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Display the lake

To calculate the lake's change in area over time, you'll compare imagery of the lake taken by Landsat satellites between 1984 and 2014. The Landsat satellite program has been in operation over 40 years, making its imagery vital for monitoring major planetary changes. You'll classify the pixel values of the imagery into categories based on land cover. Then, you'll display only land cover of Lake Poyang, isolating the lake from the rest of the image.

Open the project

First, you'll download the project and open it in ArcGIS Pro.

  1. Go to the Classify Land Cover to Measure Shrinking Lakes group.

    The group has one item: a project package called Lake Poyang Project by Learn_ArcGIS.

  2. Click the thumbnail to download the project package.

    Lake Poyang Project thumbnail

  3. Save the downloaded project package in a location you can easily access and remember, such as your Documents folder or an external flash drive.
  4. Start ArcGIS Pro. If prompted, sign in using your licensed ArcGIS account.
    Note:

    If you don't have ArcGIS Pro or an ArcGIS account, you can sign up for an ArcGIS free trial.

    When you open ArcGIS Pro, you're given the option to create a new project or open an existing one. If you've created a project before, you'll see a list of recent projects.

  5. Click Open another project (if you've used ArcGIS Pro before) or Open an existing project (if you haven't.)

    Next, you'll search for the project you downloaded.

  6. Under Open, click Computer. Click Browse.

    Open project

  7. In the Open Project window, browse to the location where you downloaded the Lake_Poyang_Project.ppkx file. Double-click the package to open it.

    Default project

    The project opens to central-eastern China. Three layers (and the basemap) are turned on: the Three Gorges Dam, the Yangtze River, and Lake Poyang. Lake Poyang is several hundred miles downstream of the Three Gorges Dam.

Compare Lake Poyang over time

The project also contains three imagery layers, which are currently turned off. These layers show Lake Poyang at the peak of its rainy season during three different years: 1984, 2001, and 2014.

  1. In the Contents pane, right-click the Lake Poyang layer and choose Zoom To Layer.

    Lake Poyang

    Lake Poyang is mostly long and narrow, extending south from the Yangtze River. Its narrow shape means that even small shrinkages in lake surface area can lead to fragmentation of aquatic habitat. Additionally, several cities around the lake depend on the fishing and transport trades provided by the lake. On a national level, the lake is China's largest source of freshwater. The shrinking of the lake can have devastating ecological and economic ramifications.

  2. In the Contents pane, uncheck the box next to the Lake Poyang layer to turn it off.
  3. Check the box next to the June 1984.tif layer to turn it on. Pan and zoom the map so that the entire image is visible.

    Lake Poyang in 1984

    This image shows the lake in June 1984. The image was acquired by the Landsat 5 satellite. The Landsat satellite program is a joint initiative between two American government agencies: the United States Geological Survey and the National Aeronautics and Space Administration.

    The imagery shows a bright and clear distinction between the blue lake and the green vegetation nearby. Although these colors may seem natural, they are actually a combination of colors on the electromagnetic spectrum that are normally invisible to the human eye.

  4. In the Contents pane, click the arrow next to the June 1984.tif layer.

    Lake Poyang band combinations

    The colors on the electromagnetic spectrum, known as bands, are listed beneath the layer. Imagery is usually depicted with a combination of three bands, from which an RGB (red, green, blue) composite is created. This image uses Near Infrared 2 for the red color, Near Infrared 1 for the green color, and Red for the blue color. The band designations for various Landsat satellites are listed in the following table:

    Band Landsat 5 Landsat 7 Landsat 8

    1

    Blue

    Blue

    Coastal aerosol

    2

    Green

    Green

    Blue

    3

    Red

    Red

    Green

    4

    Near Infrared 1

    Near Infrared 1

    Red

    5

    Near Infrared 2

    Far Infrared

    Near Infrared

    6

    Thermal

    Thermal

    Far Infrared 1

    7

    Mid Infrared

    Far Infrared 2

    Far Infrared 2

    8

    Panchromatic

    Panchromatic

    9

    Cirrus

    10

    Thermal Infrared 1

    11

    Thermal Infrared 2

    A list of what each band shows best can be found in the lesson Assess Burn Scars with Satellite Imagery. All three images in your project use band combinations that emphasize vegetation, making the boundaries between the lake and the surrounding landscape more clear and distinct. Next, you'll compare the 1984 imagery to the later imagery to see how the lake has changed.

  5. Check the box next to the June 2001 layer to turn it on.

    Lake Poyang in 2001

    This image was taken by Landsat 7 instead of Landsat 5, so its colors are different. Without a side-by-side comparison, it's difficult to see exactly what has changed. You'll use the Swipe tool to compare the images side-by-side.

  6. In the Contents pane, click the June 2001.tif layer to select it.
  7. On the ribbon at the top of the application, click the Appearance tab. In the Effects group, click Swipe.

    Swipe tool

    When you move the pointer over the map, it changes to an arrow.

  8. Drag the map in the direction the arrow is pointing.

    Swipe on map

    The selected layer is hidden where you drag the cursor. You can now compare the two images. As you drag the Swipe tool back and forth (or up and down), you'll see that most of the change happens on the southern and eastern ends of the lake. Areas where the lake has receded are generally a dull orange, because there is no vegetation there. Overall, in 2001, the lake had visibly less surface area than in 1984. Both of these images were taken before the Three Gorges Dam's opening in 2008, so the cause of the lake's decline during this period may be due to drought or other meteorological trends.

