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ForWarn II: How to Find Recent Satellite Imagery

Having the ability to pull up recent satellite imagery has been a real game changer for my time spent learning about my local forests. While there are other sites that have recent imagery, only the ForWarn II tool provided by the U.S. Forest Service has been able to provide imagery with enough clarity to be useful.

Sure, part of this fascination with this tool is just how cool I think it is to be able to pull up this recent imagery, but there are many practical reasons to care about this as well. From locating specific types of trees in the fall based on their colors to finding areas that have been recently logged, there’s a lot we can do with this functionality.

In this post, we’ll go into detail on exactly what you need to know about finding recent satellite imagery while using the ForWarn II tool.

Accessing The Different Layers That Have Recent Satellite Imagery

Let’s get to it: there are three different types of layers in ForWarn II that provide recent satellite imagery:

  • Planet.com Imagery
  • High-Resolution Sentinel Imagery
  • Medium-Resolution Landsat 8 Imagery

Each of these layers comes with their own quirks and functionalities, and below we’ll show you everything you need to know about using them.

A quick note before we get started: ForWarn II allows you to share custom URLs that show the exact map that you’re viewing, which consists of the data layers turned on and the positioning of the map on the screen. Each link that I list below will be custom to include only the data layer that we’re talking about.

Planet.com Imagery Data Layer

Here’s a link to open ForWarn II with the Planet.com Imagery Data Layer toggled on.

As soon as you open this link you’ll see something like this:

forwarn ii application with the planet.com imagery data layer turned on

You might catch in the screenshot above that there are two things worth nothing. First, that the imagery is only available for a few states in the Midwest (the exact list: IL, IA, IN, KS, NE, OH, WI, ND, and SD). Second, you’ll likely see that there are gaps in the coverage of these states.

As far as I know, these gaps correlate to cloud coverage that made it difficult to compile satellite imagery for these areas in this time-frame, but I don’t know for certain.

If you look to the menu on the left side of the screen you’ll notice that there are two different layers in this section:

menu options for planet.com imagery data layer in forwarn ii application

You’ll notice that these two layers cover the two most recent time periods of the imagery that is available for this layer. To turn on the older layer all you have to do is toggle off the first box and then check the box below that, like so:

screenshot highlighting previous period satellite imagery from planet.com data layer in forwarn ii application

You might catch in the screenshot above that the coverage for this layer is much better when compared to the first layer, so it may be worth checking.

Once you’re at the layer that provides the best coverage, you can zoom in and out by using the tools at the top menu, by scrolling with your mouse wheel, or by double-clicking in.

High-Resolution Sentinel Imagery Data Layer

Here’s a link to ForWarn II with the High-Resolution Sentinel Imagery Data Layer activated.

You’ll immediately notice that we don’t seen any sort of satellite imagery when we load this zoomed out view of the continental United States. This is because this specific layer doesn’t load until you zoom in close enough, which you’ll be able to see in it’s description in the Legend section:

legend entry for sentinel satellite imagery indicating need to zoom in with forwarn ii application

Once we zoom in close enough you’ll see the screen buffer and then it should load satellite imagery like the following:

screenshot showing recent satellite imagery from the sentinel imagery layer in forwarn ii application

You can spot in the left-hand portion of the above screenshot that the menu for this section is organized differently than the previous option. Here you’ll find that this set of layers is organized according to two ideas:

  • A ‘True Color’ vs. ‘Ag False Color’ Distinction
  • Offering the current year and the imagery from one year ago

We’re only going to be interested in the ‘True Color’ layers, as the ‘Ag Color’ layers are for more technical applications that don’t matter here.

We’re also going to default to the true color layer of the current year, but it can be helpful to keep the prior year layer in mind in case we run into coverage issues.

Medium-Resolution Landsat 8 Imagery Data Layer

Here’s the link to open up ForWarn II with the Medium-Resolution Landsat 8 Imagery Data Layer turned on.

When you open this link it will look a little different than the High-Resolution layer, but it behaves the exact same way. Until you zoom in close enough, the map will be all white. Once you’re close enough to bring up satellite imagery, you should see something like this:

screenshot showing recent satellite imagery from landsat 8 imagery data layer in forwarn ii application

You’ll notice that the clarity of this imagery is not as nice as the previously discussed layers, but it still may be suitable for your purposes. The different options in this section are ordered exactly like the High-Resolution section, where they have true/false options as well as the current and prior years.

Just like with the High-Resolution layers, we’ll be interested in the ‘True Color’ layer for the current year primarily, but we can consider the layer for the prior year as a backup option.

How the Different Layers Compare

Now that we’ve covered how you can access each of these different layers of recent satellite imagery, let’s give a brief summary of how they compare.

Accessibility and Coverage

There’s no way around it: each of these layers is bound to run into issues with cloud cover, they just happen to handle it differently. The Planet.com layer and the Medium-Resolution layer will remove areas impacted by cloud cover, while the High-Resolution Sentinel layers will merely show the clouds in the data layer.

The other thing to keep in mind is that the Planet.com layer is only available to a select list of states in the Midwest, while the other two layers offer mostly complete coverage throughout the continental United States, depending on cloud cover.

Clarity of the Satellite Imagery

While I find that the Planet.com layer has the best imagery in the ForWarn II tool, I also think that the High-Resolution Sentinel Imagery data layer isn’t far behind.

With that being said, I do think that the Medium-Resolution imagery layer is useful, it’s just lacking in clarity enough that I prefer to treat it as a last resort option.

How They Function in the Tool

As we discussed earlier, the different layers are organized differently and therefore provide different types of functionality.

The Planet.com imagery section provides the two most recent 7-day periods of time, and this allows you to get a feel for how things are progressing over time. This may not be useful in the middle of summer when everything is green, but it’s very helpful if you’re using this imagery around seasonal changes in spring and fall.

The High-Resolution layer and Medium-Resolution layer offer imagery for the current year and the prior year, and this may be helpful if you’re running into coverage issues.

For example, this area has a fair amount of coverage issues from cloud cover in the current year imagery:

screenshot showing a recent satellite image with high levels of cloud cover in forwarn ii application

However, if you switch to the prior year you’ll find the coverage is much better:

screenshot showing prior year satellite imagery with low levels of cloud cover in forwarn ii application

This is very helpful and definitely worth keeping in mind if you’re running into issues caused by cloud cover.

How to Best Use These Layers

Assuming that you’re most interested in finding satellite imagery most accurately reflecting the current conditions of a nearby forest, here’s the exact order I would use:

  1. Check the most recent Planet.com layer if you have coverage
  2. Check the other Planet.com layer for a slightly delayed set of imagery
  3. Switch to the High-Resolution imagery layer for the current year
  4. Check the prior year imagery of the High-Resolution imagery section
  5. Use the Medium-Resolution Imagery option if everything else failed

This should get you access to the most accurate and clear sets of imagery first, even if you’re checking imagery from prior years before moving to the Medium-Resolution imagery. As trees have relatively consistent behavior from year to year (barring significant differences in weather), the prior year imagery should still be suitable for our purposes.

Final Thoughts

I hope you benefited from this article and came away having learned a few things. There are a lot of really cool things that can be done with recent satellite imagery, and I don’t know of a better way to get it than with ForWarn II.

If you’d like to read on other related topics, feel free to check out the below articles:

By Drew Meulemans

I've long admired forests and devote much energy to learning about them and exploring. I enjoy sharing what I learn and wish to inspire others to do the same.