Accueil Géoportail > FAQ > Summary
> How are the maps made?
> How are the aerial photos made?
> When were the aerial photos taken?
> What is the “Maps” layer?
> What is the “Aerial photos” layer?
> What is the “Roads” layer?
> What is the “Relief” layer?
> What data can I view?
> What is the BD ORTHO®?
> What is an IGN SCAN?
> What can you make out at one pixel per 50 cm?
How are the maps made?
To find out more about maps, how they are made, obtained and produced, go to the IGN website.
> Institut Géographique National – (National Geographical Institute)
How are the aerial photos made?
To find out more about aerial photos, how they are made, obtained and produced, go to the IGN website.
> Institut Géographique National – (National Geographical Institute)
When were the aerial photos taken?
Les dates des prises de vues aériennes sont listées par département dans un tableau récapitulatif.
What is the “Maps” layer?
The maps come from the IGN ‘s SCAN database:
- SCAN 1 000®
- SCAN 500® (French Guyana)
- SCAN Régional®
- SCAN 200® (New Caledonia)
- SCAN Départemental®
- SCAN 100®
- SCAN 50®
- SCAN 25®
They let you view the digital version of the printed national IGN maps with which you are already familiar.
What is the “Aerial photos” layer?
The photo layer consists of orthophotos from the BD ORTHO® database.
It enables you to view France in orthocorrected aerial photographs.
Consult the glossary
> Orthocorrected aerial photos
What is the “Roads” layer?
The “Roads” layer consists of the GEOROUTE® database, rasterised and marketed by the IGN under the name GEOROUTE® Raster.
What is the “Relief” layer?
The "Relief" layer consists of a DTM (Digital Terrain Model) displayed in hypsographic tints sourced from BD ALTI®.
Consult the glossary
> Hypsographic shading
What data can I view?
Data available for viewing on Géoportail consist of the IGN’s "image" data, i.e.:
- scanned maps at various scales, ranging from national maps at a scale of 1 : 1 000 000, and regional and departmental maps, to walking and hiking maps at 1 : 50000 and 1 : 25 000.
- orthocorrected aerial photos (BD ORTHO®) at a resolution of one pixel per 50 cm.
To find out more, see the following FAQs:
> What is the "maps" layer?
> What is the "Photos" layer?
What is the BD ORTHO®?
This is a collection of digital aerial photographs taken by the IGN. These aerial photographs have been orthocorrected (so that they can be superimposed on a map). They are also geo-referenced, i.e. they include geographical coordinates (latitude, longitude, altitude).
> Find out more on the IGN website
What is an IGN SCAN?
This is a geo-referenced IGN digital map. A geo-referenced map has been processed by a given projection system so as to make it compatible with any geographical information contained in the system. It makes it possible to localise information very accurately, covers the entire French territory and is compatible with IGN vector data.
> Find out more on the IGN website
What can you make out at one pixel per 50 cm?
A digital image consists of pixels: these are basic squares (pixel = picture element). One pixel per 50 cm means that the average size on the ground of each element of the square is 50 cm by 50 cm, or 0,25 m². The colour of the pixel is the average of all the colours present in these 50 cm of reality.
Recognising a detail depends far more on its contrast with its surroundings than on its size. This means that very light-coloured objects much smaller than 50 cm can be seen against a dark background (e.g. the white lines on a tennis court against the clay’s deep red. Its linear shape also helps the eye).
On the other hand, a man-hole cover, measuring about 50 cm on the ground, is not easy to make out. For a detail to be recognised in an aerial photograph, it must be big enough for the picture’s resolution to allow the shape to be seen.