  9. In the Contents pane, turn on the May 2014.tif layer.

    Lake Poyang in 2014

    This image was taken by Landsat 8. The orange areas that indicated bare earth from the receding lake in the 2001 image now show up as bright green due to vegetation growth, indicating long-term water level change.

  10. In the Contents pane, click the May 2014 layer to select it.
  11. Use the Swipe tool to compare the 2014 image to the 2001 image.

    The lake appears to have undergone additional surface area loss, mostly in the southern and western parts of the lake. Visually, it's clear that the lake has diminished between 1984 and 2014 (at least during the rainy season, when all three of these images were taken), although the exact amount is unknown.

Classify land cover in 1984

To quantify the change in lake surface area over time, you'll classify the land cover in each image. Every individual pixel, or cell, in an image has a value for each band. In Landsat imagery, these values correspond to colors. As you can see from the vibrant imagery of Lake Poyang, however, there are many possible color values for all varieties of shades and hues. By classifying the image, you'll group similar values together into a single value that represents a locational feature, or class, such as water, vegetation, or urban areas. You can use these classes to find the area of the feature you want (in this case, Lake Poyang).

You'll use a particular type of classification technique, called an unsupervised classification, in which the software uses statistical analysis to decide which values to group together into classes. You'll only need to specify how many classes there will be. To make this classification, you'll use the Iso Cluster Unsupervised Classification tool.

  1. On the ribbon, click the Analysis tab. In the Geoprocessing group, click Tools.

    Tools button

    The Geoprocessing pane opens.

  2. In the Geoprocessing pane, type Iso Cluster Unsupervised Classification in the search box. Click the result with the same name.

    Search for Iso Cluster tool

    The Iso Cluster Unsupervised Classification tool opens. This tool runs an unsupervised classification on the imagery, or raster, layer of your choosing. It uses the Iso Cluster algorithm to determine the characteristics of natural groupings of cells and creates an output layer based on the number of classes you want. You'll run the tool three times, once for each of the image layers in your map.

  3. In the Iso Cluster Unsupervised Classification tool parameters, for Input raster bands, choose June 1984.tif.
  4. For Number of classes, type 4.

    You really just want to see the lake, so there is no need for a large number of classes.

  5. Change the name of the Output classified raster to Iso_1984. Leave the other parameters unchanged.

    Paramaters of Iso Cluster tool

  6. Click Run.

    After the tool finishes, the output layer is added to the map. Your layer may have different colors than the example image.

    Classified 1984 image

    The new layer resembles the original June 1984 imagery, but the colors have changed and it looks much more pixelated. All image layers comprise grids of pixels, also known as cells, but in the original imagery the pixels had thousands of different colors. The Iso Cluster Unsupervised Classification tool took all the pixels in the original image and sorted them into four value classes, each with its own color (the output colors are randomly generated). All water values were classified into a single value, while the other values represent vegetation or cloud cover.

  7. In the Contents pane, for the Iso_1984 layer, right-click Value 1 and change the color to Yogo Blue.

    Yogo Blue color

  8. Change the other values (2, 3, and 4) to No color.

    Only the water value remains visible. You'll compare it to the original June 1984 image to make sure the classification is correct.

  9. In the Contents pane, turn off the May 2014.tif and June 2001.tif layers.

    Compare classification and imagery

  10. In the Contents pane, click the Iso_1984 layer to select it. Use the Swipe tool to compare the two 1984 layers.

    Although the lake boundaries mostly match up, the classified value also includes smaller bodies of water around the lake. You'll remove some of these smaller water bodies in the next lesson. There is also a portion of the lake that was not classified into the same value as the rest of the lake due to cloud cover.

    Clouds in image

    Clouds often obscure ground features in satellite imagery. The cloud cover in this image is relatively minor, so it won't have a major impact on the analysis, but the analysis could be improved by using imagery with even less cloud cover.

  11. Turn off the June 1984.tif layer.

Classify land cover in 2001 and 2014

Next, you'll classify the other two images to see how the lake has changed over time. The Geoprocessing pane is already open to the Iso Cluster Unsupervised Classification tool, so you'll only change the parameters before running the tool again.

  1. In the Geoprocessing pane, for Input raster bands, change June 1984.tif to June 2001.tif.
  2. For Output classified raster, change the output name to Iso_2001. Leave the other parameters unchanged.

    Parameters for Iso Cluster tool

  3. Click Run.

    Classified 2001 image

    As before, the water values were classified in Value 1 of the new layer.

  4. In the Contents pane, for the Iso_2001 layer, change the color of Value 1 to Light Apple.

    Light Apple color

  5. Change the other values (2, 3, and 4) to No color.

    Compare 2001 and 1984

    The visible blue areas indicate areas that were water in 1984 but not in 2001, more clearly showing the lake's reduction between the two years. Lastly, you'll classify the 2014 image.

  6. In the Geoprocessing pane, change the Input raster bands to May 2014.tif. Change the name of the Output classified raster to Iso_2014.
  7. Click Run.

    Classified 2014 image

  8. In the Contents pane, for the Iso_2014 layer, change the color of Value 1 to Mango.

    Mango color

  9. Change the other values (2, 3, and 4) to No color.

    Compare 2014 to 2001 and 1984

  10. On the ribbon, click the Project tab and click Save As. Save the project with the name Poyang Land Cover.

In this lesson, you visually compared imagery of the same lake from three different times spanning a period of 30 years, noticing a trend of water loss. You then classified each image to show land cover, creating a single value for water from the many water values in the original image. In the next lesson, you'll smooth out the lake boundaries and remove many of the smaller water features that were classified alongside Lake Poyang. You'll then calculate the area of the lake for each year and determine the rate at which the lake is decreasing.