Earth Engine Explorer (EE Explorer) is a lightweight geospatial image data viewer with access to a large set of global and regional datasets available in the Earth Engine Data Catalogue. It allows for quick viewing of data with the ability to zoom and pan anywhere on Earth, adjust visualisation settings and layer data to inspect change over time. Show This tutorial will cover the use of the EE Explorer application, including:
The goals of this tutorial are to enable you to use EE Explorer, inspire you to discover and view new data and provide a starting point to imagine how you might expand your exploration using the other more powerful Earth Engine platform tools to answer questions about the current state and ongoing changes affecting the Earth.
Follow this link (https://explorer.earthengine.google.com) to take you to the EE Explorer application. EE Explorer is composed of an integrated Data Catalogue and Workspace. The Workspace is where you’ll view data, and the Data Catalogue is where you‘ll discover and import data to the Workspace. Common to both of these components are a set of buttons to toggle between the Data Catalogue and the Workspace and a search bar where you can find datasets and places by keyword and location name. Following the above link takes you to the Workspace, as shown in the image below. This is where we’ll spend most of our time in this lesson, but before we get started, we’ll need some data. Let’s check out the Data Catalogue.
The Data Catalogue lists the datasets available for viewing and analysis in Earth Engine proper. A subset of the data are available for display in EE Explorer.
On the Data Catalogue page, you will see a list of popular tags, linking to datasets that have those tags applied. Below that is a list of various data types and multi-day mosaics, including brief descriptions of, and direct links to, a handful of the available datasets. To access other datasets, use the search bar at the top of the page.
This page shows details about the selected dataset, including its name, a brief description, a sample image and information such as which dates are available, the provider’s name and any tags for that dataset. There is also a blue Open in workspace button which can be used to add the dataset to your current workspace (more on that below). You can return to the Data Catalogue page by clicking your browser Back button twice, or by clicking the Data Catalogue button in the upper right.
The Workspace is where you manage and visualise datasets in EE Explorer.
On the Workspace page, you will see a map on the right, and space for a list of data layers on the left. Unless you have already added a dataset to your workspace, your Data list will be empty, and the map will show the Google Maps terrain layer, as shown above. As a reminder for navigating the Google Maps interface, the following points provide some basics.
After adding the data, you will see that the dataset is overlain on the Google Maps base layer, the dataset name is added to the Data list and the layer’s visualisation settings dialogue is open and attached to the right of the dataset name.
Layer visibility
Note that some datasets can only be shown at certain zoom levels. For example, if you are zoomed all the way out to a global view with Landsat 8 dataset, it will not be visible on the map. Don’t worry, it’s not broken. A yellow bar appears at the top of the map saying that you need to zoom in to view the data. Also, note that some datasets contain missing data as a result of quality masking and absent observations. Pixels representing these data are set to 100% transparent, allowing the Google Maps base layer to show through. An added layer will generally default to the most recent temporal representation. You can adjust the date(s) displayed through the layer’s visualisation settings dialogue.
Adding multiple layersYou can view multiple data layers on your map at the same time by adding additional datasets. To add additional datasets, return to the Data Catalogue and simply select another dataset. You can access the Data Catalogue any of three ways:
Though tempting, avoid using the search bar, as it will return many results that are not compatible with EE Explorer (only accessible through Earth Engine proper). For datasets having either “open in workspace” following the data name or Open in Workspace button in their description page, click the link to add it as an additional layer to your Workspace data list. The new data layer will appear above your current data layer(s) both in the data list and the map. See below for changing the order of the layers. Duplicate datasetsDuplicate datasetsYou can also add the same dataset twice, as two separate layers in your Workspace. One reason to do this would be to view two different time slices of the same dataset, to view change over time. For more on this, see the Visualising change over time section below. Reordering layers (adsbygoogle = window.adsbygoogle || []).push({});When you have more than one dataset visible on your map, the one listed at the top of the Data list will be drawn on top of those below it. To change the order, left-click + hold + drag the layer handle found to the left of the dataset name in the Data list. Note that in the example image below, the SRTM Digital Elevation Data Version 4 dataset has been added. Try adding a new layer and change layer visibility by reordering the layers, and alternating visibility with the visibility icon. Removing layers
Now that you know the basics, let’s explore a few of the more powerful things you can do in the EE Explorer workspace. In the sections below, we’ll go over adjusting a layer’s visualisation parameters and visualising change over time.
In the Layer Settings dialogue, you will see a Visualisation Parameters dropdown. Each dataset has different default values, but you can modify them to change how you visualise the dataset.
Data band displayData can be viewed as either single-band greyscale, single-band pseudo-colour and three-band RGB.
Single-band greyscaleSome data have only a single band and will default to single-band display. Multi-band data will default to three-band display, but you can choose to view just a single band as greyscale. Note that you can also display single-band data as three-band RGB display, but the map appearance will not change. To demonstrate visualising a single band, let’s try it with an elevation layer and then try looking at a single band of the multi-band MCD43A4 Nadir reflectance data.
