ALERTS HOW-TO GUIDE
View Satellite Imagery
The satellite images currently in Alerts come from Sentinel-2 and Sentinel-1 — Earth observation missions from the European Union’s Copernicus Program. Copernicus systematically acquires optical imagery at high spatial resolution (10 to 60 meters) over land and coastal waters. SkyTruth uses Sentinel-Hub, a platform for handling and delivering satellite data, to make the imagery available in Alerts.
What you can see with 10 meter resolution imagery
The 10m resolution images from Sentinel 2 satellites should work well if you’re searching for new roads, expansion of large disturbance areas, or changes in natural boundaries. But you’ll be disappointed if you’re trying to identify tree cover or the type of vehicle that’s parked in your driveway.
In a 10m resolution image, one pixel represents a 10 meter by 10 meter area, so objects will need to be considerably larger than that for any detail to be discernible.
Here are two 10m images over a gas drilling site in Pennsylvania, taken one year apart.
- Clicking the My Areas tab
- Clicking the Explore a New Area button
- Moving to the map to the area you want to view
- Clicking the Current Map button
Once you’ve got your AOI, you’ll see a Sentinel Imagery checkbox to click.
Selecting the Image Type
The default image type is True color Sentinel-2, which will be the only type that most Alerts subscribers will use.
Geo-Spatial Analysts and other scientists may be interested in other selections that provide special processing for monitoring agriculture, forests, land-use and coastal and inland waters. However, these additional image types are provided without classification; we make them available in Alerts to generate ideas for future use. All of the image types are from the Sentinel-2 satellite with one exception: Sentinel-1. Sentinel-1 is a radar imaging satellite used to monitor oil spills, sea ice and land-use change, among other uses. Because Sentinel-1 is a different satellite than Sentinel-2, a different set of of map dates will be available as you change your selection to and from Sentinel-1.
Selecting a Map Date
The Select a map date drop-down list is populated based on the filter settings (max cloud cover and date range). By default, the most recent images with a maximum of 50% cloud coverage are selected. You can change the map dates available by adjusting the Max Cloud Cover slider or entering a date range, and then clicking the Filter button.
What Exactly am I Seeing?
In Alerts, imagery from Sentinel-Hub is made up of a mosaic of “scenes” based on zoom level, the size of your AOI and the imagery available. If your AOI is small, chances are the image will be from a single scene. However, for larger AOIs the satellite image you see is created from several stitched together scenes, which may or may not be from the same date. For this reason, we include the date of the imagery on top of each scene so it’s clear what’s being presented.
When you select a map date, the corresponding scene for that date may only represent a small part of the AOI. Sentinel-Hub will back-fill the rest of your map view with the most recent imagery available, regardless of cloud cover.
In some cases, especially for very large AOIs, the scenes presented might even change based on zoom level. This is because Sentinel-Hub’s image vault may not have the same view available for each zoom level.
About the Maximum Cloud Cover…
The cloud coverage percentage shown for each map date is based on 100km x 100km scenes, which may not be very useful if you are looking at a field that’s only a couple of acres in size.
I Don’t See Any Scenes for the Map Date I Selected
A scene may make up only a very small portion of your AOI. Or, not be available at all zoom levels.
Some of the imagery can be enhanced by using the Gain and Gamma controls to adjust the brightness/contrast. The default values are 2 for Gain and 1 for Gamma.
There are check-boxes that can be used to Spotlight your AOI and Show Map Labels over your satellite image. Here’s the same image as above, but with both these boxes checked.