Jessie Norriss

The View from Above: How Satellites and Drones Can Complement Monitoring

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During grad school, I started working with an organization called Living Observatory that focused on cranberry bog restoration. We formed collaborations with researchers and practitioners across dozens of universities and organizations to understand best practices and outcomes of wetland restoration in former cranberry bog areas.

It was clear based on historical maps in the archives of University of New Hampshire that these landscapes had experienced major transitions since the 1800’s as a result of population growth and farming. However, gathering timely and detailed information on the progress of recent restoration activity proved challenging. From the hours I’ve spent inspecting aerial and satellite imagery and trudging through muddy bogs to recover crashed drones, I’ve learned much about how various data collection methods can provide insights about changing landscape conditions. This article provides an overview of the strengths and weaknesses of satellites and drones to help organizations determine what sources make sense for the questions they aim to answer.

Jessie Norriss with a drone
Jessie Norriss recovering a fallen drone at the cranberry bog restoration site in Plymouth, MA

Satellites and drones operate at different distances from the earth, which leads to tradeoffs in spatial resolution and how large of an area can be captured. In some ways, drones can democratize data collection by allowing people and organizations to collect detailed data themselves. However, there is a ceiling to drones’ data collection capacity, making it difficult to scale this approach across large areas in a cost-effective and efficient way. Projects which require large-scale assessments and consistent data collection may be a job best suited for satellites. To suss out the differences a bit more, below are a few guiding questions which unpack the fundamental differences between satellite and drone data collection and can help your organization determine the best method for your needs.

How frequently do you need to monitor an area?

Satellites have consistent flight patterns, and some sources have a revisit time (i.e. come back to capture an image of the same area) within days or weeks, and some have been in orbit for decades. The number of earth observation satellites and their spatial resolution continues to accelerate, making it possible to efficiently monitor up-to-date changes on the landscape.

Alternatively, drone flights are conducted on a one-time basis and require significant planning and coordination in advance. A licensed pilot is needed, often with staff on the ground to support during a flyover. The upside of this approach is that you have a lot of agency in when and where you capture your data, but it can be time- and resource-intensive to capture several flights over the year for comparisons and consistent monitoring.

Weather is another consideration which impacts both satellites and drones: bad weather can thwart flyover plans, making it impossible to collect good data or risking damage to the drone itself. Similarly, clouds can block satellites’ ability to capture data in certain areas, resulting in patchy images. In sum, if you are looking for a few captures to document detailed changes, drones might be the right fit. If you are aiming to evaluate historical trends and patterns, and consistently monitor into the future, satellites are your answer. In both cases, you have the opportunity to visualize your property with a new perspective that is sure to amaze!

How large of an area are you looking to monitor?

Drones have a limited flight range due to battery life, and, per FAA regulations, must be kept in line of sight at all times. This means if you are in mountainous or heavily forested terrain, it can be difficult to find a good location to operate the drone from in order to capture the full area of interest. Larger properties will often be broken into sections, trading out batteries to capture the area in complete over multiple flights. On the other hand, satellites have global coverage and are not restricted by the type of terrain. If you have many properties that are hard to access because they are remote or difficult to survey on foot, satellites become a very useful tool in the toolkit, as they are great for monitoring landscape-level trends. So if you’re looking to monitor hundreds of acres or more, satellites may be your best bet. But if your needs are a smaller, more thorough understanding of the landscape, drones might be ideal.

What spatial resolution do you need?

Though satellite data resolution has increased significantly in recent years, drones still provide more crisp imagery and detail overall. The question then becomes: what resolution is needed for a particular goal? For example, satellites can provide insight into changing landscape conditions, such as vegetation or surface water presence. The illustrative graphic below compares the resolutions between drones and various types of satellites, with each of those measurements translating into one pixel in an image. Note: The highest quality drone can capture imagery at 5 cm spatial resolution and the highest spatial resolution of a satellite is 30 cm.

Overview of spatial resolutions
An overview of spatial resolution from satellites. Imagery © 2018 Planet Labs Inc., Maxar (formerly Digital Globe), European Space Agency. All rights reserved. Post processing and analysis by Upstream Tech.

What type of analysis are you looking to perform?

Another question to consider is, are you looking for pretty images for marketing or are you aiming to analyze the data to glean new insights about the property? While these don’t have to be mutually exclusive, there are some key considerations when making the decision about how to approach one or the other.

On any given sunny day, folks with a drone can fly to capture beautiful images or video of their properties, and it can be a bit more difficult, and potentially more expensive, to procure high resolution satellite imagery. If the goal is a few nice photos or videos for marketing or simple visualizations, drones are likely the more fun route to take if your area of interest is less than 100 acres. If properties are more expansive, it could take several hours (or even days!) to capture the site with a drone and satellites might do the trick in a more efficient and affordable way.

However, this gets more complicated if you plan to use the drone data to track changes over time.

Since each drone flight is unique, it is critical to align the images by properly georeferencing them for comparison between the captures. This is done through setting up ground control points distributed across the site you are flying. When comparing multiple points in time, or specific locations, a semi-permanent ground control (like rebar) can improve precision. You could also invest in an RTK system, which acts as a highly accurate GPS to precisely locate your drone imagery without the need for ground control, but they are much more expensive.

Satellites have much more consistent flight patterns and data collection methods, so comparing images over time becomes much easier (as shown in the image below). That said, folks often need specialized computer programs and the requisite training to view, process and analyze the data — it often takes multiple perspectives to fully interpret and analyze the data, remote sensing specialists who understand the methodology to analyze it, and and environmental experts to glean insight from the data on ecological conditions.

Takeaway: We recommend carefully weighing the balance between both sources based on the questions outlined above. Lower resolution satellite data is a good place to start for consistent monitoring across large areas, and higher resolution sources can supplement that baseline — either commercial satellite imagery or drones based on your specific project goals. Additionally, the historic satellite data can become a helpful reference point if drone data was not captured.

We hope you have a better understanding of the benefits and considerations of satellites and drones. Each operates at different scales to help explain and monitor the natural environment. As you navigate which heights to take your monitoring to, we are here to help!