IGNF register and shift grid NTF-RGF93/fr - Revision history

⚠️Thibault.lemaitre: update : updating discussion link

2011-01-03T11:56:37Z

update : updating discussion link

← Older revision		Revision as of 11:56, 3 January 2011
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	Pour être sur que votre localisation est correctement définie, répétez les étapes de la section [[#Description_du_problème\|Description du problème]].		Pour être sur que votre localisation est correctement définie, répétez les étapes de la section [[#Description_du_problème\|Description du problème]].
	*Si la différence est inférieure à 1 cm, c'est bon.		*Si la différence est inférieure à 1 cm, c'est bon.
	*Autrement, cela signifie que la grille de transformation n'est pas bien prise en compte. Utilisez s'il vous plaît [[Talk:IGNF_register_and_shift_grid_NTF-RGF93\|l'onglet discussion de cette page]] pour signaler vos problèmes.		*Autrement, cela signifie que la grille de transformation n'est pas bien prise en compte. Utilisez s'il vous plaît [[Talk:IGNF_register_and_shift_grid_NTF-RGF93/fr\|l'onglet discussion de cette page]] pour signaler vos problèmes.

⚠️Thibault.lemaitre: update : end of the translation

2011-01-03T11:53:14Z

update : end of the translation

← Older revision		Revision as of 11:53, 3 January 2011
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	~~{{Trans\|French\|English}}~~
	~~[[Category:Tutorial]]~~
	~~[[Category:Languages/fr]]~~

	Cette page décrit comment correctement utiliser les systèmes de projections définis par l'[http://fr.wikipedia.org/wiki/Institut_g%C3%A9ographique_national_%28France%29 Institut géographique national français] avec GRASS.		Cette page décrit comment correctement utiliser les systèmes de projections définis par l'[http://fr.wikipedia.org/wiki/Institut_g%C3%A9ographique_national_%28France%29 Institut géographique national français] avec GRASS.

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	</blockquote>		</blockquote>

	Pour être sur que votre localisation est correctement définie, répétez les étapes de la section [[#~~Description_du_problême]].~~		Pour être sur que votre localisation est correctement définie, répétez les étapes de la section [[#Description_du_problème\|Description du problème]].
	~~To be sure your location is well defined, repeat the step of the section [[#Problem_description~~\|~~Problem description~~]].		*Si la différence est inférieure à 1 cm, c'est bon.
	*~~If you have a difference lower than~~ 1 cm, it'~~s ok~~		*Autrement, cela signifie que la grille de transformation n'est pas bien prise en compte. Utilisez s'il vous plaît [[Talk:IGNF_register_and_shift_grid_NTF-RGF93\|l'onglet discussion de cette page]] pour signaler vos problèmes.
	*~~Otherwise~~, ~~it means the shift grid is not well taken in account~~. ~~Please use the~~ [[Talk:IGNF_register_and_shift_grid_NTF-RGF93\|~~Discussion tab of this~~ page]].


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	[http://www.epsg-registry.org/ http://www.epsg-registry.org/]		[http://www.epsg-registry.org/ http://www.epsg-registry.org/]

	[http://lambert93.ign.fr http://lambert93.ign.fr (fr)] : site ~~on the new French geodetic system~~ RGF		[http://lambert93.ign.fr http://lambert93.ign.fr (fr)] : site de l'IGN sur le nouveau système géodésique RGF

			[[Category:Tutorial]]
			[[Category:Languages/fr]]

⚠️Thibault.lemaitre: update : progression of the translation

2011-01-03T11:36:01Z

update : progression of the translation

← Older revision		Revision as of 11:36, 3 January 2011
Line 5:		Line 5:
	Cette page décrit comment correctement utiliser les systèmes de projections définis par l'[http://fr.wikipedia.org/wiki/Institut_g%C3%A9ographique_national_%28France%29 Institut géographique national français] avec GRASS.		Cette page décrit comment correctement utiliser les systèmes de projections définis par l'[http://fr.wikipedia.org/wiki/Institut_g%C3%A9ographique_national_%28France%29 Institut géographique national français] avec GRASS.

	L'IGN définit les projections utilisées en France et fournit le registre IGNF, qui définit celles-ci en détails. Il fournit de plus une grille pour les ~~transformations~~ entre les deux systèmes géodésiques français que sont le NTF et le RGF93. Les projections définies par les codes EPSG ne sont pas exactement les mêmes que celles définies par l'IGN. Les différences rencontrées allant de 2 cm à 5 m, avec une valeur moyenne de 1,3 m.		L'IGN définit les projections utilisées en France et fournit le registre IGNF, qui définit celles-ci en détails. Il fournit de plus une grille de transformation pour les changements entre les deux systèmes géodésiques français que sont le NTF et le RGF93. Les projections définies par les codes EPSG ne sont pas exactement les mêmes que celles définies par l'IGN. Les différences rencontrées allant de 2 cm à 5 m, avec une valeur moyenne de 1,3 m.


