Visualizing Images and Image Bands

Now that you're ready to begin writing Earth Engine JavaScript, start by copying the following code into the Code Editor:

// Instantiate an image with the Image constructor.
var image = ee.Image('CGIAR/SRTM90_V4');

// Zoom to a location.
Map.setCenter(-112.8598, 36.2841, 9); // Center on the Grand Canyon.

// Display the image on the map.
Map.addLayer(image);

Click the Run button at the top of the Code Editor and observe that a very gray image appears on the map. Don't worry, you'll make it look better soon. If the syntax of any part of this example in unfamiliar, be sure to review the JavaScript for Earth Engine tutorial.

Image Constructor

The first new thing in this example is the image constructor ee.Image(). The argument provided to the constructor is the string ID of an image in the Earth Engine data catalog. (See the Docs tab, at the left of the Code Editor, to see a full list of possible arguments to the image constructor. The Docs tab represents the most up to date information about what Earth Engine can do.)

To discover an image ID, search in the Earth Engine data catalog using the search tool at the top of the Code Editor. For example, type 'elevation' into the search field and note that a list of rasters is returned. Click the 'SRTM Digital Elevation Data Version 4' entry to see more information about that dataset. On the right side of the dataset description is an Image ID field. Observe that the image ID in the example is copied from the image ID shown in the dataset description.

An alternative to copying and pasting image IDs is to use the Import button on the dataset description or the import link at the right side of the search results. If you click the import link or button, a variable is automatically created in a special section, named 'Imports', at the top of your script. You can rename the variable by clicking on its name in the imports section.

Configuring the Map

The second new part of this example is the Map.setCenter() call. This method on the Map object, which represents the Map display in the Code Editor, centers the map at the given longitude, latitude (in decimal degrees) and zoom level where 1 is zoomed out so that the map shows the entire Earth's surface. Larger numbers zoom in from there. Discover all the methods on the Map object by checking the Map section in the Docs tab on the left side of the Code Editor.

Adding a layer to the Map

The last line in the example says: use the Mapobject's addLayer() method to add an image to the map display in the Code Editor.

Congratulations! You've created your first Earth Engine script. In the next section, you'll learn how to make that image look a little better.

Digression: Images in Earth Engine

Images in Earth Engine (see this page for more details) are made up of one or more bands. Each band in an image has its own name, pixel values, pixel resolution, and projection. As you'll soon discover, the SRTM image has one band: 'elevation'.

When you add an image to a map using Map.addLayer(), Earth Engine needs to determine how to map the values in the image band(s) to colors on the display. If a single-band image is added to a map, by default Earth Engine displays the band in grayscale, where the minimum value is assigned to black, and the maximum value is assigned to white. If you don't specify what the minimum and maximum should be, Earth Engine will use default values. For example, the image you just added to the map is displayed as a grayscale image stretched to the full range of the data, or signed 16-bit integer [-32768, 32767]. (float bands are stretched to [0, 1] and byte bands are stretched to [0, 255] by default).

You can discover the data type of the image by printing it and inspecting the image object in the Console tab. For example, paste the following after the previous code:

print('SRTM image', image);

When you click run, note that an object appears in the console. To investigate the object properties, expand it by clicking on the zippy () to the left of the object or property. Expand the image object, the 'bands' property, the 'elevation' band at index '0' and the 'data_type' property of the 'elevation' band to discover that it is a signed int16 data type.

Customizing layer visualization

To change the way the data are stretched, you can provide another parameter to the Map.addLayer() call. Specifically, the second parameter, visParams, lets you specify the minimum and maximum values to display. To discover what values to use, activate the Inspector tab and click around on the map to get an idea of the range of pixel values. Alternatively, use the Layer manager to interactively stretch the data, then observe the minimum and maximum corresponding to percentiles or standard deviation stretches. Suppose that through such experimentation, you determine that the data should be stretched to [0, 3000]. To display the image using this range, use:

Map.addLayer(image, {min: 0, max: 3000}, 'custom visualization');

Note that the visParams parameter is an object, with properties specifying the min and the max. (Learn more about JavaScript objects from the JavaScript tutorial or this external reference). Note that the third parameter for Map.addLayer() is the name of the layer that is displayed in the Layer manager. The result should look something like Figure 1. Hover the mouse over the Layers box on the right to see the effect of renaming that layer.

Tutorial_api_01_elevation.png
Figure 1. Elevation image as grayscale, stretched to [0, 3000].

To display a single band using a color palette, add a palette property to the visParams object:

Map.addLayer(image, {min: 0, max: 3000, palette: ['blue', 'green', 'red']},
    'custom palette');

The result should look something like Figure 2.

Tutorial_api_02_palette.png
Figure 2. Elevation image as a color ramp from blue to red, stretched to [0, 3000].

Digression: Palettes

Palettes let you set the color scheme for single-band images. A palette is a comma delimited list of color strings which are linearly interpolated between the maximum and minimum values in the visualization parameters (or defaults according to the band type, as described previously). For example, pixels less than or equal to the minimum will be displayed with the first color in the list; pixels greater than or equal to the maximum will be displayed with the last color in the list. Intermediate colors are linearly stretched to intermediate pixel values.

The colors are defined using the web standard CSS color value scheme (see this external reference to learn more). Colors can be specified by name or as hexadecimal strings indicating the combination of red, green and blue. The lowest value in any of the three positions is 00 (representing the decimal number 0), while the highest is FF (representing the decimal number 255). The string '000000' represents the color black, 'FFFFFF' is white, 'FF0000' is red, '00FF00' is green, and '0000FF' is blue. See the Color palettes section for more detail. Other stretches are possible by using Styled Layer Descriptors, as described in this section.

Later in this tutorial, you'll learn how to display multi-band imagery. But first, learn about performing computations with images.

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Google Earth Engine API