Overview
Using satellite images to scout ice can be super helpful. The data that’s now freely available is absolutely incredible. But if you’re not up-to-speed on how to acquire and interpret the images, they can be a little confusing at first. Thus, I’ve written this primer.
This has come up on the Nordic skating email lists a bunch of times over the years, but the information is a little scattered. Luc Mehl has some relevant and helpful resources, but I think what’s appropriate for big trips in Alaska (or sea ice off the Swedish archipelago, for that matter) is not quite the same as what’s needed for smaller bodies of water in the northeast.
In what follows I use Lake Winnipesaukee as my main example, because I’ve been on it recently and have had the chance to compare the images to what I found in real life, and because it’s got a good diversity of conditions at the moment to illustrate some of what we can look for in satellite scouting.
Contents
- Getting the images on your computer (this article)
- Visualizing the data
- Examples: Lake Winnipesaukee; Lake Sunapee
- Getting the images while on the ice
- Additional information
NB: Click any image to view it larger. There’s lots of detail to examine in them.
Caveat: The process outlined here is not the only way to do it, it’s just the way I’m doing it currently.
BIGGER CAVEAT: Looking at pretty pictures on the internet is no substitute for first-hand knowledge. Read reports, talk to people, test the ice, make your own observations. Anything else is not enough.
1. Getting the images on your computer
The data we want comes primarily from the Sentinel-2 satellites (thanks Europe!), which provide frequently updated (at least every 5 days), very detailed (up to 10m resolution, which is amazing), and freely available images (free as in beer).
There are a few ways to get it; the overall best is to use the Sentinel Hub EO Browser. Click on “Start exploring!” or go directly to the app.
(I describe some other ways to get the images in part 4.)
This service is free to use and does not require an account. You just need to agree to the terms of service (the usual stuff, and free usage has to be for non-commercial purposes). You can create an account, which gives you some additional functionality (for example, saving pinned maps and downloading high resolution maps), but you definitely don’t need to. When you first open the EO Browser, it’ll give you links to the user guide and to a nice overview tutorial.
This website requires an up-to-date browser and a fast internet connection. If you’re using an old browser, or an old operating system, or a very old computer, or a very slow connection, it might not work. It’s loading and processing huge amounts of data.
NB: The order of the following steps matters. If you’re having trouble, make sure you’re doing things in the order listed here.
The EO Browser opens up to a map and the Discover tab of the control panel. In the top right, you’ll see a search box labeled “Go to place”. Type a location in. For example, type “Lake Winnipesaukee”. Then select the best matching item from the drop-down that appears.
Once you see the map update to show the correct location, choose the types of satellite images you’re interested in, using the control panel on the left. Make sure “Sentinel-2” is selected (it should be checked by default), then also click the checkbox next to “Harmonized Landsat Sentinel” to select it as well. Then click the “Search” button on the control panel.
A list of results will appear.
They’re sorted chronologically, with the most recent satellite passes at the top. Each result will be labeled either “Harmonized Landsat Sentinel (HLS)” or “Sentinel-2 L2A”, then a date, then a time (in UTC, which is 5 hours ahead of Eastern Standard Time), then a percentage of cloud cover. You can either hover your pointer over a result in the list, and the swath it corresponds to on the map will turn blue (all others will be a pale cyan), or you can click on an area on the map and the result it corresponds to on the list will be selected. (Or if it corresponds to multiple results, you’ll be allowed to choose between them.)
The simplest thing to do is to start at the top and work backwards in time until you find what you need. Best case scenario would be Sentinel-2 imagery from today with a low cloud cover percentage.
In this example, since both the latest HLS data and the latest Sentinel-2 data (which are kind of the same; see the question in part 5 about the difference) are from 2 days ago, I’m going to choose the Sentinel-2 result to get better resolution. (HLS images more often but at a slightly lower resolution.)
Click the green “Visualize” button on the result.
You should see something like this.
If clouds are covering the spot you’re interested in, click the “Discover” tab in the control panel (almost the top left corner) and choose a different result from the list.
Now that you have a satellite image, continue on to part 2, visualizing the data.
— Christopher Boone, some rights reserved (CC BY-NC-SA 4.0)