	== ~~Problem description~~ ==		== Description du problème ==
	~~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 previously introduced~~.		Le logiciel [http://geodesie.ign.fr/index.php?page=circe Circé] est considéré comme une référence pour ce qui est de la transformation entre les différents systèmes de projection français. Nous allons l'utiliser pour illustrer le problème décrit en introduction.

	~~The~~ Linux ~~users can install and use~~ Circé ~~with~~ [http://en.wikipedia.org/wiki/Wine_%28software%29 Wine].		Les utilisateurs de Linux pourront installer et utiliser Circé à l'aide de [http://en.wikipedia.org/wiki/Wine_%28software%29 Wine].

	#~~Define two~~ GRASS ~~locations with the~~ EPSG ~~code~~ 2154 (~~for the~~ projection RGF Lambert-93) ~~and~~ 27572 (~~for the~~ projection NTF Lambert 2 étendue).		#Définissez deux localisations GRASS ayant les codes EPSG 2154 (pour la projection RGF Lambert-93) et 27572 (pour la projection NTF Lambert 2 étendue).
	#~~Open~~ GRASS ~~in one of these locations and define some~~ points. ~~Project them to the other location with~~ GRASS.		#Ouvrez GRASS dans une de ces localisations et créez-y quelques points. Projetez ces points dans l'autre localisation à l'aide de GRASS.
	#~~Do the same conversion with~~ Circé.		#Effectuez la même transformation à l'aide de Circé.
	#~~Compare the results obtained with~~ GRASS ~~and~~ Circé : ~~you will see an abnormal difference~~ (~~between~~ 2 cm ~~and~~ 5 ~~meters for me~~).		#Comparez les résultats obtenus avec GRASS et Circé : vous constaterez alors une différence anormale (comprise entre 2 cm et 5 m pour moi).

	~~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.		L'exemple portait ici sur deux codes EPSG particuliers, mais ce problème a lieu chaque fois que vous devez effectuer une transformation entre les systèmes géodésiques NTF et RGF.

	~~There is then 2 cases~~ :		Vous vous trouvez dans l'un des deux cas suivants :
	*~~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 difference in each of your transformation~~.		*Si la différence évoquée précédemment est négligeable pour votre travail, vous pouvez continuer à utiliser les localisations telles qu'elles ont été définies précédemment. Cependant, n'oubliez pas que vous avez cette différence à chacune des transformations concernant la localisation NTF Lambert 2 étendue.
	*~~If this difference is too important for your application~~, ~~follow the next steps to obtain a precision~~ comparable to Circé.		*Si la différence est trop grande pour votre travail, suivez les étapes suivantes pour obtenir une précision comparable à Circé.


	== ~~Get the register and the shift grid~~ ==		== Obtenir le registre et la grille de transformation ==
	~~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.		Pour les utilisateurs de Linux, une fois GRASS installé, vous pouvez trouver le registre et la grille sur votre ordinateur. Le registre se nomme IGNF et se trouve dans le dossier /usr/share/proj/. La grille se trouve dans dans le même dossier et se nomme ntf_r93.gsb.

	~~For the other users~~, ~~or if you don't find the file on your computer~~, ~~you can 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~~.		Pour les autres utilisateurs, ou si vous ne trouvez pas les fichiers sur votre ordinateur, vous pouvez sauvegarder [http://lambert93.ign.fr/fileadmin/files/IGNF le registre] et [http://lambert93.ign.fr/fileadmin/files/ntf_r93.gsb la grille] sur votre ordinateur.


	== ~~Differences between~~ IGNF ~~register and~~ EPSG ~~code~~ ==		== Différences entre le registre IGNF et les codes EPSG ==
	~~Open the~~ IGNF ~~register with a text editor and search for~~ "<LAMBE>". ~~You will then read the informations about the previous quoted~~ projection NTF Lambert 2 étendue, ~~which as the~~ code LAMBE ~~in the~~ IGNF ~~register~~. ~~Below~~, ~~a copy of these informations~~.		Ouvrez le registre IGNF avec un éditeur de texte et rechercher "<LAMBE>". Vous pourrez alors lire les paramètres concernant la projection NTF Lambert 2 étendue précédemment citée, qui a le code LAMBE dans le registre IGNF. Ci-dessous, une copie de ces paramètres.