Note that you can preview changes by clicking the Apply button, which will change the map to reflect your changes, while keeping the Layer Settings dialogue open and ready for adjustments. Three-band true colour Let’s work with the MCD43A4 data again to understand three-band display, as well as changing the colour assignment to various bands.
If you visit the data description page for this layer, you’ll see that these three bands are associated with reflectance in particular wavelength ranges. In this case, band 1 represents reflectance intensity in the blue part of the electromagnetic spectrum, 4 in the green range and 3 in the red range. Pairing red, green and blue reflectance bands to red, green and blue display colour will produce a natural colour image very similar to what our eyes see as we view the landscape from a plane, for instance. Three-band false colour Natural colour is nice because it is familiar, but assignment of reflectance bands outside of what we can see with our eyes to RGB colour allows us to “see” landscapes in a whole new way. These types of data representations are called false colour composites. They often include representation from the visible, near-infrared (NIR) and shortwave (SWIR) ranges of the electromagnetic spectrum. False colour display can accentuate inter- and intra-landscape feature type contrast, improving some aspects of image interpretation. To demonstrate, let’s visualise the “standard” false colour composite, where NIR, red and green reflectance bands are assigned to display colours red, green and blue, respectively.
Contrast, brightness and opacityData range The contrast and brightness of an image can be adjusted using the Range (min and max) and Gamma parameters. Visualisation of data requires that a given value range be scaled between 0 and 255 for each band being displayed. The range parameter allows you to adjust the range of values to display. The defined min value will be drawn to 0 and the max to 255, all data values in between the defined min and max range are scaled linearly. Data outside the min and max range are set to either 0 or 255, depending on whether they are less than or greater than the provided range. Let’s try to add some more contrast to a vegetated region to better distinguish subtle differences in vegetation cover.
You should now see greater contrast – areas of red look less saturated. We have narrowed the visible data range and exaggerated the disparity between high reflectance in red and low reflectance in near-infrared. Gamma Gamma represents the relationship between a value and the luminance used to represent it. Roughly speaking, increasing gamma increases the intensity of values in the middle of the visualisation range. It adjusts image brightness and contrast.
Opacity Opacity is the condition of lacking transparency. It is on a scale from 0 to 1, where 0 is transparent and 1 is opaque. It can be helpful for maintaining some visibility of the top data layer while also displaying information from underlying layers. In the example below, opacity has been set to 0.6, which faintly reveals the underlying Google Maps terrain layer. With this data view it is possible to determine which states have the greatest vegetation response for the given time period of the image (May 23rd, in this case). Note that setting the data Range, Gamma and Opacity applies to both three-band and single-band displays. Palette A palette allows you to assign colours to the range of values in a dataset to single-band (greyscale) display. A palette is a series of hexadecimal colour values. Providing two values sets the colours of the defined min and max value of the dataset explicitly, and all values in between are mapped to a linear interpolation of the colour gradient. For example, the SRTM digital elevation model is displayed in shades of grey by default, but we can display it in shades of green instead, where the lowest elevation pixels are black and the highest elevation pixels are green (“lowest” and “highest” are defined by the Range parameter).
Additional colours can be added to the palette. Try visualising SRTM elevation with this palette: 000004, 2c105c, 711f81, b63679, ee605e, fdae78, fcfdbf, fdffe5
{palette 2} Visualising change over timeOne of the interesting things you can do in EE Explorer is visualise change over time. To do this, you will need to add the same dataset to your Workspace as two separate layers and then set them to show different time slices. The example below will show you how to visualise the rapid urban expansion of Las Vegas, Nevada.
As you can see, when looking between the two images above, the 2011 image has far greater urban area than the 1986 image. Note that in this example I’ve set the display to a false colour representation of Landsat 5 bands 5, 4, 2 / red, green, blue, respectively. This representation enhances the contrast between vegetation and barren desert. The following graphic is a practical example of an application of date-to-date comparison. Here, two screen clips from EE Explorer were merged together and annotated to convey information about land cover change recorded by Landsat. This example shows expansion of an open pit coal mine and forest clear cutting near Elkford, British Columbia, Canada from 1984 to 2011.
The following are a list of possibly unintuitive behaviors and characteristics of EE Explorer and datasets that you should be aware of.
Google Earth Engine has more advanced features such as classifying land cover, downloading datasets and the ability to build your own data analysis algorithms. To start using these advanced features of Earth Engine, sign up at earthengine.google.com/signup. Feedback for us?If you have any feedback on the functionality or user interface, please let us know so that we can take it into account as we continue to develop and improve Google Earth Engine. The best way to provide us feedback is through the Send Feedback link in the upper right-hand corner of every Earth Engine page. |