and 27572 (for the projection NTF Lambert 2 étendue).
2. Open GRASS in one of these locations and define some points. Project them to the other location with GRASS.
3. Do the same conversion with Circé.
4. Compare the results obtained with GRASS and Circé : you will see an abnormal difference (between 2 cm and 5 meters for me).

The example was here with two particular EPSG codes, but this problem occurs each time you have to do a transformation between the geodetic systems NTF and RGF.

There is then 2 cases :

• If the previous difference is negligible for your work, you can continue to use the previous defined locations as they are currently. However, don't forget that you have this difference in each of your transformation concerning the location NTF Lambert 2 étendue.
• If this difference is too important for your work, follow the next steps to obtain a precision comparable to Circé.

## Get the register and the shift grid

For the Linux users, once you have installed GRASS, you can find the register and the grid on your computer. The register is named IGNF and is in the folder /usr/share/proj/. The grid is in the same folder and is named ntf_r93.gsb.

For the other users, or if you don't find the files on your computer, you can save the register and the grid on your computer.

## Differences between IGNF register and the EPSG codes

Open the IGNF register with a text editor and search for "<LAMBE>". You will then read the parameters concerning the previous quoted projection NTF Lambert 2 étendue, which as the code LAMBE in the IGNF register. Below, a copy of these parameters.

<LAMBE> +title=Lambert II etendu +proj=lcc +nadgrids=ntf_r93.gsb,null +towgs84=-168.0000,-60.0000,320.0000 +a=6378249.2000 +rf=293.4660210000000 +pm=2.337229167 +lat_0=46.800000000 +lon_0=0.000000000 +k_0=0.99987742 +lat_1=46.800000000 +x_0=600000.000 +y_0=2200000.000 +units=m +no_defs <>

The bold text makes the important difference between the EPSG and the IGNF. The IGN recommend to use the shift grid previously downloaded, and the EPSG no. As we define our GRASS location with the EPSG codes, this explains the difference we encounter between GRASS and Circé. That's the same for each projection using the NTF geodetic system.

If you look precisely, there are also other differences between IGNF and EPSG, but they have lesser importance.

## Define a GRASS location using the shift grid

We will here show how to use the IGN shift grid for a specified GRASS location. For this, we will put the grid in a place where GRASS can use it and modify the file PROJ_INFO of the concerned location.

First, you have to copy the grid in the folder etc/nad/ of GRASS. For example, with the version 6.4 on Linux, the concerned folder is /usr/lib/grass64/etc/nad/. You can also create a link instead of a copy with this command :

gksudo ln -s /usr/share/proj/ntf_r93.gsb /usr/lib/grass64/etc/nad/ntf_r93.gsb

Note that you must have the administrator rights to do this on Linux. For the Windows users you don't need to be an administrator and the folder is something like C://Program Files/grass64/etc/nad/.

If we continue with the previous projection NTF Lambert 2 étendue, which corresponds with the GRASS location NTF_L2E in my case, the file PROJ_INFO is in the folder grassdata/NTF_L2E/PERMANENT/. Open this file with a text editor. Add the following line at the end of this file :

nadgrids: ntf_r93.gsb

To be sure your location is well defined, repeat the step of the section Problem description.

• If you have a difference lower than 1 cm, it's ok.
• Otherwise, it means the shift grid is not well taken in account. Please use the discussion tab of this page to report your problems.

## Links

http://lambert93.ign.fr (fr) : IGN's site on the new French geodetic system RGF