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Introduction to Remote Sensing

Objective: The objective of this tutorial is to show you

1) How to create NDVI images to map the amount of green vegetation in an area

2) How to conduct a supervised classification to map land cover

Note: This tutorial is written for PCs running Windows 7, so some parts of the tutorial might not work for

you if you are using a Mac or running a different version of Windows. You will be using MultiSpec for

this tutorial. You can access MultiSpec by double-clicking the program shortcut on the desktop of any

MOSS computer. If you are using your own computer, you can download MultiSpec for free from the

Internet at: https://engineering.purdue.edu/~biehl/MultiSpec.

The Normalized Difference Vegetation Index (NDVI)

STEP 1: Basics

The Normalized Difference Vegetation Index (NDVI) can be used to map the amount of green vegetation

in an area. The ability of NDVI to measure the amount of green vegetation is based on the principle that

green vegetation absorbs radiation in the visible region of the electromagnetic spectrum to use it for

photosynthesis – a process during which light energy is converted into chemical energy. In contrast to

visible light, near infrared (NIR) light is not used for photosynthesis and is strongly reflected by the plant.

The NDVI is typically calculated as follows:

NDVI = (NIR – R)/(NIR + R)

where R is the reflectance in the red region of the electromagnetic spectrum and NIR is the reflectance in

the near infrared spectral region. Use Fig. 1 below to calculate NDVI of the soil, 1 leaf layer, and 2 leaf

layers. Write the results of your calculations in the space provided below. For this exercise, let’s assume

that red (R) is at 680 nanometers (nm) and NIR at 800 nm. You should be able to get all the reflectance

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values you need for your calculations from Fig. 1. Please feel free to use the calculator on your computer

for calculating NDVI.

Figure 1. Spectral response curve of soil, 1 leaf layer, and 2 leaf layers.

NDVI (Soil): _________________________________________________

NDVI (1 leaf layer): _________________________________________________

NDVI (2 leaf layers): _________________________________________________

Which of the above has the lowest NDVI value and which has the highest NDVI value? Why do you

think that is? Please explain?

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Based on what you learned above, which of the below NDVI values is associated with most vegetation.

Please circle the right answer.

o NDVI 0.71

o NDVI 0.46

o NDVI 0.70

o NDVI 0.75

Next, we will use an aerial image to create an NDVI map of Ponderosa State Park.

Before we do that, use the Internet to find out what an orthoimage is. Write your answer in the space

provided below. Please make sure you use your own words rather than copying and pasting the exact

words you read.

What is an orthophoto:

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NOTE: For those of you following along at MOSS, please disregard the next section. For those of you

following along at home, you will need to complete the next section.

STEP 2: Download an aerial image of Ponderosa State Park from the Inside Idaho Website

Before you can create a NDVI map of Ponderosa State Park, you have to get an aerial image of the

park. To get aerial imagery for the entire State of Idaho, you can go to the Inside Idaho website

(http://inside.uidaho.edu). On the website, go to Find Data/Popular Data.

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Next, click on the 2011 NAIP link that will allow you to download digital orthoimagery.

On the next site, click on the Online Map link. This will bring you to a map. Zoom into the McCall

area so you can see Ponderosa State Park. Place your mouse cursor somewhere within the Ponderosa

State Park and double click. This will highlight the park area and some of the surrounding area in

yellow and a Tile Information window will pop up. Within the Tile Information window, click on

Download Data and save the orthoimagery somewhere on your computer. Please be advised that the

download can take upwards of 20 minutes due to the size of the file.

After your download is complete, navigate to the location where you saved the file. Since the image

data is contained within a zip file, you have to unzip it in order to access the data. To unzip the file,

simply double click on it. This action should unzip the files into a new folder with the same name as

the original .zip file. You should now be set to go ahead with step 3.

