Ground sampling distance (GSD) and its impact on drone surveys – heliguy ™

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  • A guide to Ground Sample Distance (GSD) for drone surveying: what it is and why it matters;
  • Factors impacting GSD, such as flight altitude and your camera specifications;
  • How to calculate GSD, using online or manual methods;
  • Sample data sets from the DJI M300 RTK-P1.

If you are using drones for surveying, understanding GSD (Ground Sample Distance) is crucial.

Simply put, your GSD has an impact on the accuracy of your surveys. Without it, you risk collecting inaccurate data or having an unnecessary map.

This guide will teach you more about GSD, how to calculate it, and why flight altitude and a good quality camera are important.

What is GSD and why is it important for drone surveys?

The GSD is defined as the length (in inches, centimeters, or millimeters) between the centers of two consecutive pixels on your map.

Or, to put it another way, GSD can be thought of as the length of a pixel in your map.

So if a drone achieves a GSD of 5cm / px, that equates to a pixel on your digital map corresponding to 5cm in reality.

Therefore, the smaller the GSD, the higher the accuracy.

Take the images below as an example: The orthomosaic with a 5cm GSD (left) is much more detailed than the right one with a 30cm GSD.

In fact, the greater the value of the GSD image, the lower the spatial resolution of the image and the less visible the details.

With that in mind, GSD is an important computation for aerial photography and photogrammetry, which is a technique commonly used to create 3D topographic maps.

Understanding the GSD as a centimeter, inch, or millimeter-to-pixel relationship makes it easy to calculate the size of the ground and features captured in your drone image.

Not only that, but knowing the size of each pixel is necessary to grasp the full scale of your map and make decisions based on clear information.

An error of one centimeter or less may seem minor, but this error over hundreds of thousands of pixels will create a serious mismatch between your map and reality, making measurements nearly impossible.

To be safe, land surveyors always use the lowest possible value when calculating the GSD.

GSD – Flight Altitude and Your Camera

Two key factors impact your GSD: Camera quality – especially camera focal length and resolution – and flight altitude.

As a general rule, the lower the altitude, the lower the GSD.

Take the DJI Phantom 4 RTK, for example. This table shows the impact of flight altitude on the GSD, i.e. the lower the flight, the lower the GSD.

Flight altitude GSD
10 meters 0.27 cm / px
30 meters 0.82 cm / px
50 meters 1.37 cm / px
80 meters 2.19 cm / px
100 meters 2.74 cm / px
120 meters 3.29 cm / px

In addition to the flight altitude, a higher resolution camera further improves the GSD.

For example, look at what happens when the P4 RTK – with its 20MP sensor – pits against the DJI M300 RTK and P1 camera, with its 45MP full frame sensor:

Altitude P4 RTK M300-P1
50m 1.37 cm GSD 0.63 cm GSD
80m 2.19 cm GSD 1.18 cm GSD

As you can see, the P1 achieves a lower GSD, thanks to its high resolution camera. And at 50m it actually achieves an extremely impressive GSD of less than 1cm.

For the record, the images below show how a sub-centimeter GSD can help create very detailed and crystal-clear maps.

The orange circle highlights a section of this orthomosaic which was captured at an altitude of 50m …

… and the resolution is so high that when you zoom in on this section you can clearly count the number of holes in the bricks or identify specific toys in the sandbox.

This level of detail is extremely important for projects requiring complex details or measurements.

Having a higher resolution camera also increases the efficiency of mapping, as it allows the drone to fly higher – covering more ground in the process – while capturing very detailed data.

As an example, we’ll explore this theory again using DJI’s two photogrammetry solutions: the Phantom 4 RTK (below, left) and the M300 RTK and P1 cameras.

This table shows that the P1 – with its three interchangeable lenses – can fly at a higher altitude than the P4 RTK while still achieving the equivalent GSD.

M300 RTK-P1 P4 RTK
24mm, GSD = H / 55;
35mm, GSD = H / 80;
50 mm, GSD = H / 120.
H / 36.5

It’s not just the higher resolution of the P1 that generates efficiency gains.