	<blockquote>		<blockquote>
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	</blockquote>		</blockquote>

	~~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~~.		Le texte en gras fait l'importante différence entre l'EPSG et l'IGNF. L'IGN recommande d'utiliser la grille de transformation que vous venez de télécharger, alors que l'EPSG non. Comme nous avons définis les localisations GRASS à l'aide des codes EPSG, ceci explique la différence que nous avons eu entre GRASS et Circé. Il en est de même pour toute projection utilisant le système géodésique NTF.

	~~If you look precisely~~, ~~there are also other differences between~~ IGNF ~~and~~ EPSG, ~~but they don't really have~~ importance.		Si vous regardez plus précisément, il y a d'autres différences entre l'IGNF et l'EPSG, mais elles ont une importance moindre.


	== ~~Define a~~ GRASS ~~location using the shift grid~~==		== Définir une localisation GRASS utilisant la grille de transformation ==
	~~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 ~~in the folder of the mapset PERMANENT~~.		Nous allons maintenant voir comment utiliser la grille de transformation de l'IGN pour une localisation GRASS choisie. Pour cela, nous allons placer la grille à un endroit permettant à GRASS de l'utiliser et modifier le fichier PROJ_INFO de la localisation concernée.

	~~First, you have to place 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~~ :		Vous devez tout d'abord copier la grille dans le dossier etc/nad de GRASS. Par exemple, pour la version 6.4 sur Linux, le dossier en question est /usr/lib/grass64/etc/nad/. Vous pouvez aussi créer un lien à la place d'une copie avec la commande suivante :
	<blockquote>		<blockquote>
	gksudo ln -s /usr/share/proj/ntf_r93.gsb /usr/lib/grass64/etc/nad/ntf_r93.gsb		gksudo ln -s /usr/share/proj/ntf_r93.gsb /usr/lib/grass64/etc/nad/ntf_r93.gsb
	</blockquote>		</blockquote>
	~~For the~~ Windows ~~users, that~~'~~s something like~~ C://Program Files/grass64/etc/nad/.		Notez que sur Linux vous devez avoir les droits d'administration pour faire cela.
			Pour les utilisateurs de Windows vous n'avez pas besoin d'être un administrateur et le dossier est quelque chose comme 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~~. ~~For the Linux users, add the following line at the end of the file~~ :		Si nous continuons avec la précédente projection NTF Lambert 2 étendue, qui correspond dans mon cas à la localisation GRASS NTF_L2E, le fichier PROJ_INFO se trouve dans le dossier grassdata/NTF_L2E/PERMANENT/. Ouvrez ce fichier avec un éditeur de texte. Ajoutez la ligne suivante à la fin de ce fichier :
	<blockquote>		<blockquote>
	nadgrids: ntf_r93.gsb		nadgrids: ntf_r93.gsb
	</blockquote>		</blockquote>

			Pour être sur que votre localisation est correctement définie, répétez les étapes de la section [[#Description_du_problême]].
	To be sure your location is well defined, repeat the step of the section [[#Problem_description\|Problem description]].		To be sure your location is well defined, repeat the step of the section [[#Problem_description\|Problem description]].
	*If you have a difference lower than 1 cm, it's ok		*If you have a difference lower than 1 cm, it's ok

⚠️Thibault.lemaitre: begin of the translation : introduction

2010-12-16T20:10:41Z

begin of the translation : introduction

← Older revision		Revision as of 20:10, 16 December 2010
Line 3:		Line 3:
	[[Category:Languages/fr]]		[[Category:Languages/fr]]

	~~This~~ page ~~describes how to well use the~~ projections ~~defined by the French company~~ [http://en.wikipedia.org/wiki/~~Institut_G~~%C3%~~A9ographique_National IGN~~] in GRASS.		Cette page décrit comment correctement utiliser les systèmes de projections définis par l'[http://fr.wikipedia.org/wiki/Institut_g%C3%A9ographique_national_%28France%29 Institut géographique national français] avec 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.		L'IGN définit les projections utilisées en France et fournit le registre IGNF, qui définit celles-ci en détails. Il fournit de plus une grille pour les transformations entre les deux systèmes géodésiques français que sont le NTF et le RGF93. Les projections définies par les codes EPSG ne sont pas exactement les mêmes que celles définies par l'IGN. Les différences rencontrées allant de 2 cm à 5 m, avec une valeur moyenne de 1,3 m.