STEP 3: Create NDVI map from aerial digital orthoimage

This step will provide us with a visual of the amount of green vegetation in the entire image. To create an

NDVI image from the aerial image, we will be using the remote sensing software package MultiSpec. To

open MultiSpec on the MOSS computers, double-click on the desktop shortcut MultiSpecW32

. To open

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the digital orthoimage of Ponderosa State Park in MultiSpec, go to File/Open Image … on the main menu

bar and navigate to the folder where you saved your image (C:\Users\moss\My Documents\GIS

Files\image for those of you at MOSS) and open it. Once you click on Open, the Set Display

Specifications window will appear. Change the channels from the default (Red: 4; Green: 3; Blue: 2) to

the following values (Red: 3; Green: 2; Blue: 1):

This assigns the red color gun of your monitor the image color red, the green color gun of your monitor to

the image color green, and the blue color gun of your monitor to the image color blue. This way, you see

the image in its natural colors. If you do not change the values, you will see the image in false colors (aka:

False color image). After you click OK, another window pops up. Simply click OK. You should now see

an orthoimage of Ponderosa State Park.

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Next, from the main menu bar, select Processor/Reformat and select Change Image File Format.

When the Image File Format Change Options window pops up, check the Transform Data box on the left

side. The next window on the bottom of the above picture will pop up. Select the New Channel from

General Algebraic Transformation bubble. Plug in the below equation. This equation represents the

NDVI algorithm. We need to subtract channel 3(red) from channel 4 (NIR)/channel 4 + channel 1. Use

“C4” to represent channel 4 and “C3” for channel 3.

After your new equation resembles the one above, click OK. The “Image File Format Change Options”

screen will open again. VERY IMPORTANT: Change Header to GeoTIFF format and change Data value

type to 32-bit Real.

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A Save New Image File window will appear. Call the image NDVI_image and save it somewhere on your

hard-drive where you can find it later on. (Note: If you are using a MOSS computer, save it under

C:\Users\moss\My Documents\GIS Files\Student Work).

To open the NDVI image you just created, go to File/Open Image on the MultiSpec menu bar and

navigate to the location where you saved the NDVI image. In order for your image to be appear you may

have to select All Files from the Files of type dropdown menu.

Once you have selected your NDVI image and clicked Open, the Display specifications window

will appear. Make sure to change the Type to 1-Channel Thematic.

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Click OK. Another window will appear. Simply click OK again and a default color scheme will appear

that may look something like the image shown below:

The legend in the upper left will give you an accurate portrayal of the amount of green vegetation within

Ponderosa State Park (from -1 to 1).

Based on your knowledge of Ponderosa State Park, please discuss how well NDVI mapped the

amount of green vegetation in park. Also, please discuss how water managers could use the NDVI map

you created. Please provide your answer in the space provided below:

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STEP 4: Import your NDVI map into ArcGIS Explorer.

To open your NDVI image in ArcGIS Explorer, you have to save your NDVI image as a GeoTIFF image

in MultiSpec. To do this, go to File/Save Image to GeoTIFF As…. Call the image NDVI_image_arc and

save it somewhere on your hard-drive where you can find it later on. (Note: If you are using a MOSS

computer, please save it under C:\Users\moss\My Documents\GIS Files\Student Work).

After you saved the image as a GEOTIFF image in MultiSpec, open ArcGIS Explorer and select Add

Content/Raster data … from the home menu bar. Now, simply navigate to the location where you stored

your NDVI_image_arc and open. Voilà – you should now see the NDVI images you created in ArcGIS

Explorer.

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If you want to see the true color image underneath your NDVI image, for example to help you interpret

the NDVI values, you can use the swipe tool in ArcGIS Explorer. Simply select the Swipe tool under the

Tool tab menu bar.

At the end of this part of the tutorial, go to the following website: http://www.esri.com/landsat-

imagery/viewer.html. This website allows you to display NDVI increases or decreases for a specified area

over a specified period of time. In the search window, simply type in McCall, ID and hit enter.

