img credit: delair.aero
Overview
Ground Sampling Distance (GSD), as the name implies, refers to the amount of ground/surface area covered by a single image in flight.
If you are mapping a site using a drone with the camera facing down (nadir position), then the ground sampling distance is basically the amount of ground captured per image pixel in the drone's camera.
So for example, if the GSD for a drone flight is 1cm/pixel, that means that every pixel is the drone image is capturing 1cm on the ground.
If you're flying vertically and mapping a tower or a facade, then GSD is basically the amount of facade surface area covered by a single image pixel in flight.
There is a formula to calculate GSD and we have discussed this is a previous post:
In the video below, we explain GSD is and how to calculate GSD:
Why Does GSD Matter?
Let's look at it like this, GSD affects the resolution of the data, a bit like HD vs 4K.
Counter intuitively, a high GSD means lower resolution data and a low GSD means higher resolution data.
Let's try to understand why that is.
A high GSD (50cm / pixel) would imply every pixel is the image is capturing 50cm on the ground, implying that the drone is far from the ground, i.e reducing the resolution of the data. On the other hand, a low GSD (1cm / pixel) would imply that every pixel in the image is capturing 1cm on the ground, implying higher resolution of the data.
So does this mean we should always aim to get the GSD as low as possible and the resolution as high as possible?
Not quite.
GSD helps us understand how 'big' or 'small' features would look in the final images. A low GSD would make features in the image look bigger and a high GSD would make features look smaller.
img credit: heliguy.com
However, not every drone mission requires big features, and getting the GSD to be low requires you to either 1) get closer to the target 2) upgrade the camera.
Both of those options can be expensive in time, costs and safety as getting very close to the ground in certain areas might not be safe or efficient.
Other Considerations
1. Data Accuracy
If you looking to capture highly accurate data, GSD can have an effect on the overall data accuracy. This is specially true if the end goal of the drone survey is to perform measurements. As a rule of thumb, we recommend shooting for a GSD within 2-3x of your desired absolute accuracy and a GSD low enough for relative accuracy. To learn more about the different types of accuracy and how GSD can affect them, please visit:
2. Motion Blur
Motion Blue and GSD are also related concepts. Motion blue can sometimes be unavoidable, but generally speaking, it's good to keep your motion blur below 2x your GSD to collect high quality data for mapping flights. Here's more on Motion blur:
3. Mapping vs Inspection
Lastly, it's important to understand what type of flight you are undertaking and how to use GSD to achieve the right results for the right job. If you are undertaking a mapping flight, it's important to pay attention to data accuracy and data resolution, as this would directly affect any measurements you make from the processed data. On the other hand, if you are undertaking an inspection flight, then it's important to think about how 'big' or 'small' would the cracks, corrosion or other defects appear in the final photos, so that they can be annotated and accumulated in to a report. If you are unsure about the differences between drone mapping and inspection, please visit:
How To Plan GSD In Drone Flights?
The first thing you need to do when planning to use low GSD in your drone flight is to define the area you are planning on flying in, making sure to take safety into account.
Flying lower will decrease your GSD, and increase your resolution, but you may compromise safety by doing so, this is why it is always best to check your surroundings for immediate hazards.
Select your camera or drone and your flight altitude, and a software platform like Hammer Missions will automatically calculate the GSD for you:
Want to calculate this for yourself?
Get your free and comprehensive spreadsheet designed with Drone Mapping and Inspection formulas over here.
GSD should always be a good balance between data quality, accuracy and safety!
Summary
This post explains the concept behind GSD and why it matters, GSD is really a central concept to drone missions and will help improve the quality of your deliverables!
If you haven't got a Hammer account as yet and would like to try Hammer, you can get started on our free trial.
To learn more about our enterprise solutions, including mission collaboration, data processing, and AI solutions, please contact us at team@hammermissions.com.
We look forward to hearing from you.
- Team at Hammer Missions