⚠️Thibault.lemaitre: updating from the english version

2010-12-02T15:24:24Z

updating from the english version

← Older revision		Revision as of 15:24, 2 December 2010
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	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 in the folder of the mapset PERMANENT.		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 in the folder of the mapset PERMANENT.

	First, you have to place 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 ~~do this~~ with this command :		First, you have to place 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 :
	<blockquote>		<blockquote>
	gksudo ln -s /usr/share/proj/ntf_r93.gsb /usr/lib/grass64/etc/nad/ntf_r93.gsb		gksudo ln -s /usr/share/proj/ntf_r93.gsb /usr/lib/grass64/etc/nad/ntf_r93.gsb

⚠️Thibault.lemaitre: updating from the english version

2010-11-30T16:24:50Z

updating from the english version

← Older revision		Revision as of 16:24, 30 November 2010
Line 5:		Line 5:
	This page describes how to well use the projections defined by the French company [http://en.wikipedia.org/wiki/Institut_G%C3%A9ographique_National IGN] in GRASS.		This page describes how to well use the projections defined by the French company [http://en.wikipedia.org/wiki/Institut_G%C3%A9ographique_National 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. ~~If you use the EPSG codes instead~~ of ~~the IGN specifications you can encounter an error of more or less 2~~ m.		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.


Line 16:		Line 16:
	#Open GRASS in one of these locations and define some points. Project them to the other location with GRASS.		#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.		#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).		#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.		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 :		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 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 difference in each of your transformation.
	*If this difference is too important for your application, follow the next steps to obtain a precision comparable to Circé.		*If this difference is too important for your application, follow the next steps to obtain a precision comparable to Circé.

Line 44:		Line 44:

	== Define a GRASS location using the shift grid==		== 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 modify the file PROJ_INFO in the folder of the mapset PERMANENT.		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 in the folder of the mapset PERMANENT.

			First, you have to place 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 do this with this command :
			<blockquote>
			gksudo ln -s /usr/share/proj/ntf_r93.gsb /usr/lib/grass64/etc/nad/ntf_r93.gsb
			</blockquote>
			For the Windows users, that's 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. For the Linux users, add the following line at the end of the file :		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. For the Linux users, add the following line at the end of the file :
	<blockquote>		<blockquote>
	nadgrids: ~~/usr/share/proj/~~ntf_r93.gsb		nadgrids: ntf_r93.gsb
	</blockquote>		</blockquote>

	~~For the other users, place the shift grid file in a folder of your choice. You have then to change "/usr/share/proj/" by the path of the folder you have chosen.~~

	To be sure your location is well defined, repeat the step of the section [[#Problem_description\|Problem description]].		To be sure your location is well defined, repeat the step of the section [[#Problem_description\|Problem description]].
	*If you have a difference lower than 1 cm, it's ok		*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 ask the writer of this page~~		*Otherwise, it means the shift grid is not well taken in account. Please use the [[Talk:IGNF_register_and_shift_grid_NTF-RGF93\|Discussion tab of this page]].

⚠️Thibault.lemaitre: moved IGNF Register/fr to IGNF register and shift grid NTF-RGF93/fr: the same than for the English version

2010-11-30T11:01:13Z

moved IGNF Register/fr to IGNF register and shift grid NTF-RGF93/fr: the same than for the English version

← Older revision		Revision as of 11:01, 30 November 2010
(No difference)

⚠️Thibault.lemaitre: creation of the page, based on the english version

2010-11-30T10:54:36Z

creation of the page, based on the english version

New page

{{Trans|French|English}}
[[Category:Tutorial]]
[[Category:Languages/fr]]

This page describes how to well use the projections defined by the French company [http://en.wikipedia.org/wiki/Institut_G%C3%A9ographique_National 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. 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 previously introduced.

The Linux users can install and use Circé with [http://en.wikipedia.org/wiki/Wine_%28software%29 Wine].

#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 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 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 file on your computer, you can 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.

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

<blockquote>
<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 <>
</blockquote>

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 don't really have 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 modify the file PROJ_INFO in the folder of the mapset PERMANENT.

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. For the Linux users, add the following line at the end of the file :
<blockquote>
nadgrids: /usr/share/proj/ntf_r93.gsb
</blockquote>

For the other users, place the shift grid file in a folder of your choice. You have then to change "/usr/share/proj/" by the path of the folder you have chosen.

To be sure your location is well defined, repeat the step of the section [[#Problem_description|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 ask the writer of this page

== Links ==
[http://www.epsg-registry.org/ http://www.epsg-registry.org/]

[http://lambert93.ign.fr http://lambert93.ign.fr (fr)] : site on the new French geodetic system RGF