IGNF register and shift grid NTF-RGF93: Difference between revisions
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This page describes how to well use the projections defined by the | This page describes how to well use the projections defined by the French company IGN [http://en.wikipedia.org/wiki/Institut_G%C3%A9ographique_National (en)] [http://fr.wikipedia.org/wiki/Institut_g%C3%A9ographique_national_%28France%29 (fr)] in GRASS. | ||
In fact the French company IGN manages the projections used in France and defined them by the IGNF register. The IGN also provides a shift grid for 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. If you use the EPSG codes instead of the IGN specifications you can encounter an error of more or less 2 m. | |||
== Problem description == | |||
The application [http://geodesie.ign.fr/index.php?page=circe Circé] is used as a reference concerning conversions between the French projection systems. We will use it to show the problem defined in the introduction. | |||
#Define two GRASS locations with the EPSG code 2154 (for the projection RGF Lambert-93) and 27572 (for the projection NTF Lambert 2 étendue). | |||
#Open GRASS in one of these locations and define some points. Project them to the other location with GRASS. | |||
#Do the same conversion with Circé. | |||
#Compare the results obtained with GRASS and Circé : you will see an abnormal difference (between -2 and +4 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 the 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 actually. However, don't forget that you have this error in each of your transformation. | |||
*If this difference is too important for your application, follow the next step to obtain a precision comparable to Circé. | |||
== Get the register and the 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 non Linux users, or if you don't find the file on your computer, you can download and save [http://lambert93.ign.fr/fileadmin/files/IGNF the register] and [http://lambert93.ign.fr/fileadmin/files/ntf_r93.gsb the grid] on your computer. | |||
== Define the GRASS locations == | |||
Revision as of 16:13, 16 November 2010
This page describes how to well use the projections defined by the French company IGN (en) (fr) in GRASS.
In fact the French company IGN manages the projections used in France and defined them by the IGNF register. The IGN also provides a shift grid for 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. If you use the EPSG codes instead of the IGN specifications you can encounter an error of more or less 2 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 defined in the introduction.
- Define two GRASS locations with the EPSG code 2154 (for the projection RGF Lambert-93) and 27572 (for the projection NTF Lambert 2 étendue).
- Open GRASS in one of these locations and define some points. Project them to the other location with GRASS.
- Do the same conversion with Circé.
- Compare the results obtained with GRASS and Circé : you will see an abnormal difference (between -2 and +4 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 the 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 actually. However, don't forget that you have this error in each of your transformation.
- If this difference is too important for your application, follow the next step to obtain a precision comparable to Circé.
Get the register and the 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 non Linux users, or if you don't find the file on your computer, you can download and save the register and the grid on your computer.