Part 1 :
DATA ACQUISITION
Table des matières
I- Vector data..............................................................................................................................3
II- Raster data...........................................................................................................................4
1- Sentinel data.......................................................................................................................4
User account creation..........................................................................................................4
Download the Raster...........................................................................................................8
2- ALOS PALSAR data..........................................................................................................9
User account creation..........................................................................................................9
Choice of image characteristics........................................................................................10
Download the Raster.........................................................................................................12
There are major types of data in Remote Sensing and Geographic Information Systems (GIS)
that are :
- Vector data and
- Raster data
I- Vector data
There are various types of vector data such as Excel files, a shapefile. These are data that can
be generated by the user from raster data by scanning an image data.
To download free Open Street Map data for a country, you must
- Go to https://download.geofabrik.de/
- Click on the name of the continent where it is located (Africa)
- - Click on "shp.zip" in front of the country name to download. In our case choose
Ivory Coat and Benin
-
II- Raster data
There is a large number of images from remote sensing satellites. In this diversity of image
data, it is recommended to choose the satellite according to the objectives that one wishes to
achieve. There are many sites that allow you to download satellite images for free. We can
quote https://earthexplorer.usgs.gov/ of the US Geological Survey
1- Sentinel data
The Sentinel satellites are part of the European Space Agency's Copernicus program that
began in 2014. These satellites are grouped into six families and vary from optical to radar.
Each family provides data for various types of land and ocean services..
User account creation
To download Sentinel 2 data, you need a Scihub platform user account. The account is free of
charge and the data is updated as soon as the satellite photo is received. If you already have an
account, go to the next section.
1/ Access the site https://scihub.copernicus.eu/
2/ Choose « Open Access Hub »
3/ Click on (top right), then Sign up
4/ Follow the on-screen instructions to complete the registration process.
5/ Once your registration is complete, a confirmation message is sent to your email address.
6/ Click on the link in the message to activate your account.
Choice of image characteristics
1/Click on (top left of the page)
There are several ways to filter the data in order to select the desired image.
The filters to remember are :
2/ Sort By: (Sensing date) allows you to sort the images by acquisition date
3/ Sensing period: allows you to filter the images by a given period
4/ Mission :
- Choose the data according to the satellite : Sentinel 1, 2, ...
- Check Mission : Sentinel 2
- Product type : allows to choose the processed or unprocessed images:
o 1C is an ortho-rectified image in TOA (Top-of-Atmosphere) reflectance with
a cloud mask
o 2A is an ortho-rectified image in BOA (Bottom-of-Atmosphere) reflectance.
For Sentinel 2, the Cloud Coverage allows to filter the images with too much cloud coverage.
5/ Spatial filter
Firts, zoom in on your study area first. It is activated by clicking on the map:
- - Hold down the right mouse button, then grid your study area
- or by activating the « Switch to mode area »( à droite), then draw your study
area with the left click, once the area is squared double click.
An orange square is displayed to show the chosen selection.
Click on to start the search .
The results are displayed both on the map and on the tableau
We can read the name of the image, which defines precisely the image
- S2B : Acquired by the Sentinel 2 B satellite
- MSIL2A: Acquired by the MSI instrument, with 2A processing to remove the
effects of the atmosphere.
- 20151230: The picture was taken on December 30, 2015
Download the Raster
Our study area straddles two scenes, so we need to download two images.
1/ 1/ Check that the images are not « Offline»
2/ Click on to check image details such as size and cloud cover
3/ If the image meets our expectations click on to download it.
2- ALOS PALSAR data
User account creation
1/ Access the site https://urs.earthdata.nasa.gov/users/new
2/Follow the instructions on the screen to complete the registration process.
3/ Once your registration is complete, a confirmation message is sent to your email address.
4/ Click on the link in the message to activate your account.
Choice of image characteristics
*1/ Access the site https://search.asf.alaska.edu/#/
2/ Click on (top right), then Sign up
3/ Scroll down the list under "Dataset" (top left) to choose the satellite whose data you wish
to acquire. Click on " ALOS PALSAR ".
4/ Zoom in on your study area
5/ Start drawing the study area with a left click and finish with a right click
6/ Click on Filters
Dans la section Additional Filters, cliquez sur File type et choisissez le type de fichier. Dans
notre cas nous rechercherons les Modèles numériques de terrain, cochez Hi-Res Terrain
Corrected puis Update
In the Additional Filters section, click on File type and choose the file type. In our case we
will look for Digital Terrain Models, check Hi-Res Terrain Corrected and then Update
Download the Raster
the lake Koosou is to checal between 6 scenes. So we need to download six images;
1/ allows you to zoom in on the scene
2/ Add an image of each scene to the basket by clicking on . Indeed it can have several
images for the same scene
3/ Click on and of each scene to start downloading the images.
