In a previous
article, How to use Landsat images (
free ) in your GIS , we discuss how
to download Landsat 7 or 8 images. Now we will discuss
how to use them with ArcGis, when you use ArcMap or ArcGis Pro.
We left at the point where you had , in a
directory
, a series of 8 or 11
tiff images. Each of
them corresponds to a satellite band .
Landsat 7 images have 8 bands
while the Landsat 8 have 11.
For the calculation of the NDVI
you
will
need
the band corresponding to the red and the
near infra-red:
For Landsat 7 the red band is N ° 3 and the
infra- red is N ° 4
For Landsat 8 the
red is N ° 4 and the infra- red is N ° 5
We could calculate directly
the NDVI by entering its formula in the raster calculator , by
using the two corresponding tiff images . But you
ArcGis allows you to, directly, calculate NDVI on an image,
provided it contains the necessary bands and, in general
, we, also, wish to” see ” the optical
image (RGB) and not only the NDVI calculation.
How to create
a composite image from the different
bands
Therefore we will start by creating
a
composite image containing the three bands of the visible spectrum (RGB) and
the band corresponding to the infra -red.
Each of these four bands
are
in a separate image, we will use a
toolbox
tool to create a single composite image.
You will find the command “Composite channels”
in the toolbox “Data Management” -> Raster -> Raster Processing
If you use ArcGis Pro, the same
tool
is
available
in
the Geoprocessing window .
Select the four bands
whose
names end with B1, B2, B3 and B4.
Using
the arrows change the
order of the images to match the order of the optical images (RGB- Ir), ie B3
first, then B2, and B1 finally B4.
If you do not save the image in
a
geodatabase, do not forget to add the .tif extension to the
output name.
The result of the command is loaded in the ArcMap
window .
How to calculate and display the NDVI
To calculate the NDVI index , just open the ArcMap Image Analysis window:
Before proceeding with the calculation , the bands to be used have to be
configured.
- Click the “Options” icon at the left top.
- In the NDVI index tab enter 1 as the red channel and 4 as the
infra-red channel. - Check the Scientific Output tab to get the results between -1 and 1. If not, you will have the NDVI between 0 and 255.
Close the window options .
The display of the NDVI index is performed by clicking on the fourth treatment
block icon (the green Maple leaf ).
How to create a customized symbology for the NDVI display
The default symbology is in shades of gray . To apply a more “standard” symbology you have to create a color gradient in ArcMap. This “standard” symbology depicts the negative values of NDVI in shades of blue and the positive values in a red to green gradient with yellow.
This operation only has to be performed the first time .
To create the customized gradient:
In the ArcMap menu, click Customize -> Styles Manager
Open the personal styles folder (take care not to open the ESRI folder )
Select “color gradients”
Right-click the right window
Select “New” from the menu -> Blend multipart colors
We are going to create two sections for the gradient : the first for the blue
gradients, the second for the green to red gradient.
Click on “Add”
Then select ” Algorithmic Colors Gradient ”
By default, a shading is added to the window . Click on ” Properties “. In ” Color 1 select a dark blue . In color 2 select a light blue in the same column only as the dark blue.
Click again on ” Add ”
Then select ” Algorithmic colors gradient ”
Click on ” Properties “. In ” Color 1 select a bright green . In color 2 select the red in the same line as green. The yellow should be on this line .
Click OK and OK
Rename
the “NDVI” gradient
We are done! The gradient will
appear in the gradient dropdown window of the ArcMap layer
symbology .
