Ground sample distance is the most useful number in a drone mapping quote and the one clients most often skip past. It is usually written as something like 1.4 cm/px. Understanding what it means makes the difference between commissioning a survey that answers your question and one that does not.
What the number means
Ground sample distance, or GSD, is the real-world size of one pixel in the imagery. At 2 cm/px, each pixel covers a 2 by 2 centimetre square of ground. At 5 cm/px, each pixel covers 25 times the area, and everything smaller than that square simply is not resolved.
It is set almost entirely by two things: how high the aircraft flies, and the sensor and lens it carries. Fly lower, get a finer GSD, cover less ground per battery. That trade between detail and coverage is the fundamental economics of a mapping flight.
Resolution is not accuracy
These get conflated constantly, including by people selling the service.
GSD describes how much detail you can see. Accuracy describes how correctly positioned that detail is. A flight can produce beautifully sharp imagery that sits two feet from where it belongs, and a coarser flight tied to good ground control can be far more correct while looking less impressive.
You need enough resolution to identify what you care about, and enough control to trust where it is. They are separate problems with separate solutions, and a provider who answers an accuracy question with a resolution number is dodging.
A rough guide to what you need
These are general ranges rather than rules, and the right answer depends on your deliverable.
Around 1 to 2 cm/px. Detailed work. Reading paint markings, identifying rebar and small utilities, crack detection on surfaces, anything where you need to see fine features clearly. Slow to fly, heavy to process.
Around 2 to 4 cm/px. The workhorse range for most construction mapping. Sufficient for topo, earthwork volumes, progress documentation, and general site conditions. Good balance of detail against flight time.
Around 5 cm/px and coarser. Broad area work. Corridor mapping, large parcels, early planning, overview imagery. You will not resolve small features, and that may be entirely fine.
Why flying finer is not simply better
It is tempting to ask for the finest number available, but the costs are real.
Lower flights mean more flight lines, more batteries, and more time on site, which on an active job means more coordination and more disruption. Image counts climb steeply, and processing time and storage climb with them. A site flown at 1 cm/px can produce several times the data of the same site at 3 cm/px, with correspondingly longer turnaround.
More subtly, finer GSD does not improve a bare-earth terrain model as much as people expect. If the ground is obscured by vegetation, resolving that vegetation in exquisite detail does not tell you what is underneath it.
Asking the right question
Rather than requesting a specific GSD, describe what you need to be able to see or measure. The smallest feature you must identify, and the tolerance your measurements need to meet, are what should drive the flight plan.
If you need to spot a displaced manhole cover, say so. If you need earthwork quantities within a given tolerance, say that instead. A provider who understands the work will translate that into an altitude and a control plan, and will tell you plainly when the two requirements pull in different directions.
