Introduction to Remote Sensing

Transcription

1 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: 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 1

2 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? 2

3 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. STEP 2: Download an aerial image of Ponderosa State Park from the Inside Idaho Website Before we can create a NDVI map of Ponderosa State Park, we 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 ( On the website, go to Find Data/Popular Data. 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 3

4 Tile Information window will pop up. Within the Tile Information window, click on Download Data and save the orthoimagery somewhere on your computer (Note: if you are using a MOSS computer, please save the image in C:/Users/moss/Downloads). The download will take a few minutes, so in the meantime, 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: After you answered the question above and the download is complete, find the file you just downloaded. Since the image data is within a zip file, we have to unzip it. To unzip it, create a folder called Imagery (e.g. C:/Users/moss/Downloads/Imagery). Next, double click on the zip file you just downloaded. A program called WinZip will automatically open and unzip your file. WinZip may prompt you to register. If it does, click Use Evaluation Version. Another box will appear and prompt you to check for updates. Simply click OK. Two more boxes may appear. If they do, simply click OK and continue with the tutorial. 4

5 Select all files within the zip file folder by pressing Ctrl+A, then drag the files to the Imagery folder you created earlier. 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 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 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): 5

6 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. Next, from the main menu bar, select Processor/Reformat and select Change Image File Format. 6

7 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. 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/Downloads/Image). 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. 7

8 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. 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: 8

9 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 the NDVI map you created could be used by water managers. Please provide your answer in the space provided below: 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/Downloads/Image). 9

10 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. Voilá you should now see the NDVI images you created in ArcGIS Explorer. 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. 10

11 At the end of this part of the tutorial, go to the following website: 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 to 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 to 2000? Please provide your answer in the space provided below: 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 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. 11

12 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. 12

13 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 pulldown menu as shown below: and enter the new class name such as Soil into the space provided (see below). 13

14 Now, the new class will be part of the Class pull down menu the next time you create a training area. 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: 14

15 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:/Trash/Image). Next, go to File/Open Image on the MultiSpec menu bar and open your landcover image. It might look something like the image below: The text output box on our screen will tell you what percentage of the image is vegetation, soil, residential, and water. 15

16 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: Finally, open your supervised image in ArcGIS Explorer. If you cannot remember how to do this, please refer to by step 4 above when we opened the NDVI image in ArcGIS Explorer. Helpful resources for using MultiSpec Tutorials from Purdue University: 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! Disclaimer: Please 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. 16