Ground sampling distance is one of the first numbers in a drone survey quote, and one of the most often misread. On an aerial survey, it tells you how sharp the imagery will be, not how accurate the finished survey will be.
Quick Answer
Ground sampling distance (GSD) is the ground distance covered by one pixel of the imagery, set mainly by flying height and camera. It describes resolution. It does not state positional accuracy, which depends on control, overlap, camera calibration and processing. If you need an accuracy figure, ask for checkpoint results: surveyed points kept out of processing and compared to the finished model.
Quotes for the same site can differ for many reasons. Our article on drone survey cost covers what an aerial quote is priced on. This article covers one number in it.
What ground sampling distance measures
GSD is a sampling interval: the size of the patch of ground that one pixel of the source imagery represents, at the height and camera settings that were flown.
Fly lower and each pixel covers less ground. Fly higher and each pixel covers more. A higher-resolution camera gives a smaller pixel from the same height. That is all the figure describes. It is a property of the photographs before any processing has been done.
A small pixel is useful. It makes small features easier to see and pick out, such as a kerb line, a crack or a survey mark. It is one input to a good survey, in the same way that a sharp photograph is one input to a good drawing.
What it does not measure
Positional accuracy is a different question: how closely a point in the finished model matches the true position of that point on the ground, in the project coordinate system. Sharp images can still produce a model that sits in the wrong place, or one that bends away from the truth between control points, if the control or processing was poor.
Key point
Ground sampling distance describes the imagery. It does not describe where the finished model sits on the ground.
Comparing drone quotes on GSD alone?
Send us the quotes. We will show which evidence each one includes.
Ask us to compare the quotesWhat decides positional accuracy
Accuracy in the finished model depends on several things beyond the pixel size, and each one is under the control of the surveyor, not the camera.
| Factor | Why it matters |
|---|---|
| Ground control | Anchors the model to the project coordinate system |
| Control layout | Control on one side of the site leaves the rest unconstrained |
| Image overlap | Too little overlap weakens the fit between photographs |
| Camera calibration | An uncalibrated lens can bend the model |
| Processing | Settings and checks decide how errors are handled |
| Checkpoints | The only independent test of the result |
None of these appear in a ground sampling distance figure. Two surveys with the same pixel size can perform quite differently if one had good control and checks and the other did not. Our article on ground control points and checkpoints explains why the points used to build the model cannot also be its test.
Reading a GSD figure in a quote
A quote that states a ground sampling distance has told you the resolution it plans to fly, which is useful for deciding whether the imagery can show the features you need.
Read it as that, and look elsewhere in the quote for the accuracy evidence. Ask three questions. Which coordinate system will the model be delivered on? How much control will be placed, and where? Will checkpoints be withheld and reported point by point? A quote that answers those questions is offering evidence. A quote that offers only a pixel size is offering a resolution.
Choosing the resolution for the job
Choose the resolution from what the imagery has to show, not from the smallest number on offer. A smaller pixel usually means flying lower, which means more flight lines, more photographs and more processing for the same area. On a large site that adds time and cost without always adding value. If the deliverable is a ground surface for earthworks, the features that matter may be large. If it is a record of kerbs, drains or small structures, a finer pixel may be worth the extra flying.
Say what the model is for when you ask for a quote. The surveyor can then propose a flying height that shows the features you need, and plan the control and checks around it. That conversation is more useful than comparing two pixel sizes with no stated purpose.
The positional accuracy standards published by the ASPRS describe how accuracy is tested with independent checkpoints, separately from image resolution.
Key point
Use ground sampling distance to judge what the imagery can show. Use checkpoint results to judge where the model sits.
Need a drone survey with stated evidence?
Tell us the site and the output. We will quote resolution and checks separately.
Get an aerial survey quoteWhere survey cannot help you
A clear specification makes the resolution and accuracy questions easier to answer, but some limits apply however small the pixel is.
Imagery cannot show ground that the camera cannot see. Under dense vegetation, inside overhangs or below water, a drone model shows the top of whatever covers the ground. For a topographical survey in those areas, ground pick-up or 3D laser scanning may be needed alongside the flight. And a model is only current for the day it was flown. On volumetric work, state the flight date with every figure.
A drone model you need to rely on?
An independent check can test it against points it was not built from.
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Aerial surveys with resolution and accuracy stated separately, and checkpoints to prove the second.
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