If you’re working with or are considering working with drones to capture your site, you may have came across the term, ground sample distance (GSD).
It’s an important concept to understand, as it can affect the accuracy of your survey. This article will take you through what it means, how it affects your drone data, and how to calculate it.
If you don’t feel like reading and you need to calculate a GSD, feel free to jump straight into the Propeller GSD Calculator that we built for you.
What is ground sample distance or GSD?
The ground sample distance is the distance between center points of each sample taken of the ground. Since we’re talking about digital photos, each sample is a pixel.
In simpler terms, the GSD is how big each pixel is on the ground.
Below we can see an AeroPoint that was captured 30m (~100ft) above the ground with a DJI Phantom 4 Pro or Phantom 4 RTK. If we zoom all the way in, we can see individual pixels that make up the image. In this example, each of these pixel represents a 0.82cm (~0.32in) square in the real world. This is the GSD.
Why is it important to calculate GSD?
Digital images are grids of colored squares (pixels), much like chess boards. Let’s say we have a (very) big chess board and someone has parked a front-end loader on it.
We need to tell the supervisor where it is so they can send someone to move it, but we can only use the squares on the board for measurement.
What are our accuracy requirements? Enough to find the loader.
Since the loader is much bigger than the squares we’re using to locate it, we can get the operator to head to F3 where they will easily find the loader to move it.
Hold on, now, they’re saying that they need to know how wide the bucket on the loader is so that they can send the right operator to move it. Again, we can only use the squares on the board for measurement.
What are our accuracy requirements? Enough to measure the bucket to know which type of loader it is. (Let’s assume they’re relatively similar in size.)
The bucket is more than one, but less than two squares wide, which is a huge range as far as loader buckets go. So we can’t measure the bucket accurately enough with our squares. We would need much smaller squares to be able to measure the width of the bucket well enough to know which kind of loader it is.
In practice, this is how ground sample distance (GSD) works. Your survey cannot be more accurate than your GSD. In fact, if you need to measure objects within an inch of actual, you will want 0.5in GSD or less.
So before you fly, make sure you know what the survey will be used to measure and what the accuracy requirements of these measurements are. This way you can strike the balance between:
- Flying low enough to keep the GSD smaller than your required accuracy, and
- Flying high enough to keep your photo count reasonably low.
How can I calculate GSD?
We have good news and even better news!
- The good news is that the calculation isn’t particularly complicated (explained below).
- The even better news is that we’ve built the Propeller GSD Calculator to do it for you.
We’ll start with the even better news.
Calculating GSD with the Propeller GSD Calculator
- Select a drone from the list.
- Enter flight height.
- Select correct units (metric or imperial).
That’s it. You can also enter in a custom drone configuration, but all of the usual suspects are already in the calculator.
Calculating GSD example
Let’s say for example, we’d like to figure out the GSD of a Phantom 4 Pro flying 80m above the ground. As above, we:
- Select the Phantom 4 Pro.
- Enter 80 into the flight height.
- Set to meters.
GSD = 2.36cm/px.
Try it for yourself here!
Calculating GSD yourself
For those who are interested, now we’re going to jump into manually calculating GSD.
Here we have a diagram showing all of the relevant factors for calculating GSD. The figures should be publicly available for any drone that you’re working with.
When you project each pixel onto the ground they’re not perfectly square, so when we calculate the GSD, we use the greatest (worst) of:
For example, let’s figure out the ground sample distance of a Phantom 4 Pro at 80m above the ground.
We know the following from DJI’s spec:
- Image Width: 5472px
- Image Height: 3078px
- Sensor Width: 13.2mm
- Sensor Height: 8mm
- Focal Length: 8.8mm
So substituting the values (in centimeters) into the formulas:
Then we use the worst case scenario, so in this case: GSD = 2.36cm/px.