When we think of accessible data to integrate into our GIS, we hardly think of satellite data. And yet, it is possible, and just a few clicks away.
Let’s see an example. The following images represent the state of the vegetation for the islands of the Gulf of Morbihan, on the left for July 2014, on the right for March 2015. If we wish, we can obtain images every 15 days, the most just one week old.
Findin, recovering and processing them in just 5 minutes. So, why deprive yourself?
Among the available applications of satellite imagery, we will focus on the use of the normalized difference vegetation index (NDVI)
What is NDVI?
The normalized difference vegetation index, also called NDVI, is calculated using the visible red (R) and the near-infrared (IR) channels.
The normalized vegetation index highlights the difference between the visible band of red and the near infrared.
NDVI = (NIR-R) / NIR + R)
This index is sensitive to the quality (health) and quantity of the vegetation.
NDVI values range from -1 to +1, with negative values for areas other than plant covers, such as snow, water, or clouds, where red reflectance is greater than near infrared.
In the case of little or no vegetation, and, due to similar values of reflectance in both the red and infrared channels, the NDVI yields values close to 0.
Vegetation yields NDVI values, generally between 0.1 and 0.7. The highest values correspond to the densest cover.
When we observe a time series of images for a given area, we can deduce the state of stress of the vegetation by the relative variations of the index: in winter or during a drought the values will be lower than in summer or during abundance of water.
How do you calculate the NDVI?
According to the formula we need an image with a red channel and an infra-red channel. In the case of aerial photos, only the red channel is present. It is not possible to calculate the NDVI with a classic aerial photo.
You need a satellite (or overhead) image with more than three channels (bands). This is the case of Landsat (7 or 8 images), which have the advantage of being made available free by the USGS.
Once downloaded, the images appear as tiff files, one per band. Landsat 7 images has 8 channels or bands, Landsat 8 images has 11 channels.
If you combine the first three Landsat 7 bands, you get a classical optical image (RGB). To calculate the NDVI it is necessary to include the near infra-red band. You will then work with images having 4 bands, with a computer screen that can handle only three (usually RGB).
Depending on the software, you will have different ways of calculating the NDVI, and then, finally calculated, you will have to apply a color gradient that allows you to visualize the vegetation and its state of stress.
We will see, in order:
- how to download Landsat images
- how to calculate the NDVI (with arcgis and arcgis pro, with QGis)
- how to apply an appropriate symbology
How to download Landsat images
The USGS Earth Explorer Image Server can be found at http://earthexplorer.usgs.gov/
You can research the catalog, but if you want to download images you will need to login. Start by registering on the site by clicking “Register”. It’s free, easy and it only takes two minutes . You will not have to deal with ads or messages back.
Second step: define the desired period.
In the “Data Range” block, set a start date and an end date for the search.
Once these two parameters are configured, click on “DATA SETS”
The window on the left scrolls all available satellite data sources on Earth Explorer. In our case, we click on Landsat Archive-> L8 OLI / TIRS and L7 ETM + SLC off.
The different denominations correspond to periods in which satellite sensors have, or have not, operated. You do not have to be aware of all these details, because when the server detects a match between the type of image requested and the desired period, it displays a window with explanatory text and a warning of the possible problems.
What you need to know is that for Landsat 7 you will have, since 2003, on the margins of the images, black bands due to a satellite problem. If your area is in the centre band of the image, do not worry. If not, it is better to try Landsat 8.
Once ticked the right boxes, click on “Results”
At the top of the “Data Set” block you have a drop-down window with the different types of images checked previously. The thumbnails displayed only correspond to the satellite displayed in this window. To see the other images found, you must select another “data set” in the drop-down window.
By clicking on a thumbnail you can open a preview window that will allow you to see the correspondence with your work area as well as the cloud cover.
If you want to download an image, you click on the “Download options” button on the button bar of the desired image.
If you are not logged in, the window offers to connect. When connected, the product type selection window appears.
Select “Level 1 Product” to download all the bands in the image.
Once the zipped file has been downloaded, unzip it to a directory. You will get a tif image per satellite band.
That’s it! You have the satellite image on your PC. Now it will be necessary to use the data in order to display the famous NDVI of your zone of interest.
Depending on the software you are using, it’s not quite the same operations. We will see in the next article how to proceed with this work with ArcGis (ArcMap or ArcGis Pro) and in the following using QGis.