This is also the pixel size of the P1, which is 1.8 times that of the P4 RTK …

…. and its much larger sensor, which is seven times the size of the P4 RTK.

Larger sensors capture more light in less time: they allow a faster shutter speed for sharp, well-exposed results, which improves accuracy.

And this graph shows how all of these factors combine to make P1 a more efficient solution. During a 15 minute flight, and with the same GSD, the M300 RTK and P1 can cover more than four times the area of ​​the P4 RTK.

However, that doesn’t mean the P4 RTK is a bad drone – far from it! Rather, it shows how the altitude and quality of your camera can impact the GSD.

What level of GSD precision do you need?

With that in mind, what level of GSD precision do you really need? In truth, there is no right answer: it all depends on your mission.

But remember, charts with a higher GSD will be less accurate than those with a smaller GSD. However, there are implications to consider, and there are tradeoffs, as the table below shows:

Lower GSD Superior GSD
Greater precision Lower precision
Lower flight altitude Higher flight altitude
Longer flight time Shorter flight time
No more captured data to process Not so much data to process

As a general rule, if you need very complex measurements or a detailed reconstruction, a lower GSD is recommended. But that usually means a longer flight, because less area is covered.

On the other side of the coin, if you are conducting a survey over a large area that does not require very detailed results, then increasing the flight altitude and increasing the GSD is an interesting consideration, especially since it can reduce acquisition time.

The example below demonstrates this.

The shovel is located on our map – or chess board, because a digital photo is made up of many individual squares.

If we just wanted to tell someone where the excavator was, we could give any number of coordinates, such as G3 or F2.

But if we were to calculate the width of the loader, the requirement parameters would change and more precise information would be needed. Currently, the loader is spread over two squares – D2 and E2 – which is too vague for precise calculations.

Therefore, the individual grids should be smaller to give a more accurate measurement. Or, to put it another way, the GSD should be reduced.

Essentially, choosing the right GSD will be the one that allows you to take detailed images while flying high enough to avoid an excessive number of photos.

Too high a GSD, and you’ll end up with blurry pictures that mean nothing to you.

If you go too low, your survey will take extra GB and may take longer than expected.

That’s why a solution like the P1 offers the perfect balance: increasing efficiency by allowing greater flight heights while collecting very detailed data.

How to calculate the GSD

There are two options to help you calculate the GSD: use an online tool or do it manually.

The first choice is simple, and there are plenty of tools to help you, such as Propeller Aero GSD Calculator.

In this tool, you enter the drone – either preset (containing most of the big hitters in the industry) or custom – and the flight altitude, and this will provide you with the GSD for the flight.

Or, you can calculate it yourself.

For this, you will need to know the height and width of the sensor, and the height and width of the image on your drone, as well as the focal length and flight height.

Each of these stats should be available on your drone.

You can then insert each number into two basic formulas, one for GSD height and one for GSD width.

  • GSDh = flight height x sensor height / focal length x image height
  • GSDw = flight height x sensor width / focal length x image width

The corresponding GSD number will be the lower value, to ensure that you are using the worst case scenario.

Conclusion

The use of drones for surveying has grown exponentially in recent years. And such is the accessibility of technology, a mapping mission can be carried out quickly and easily with a pre-planned and automated flight route.

That being said, there are factors you need to take into consideration to ensure that you are collecting the best data for your needs: GSD is one of those factors.

Understanding ground sampling distance is essential for creating detailed and accurate maps: whether for visual representations of a site or for performing complex measurements.

Knowing the capabilities of your camera, relative to flight altitude, is important.

The DJI surveying ecosystem doesn’t disappoint in this department, and the Phantom 4 RTK is a great all-in-one low-level mapping solution, especially useful for those entering the industry.

But the all-powerful M300 RTK-P1 alternative presents a high-performance option that can truly transform surveying workflows and can compete with fixed-wing aircraft for large-area drone mapping.


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