You will now see the NDVI change between 1975 and 2000. In general is there an increase or decrease in

NDVI? What does that mean for groundwater recharge? Do you think it increased or decreased between

1975 and 2000? Please provide your answer in the space provided below:

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Supervised classification

In this part of the tutorial, you learn how to conduct a supervised classification. Supervised classification

is used by Remote Sensing scientists to map different land cover classes such as vegetation, soil, water,

and residential. It will be your task to map land cover classes within the orthoimage of Ponderosa State

Park you downloaded earlier. For this, open the orthoimage you downloaded from the Inside Idaho

website in MultiSpec. If you cannot remember how to do this, please refer to step 2 in this tutorial. Now

we are ready to do some supervised classification. In order to conduct a supervised classification in

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MultiSpec, we have to “train” the computer so the computer learns on a subset of pixels which pixels are

associated with which land-cover class such as vegetation, soil, residential, or water. First, let’s zoom into

the image by selecting the button from the MultiSpec menu bar (the allows you to zoom out

again). This allows us to better discern the different cover classes. To train the computer, we first have to

select Processor/Statistics from the main menu bar in MulitSpec. The Set Project Options window will

appear. Select OK. Next, the Project window will appear. Check the Polygon Enter box.

Now we are ready to trace areas on the image with the cursor that are associated with a given land-cover

class such as vegetation. Let’s start with vegetation and trace an area that only covered by vegetation.

After you are done tracing the area, double click and select the Add to list… box.

Next, enter the class name Vegetation and click OK.

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Repeat the last step for at least 5 more vegetated areas. VERY IMPORTANT: When the Define Class

and/or Field Description window appears, you do not have to type in the class name again - simply select

Vegetation from the class pull down menu as shown below:

After you created at least five training areas for vegetation, create at least five for the land-cover soil,

water, and residential. To create training areas for the other cover types, simply draw a polygon around

the next land-cover type you want to map such as soil. After you click on Add To List … , select ‘New’

from the Class pull-down menu as shown below:

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… and enter the new class name such as ‘Soil’ into the space provided (see below).

Now, the new class will be part of the Class pull down menu the next time you create a training area.

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After you created at least five training areas for the four land-cover types (vegetation, soil, residential, and

water), it is time to classify the image. To do this, select Processor/Classify from the MultiSpec menu bar.

The following window will appear:

VERY IMPORTANT: Check the Disk file box and select GeoTIFF from the pull down menu. Next, click

OK. Click also OK in the MultiSpec Windows Application window that appears.

MultiSpec will now classify your image based on the training data information you provided. After

MultiSpec is done with its classification, it asks you where to save your classified image. Name your

image landcover and save it somewhere on your hard drive where you can find it again later on (Note: If

you are using a MOSS computer, please save it under C:\Users\moss\My Documents\GIS Files\Student

Work).

Next, go to File/Open Image… on the MultiSpec menu bar and open your landcover image. It might look

something like the image below:

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The text output box on our screen will tell you what percentage of the image is vegetation, soil,

residential, and water.

Based on your knowledge of the McCall area, please discuss how well the supervised classification

mapped the different land-cover classes. Also, please discuss based on the land-cover map you created

where you would expect to see high runoff vs. low runoff and high groundwater recharge vs. low

groundwater recharge. Please provide your answer in the space provided below:

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Finally, open your supervised image in ArcGIS Explorer. If you cannot remember how to do this, please

refer to step 4 above when we opened the NDVI image in ArcGIS Explorer.

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Helpful resources for using MultiSpec

Tutorials from Purdue University:

https://engineering.purdue.edu/~biehl/MultiSpec/tutorials.html

Disclaimer: Please e-mail any suggestions of how to potentially improve this document to Jan Eitel

(jeitel@ uidaho.edu). Use of trade names does not constitute an official endorsement by the McCall

Outdoor Science School.

Important: If you used a MOSS computer for this tutorial, please make sure you delete all the files you

created from the computer after you are done with the tutorial. Thanks!