IGN wmts stream

The french IGN-API service offers a WMTS (OGC-compliant) stream for GIS-oriented data diffusion.

Connecting to this WMTS service requires a preliminary registration. See here for more information. This AddOn was written as a transitory solution to fetch french data within GRASS 6.4 where r.in.wms module fails. Note that this module was completely rewritten for GRASS 7.0 and now works with IGN web service, except at the present time it does not properly take into account french cartographic systems (nadgrid-based datum shifting). r.in.ign does, and works with GRASS 7.0 too.

Here we expose a simple how-to, hoping it will help users importing raster layers within GRASS GIS. It is based on the use of the r.in.ign AddOn.

Warning

r.in.ign lets you query a dataset via a contract. Beware the conditions that come with it and terms of use that apply to data... It is a quite rough module, true, but all this can be easily adapted to one's personnal needs.

Dependencies

Previously should be installed on your system:

• gdal-bin (GDALTools);
• PROJ4 and its binary tools;
• awk or gawk;
• wget;
• xmlstarlet for capabilities files analysis.

Moreover, as a reminder, your system should properly handle french cartographic projections. There should be somewhere in you PROJ system directory (usually /usr/share/proj/ on GNU/Linux OS) :

• the IGNF register, a text file called IGNF;
• the transformation grid called ntf_r93.gsb.

Learn more here for a fine tuning of this aspect.

Setting up a server connection

First thing you need is an access to the server :

• Create an account;
• And from your account page, you can create a new contract.

From the pulling list named "Type de clé" (Key type), choose "Sig" (GIS). Be cautious with special characters within your Login and Password ; r.in.ign may not correctly handle them...

Downloading the AddOn and running it

Retrieve r.in.ign, make sure the file is executable and located in a correct path directory (e.g. /usr/local/grass-addon).

You're done ! Launch GRASS in your favorite location/mapset. Define the region of interest, caring for its extent and resolution : Whether the current resolution is coarse or fine r.in.ign will request different levels of the server's tilematrix set. First run the command with -c argument to check available layers according to the contract you subscribed:

r.in.ign -c apikey=YourPersonalApiKey user=YourLogin password=YourPassword

The -m flag in the next example allows to output 3 separate-band rasters rather than a composite RGB raster:

r.in.ign -m apikey=YourPersonalApiKey user=YourLogin password=YourPassword output=orthoign wmtslayer=ORTHOIMAGERY.ORTHOPHOTOS

How it works

r.in.ign performs the following operations:

• detects the current location's projection system;
• turns the region extent into pseudo-Mercator projection system coordinates (epsg:3857);
• calculates the set of tiles to query (tilematrixset, rows & cols);
• retrieves tiles from the server;
• georeferences each tile;
• merges them all into a single image;
• then reprojects the latter in the target projection system.
• It finally runs r.in.gdal and optionnally r.composite.