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Accuracy: Relatively Right but Absolutely Wrong

A common question I hear is “how accurate are drone surveys.” If you’re a surveyor, you might argue that it’s a subject that fills whole books, classes, or even a college degree. With that said, understanding the difference between “relative” and “absolute” accuracy is something anyone making drone models should understand.

Absolute vs. Relative Accuracy

Let’s start with “relative accuracy.” Relative accuracy is the amount of error relative to itself. Maps and models with high relative accuracy are scaled well and will give you good dimensional data. You’ll be able to tell the amount of curbing to the inch or the volume of a stockpile to the cubic foot. The problem is your computer may think the model is on the wrong side of the planet! Enter in “absolute accuracy.” Absolute accuracy is the amount of error with relation to a known coordinate system. Essentially, it’s how well your model is positioned on the earth. You know your model has high absolute accuracy when comparing the model's computed locations against known ground control points (GCPs).

So What Kind of Accuracy am I Getting with my Drone?

The GPS units in most commercial drones are plus/minus 1 meter in absolute accuracy. With that as a limitation, the maps they produce generally have low absolute accuracy. However, if you use software that stitches based on image pixels and less so on GPS metadata, high relative accuracy is very common. ContextCapture does this, which is one reason I use it in my Applied Drone Technology course. For most contractor workflows, relative accuracy is what they need. If you get the right quantities, who cares where the model is located. I tell my folks all the time that as long as their maps are “relatively right, it’s fine if they’re absolutely wrong!”

How to get Both Relative and Absolute Accuracy

The most common uses where contractors want high absolute accuracy are with enhanced utility as-builts, comparing models of the same site but at different times, and reviewing work in place against a BIM file. So how can you get better absolute accuracy? One way is to use a drone with a RTK unit. RTK, or “real-time kinematic,” has a stationary base station that sends GPS correction to the drone in real-time. Essentially, it makes the drone GPS a lot more accurate. However, the best way to increase absolute accuracy, and relative for that matter, is with ground control points (GCP). GCP locates your model on the earth and improves the scaling by calculating the distance between known points. Think of your model as a sheet of rubber. The GCPs are pins that stretch it to the proper dimension and position it on the planet.

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