Accuracy and precision are 2 terms that seem to get thrown around quite a bit in the 3D GPS and grading world. Most of the time they are used interchangeably, like they mean the same thing. Catch phrases almost. High Precision Grading, High Accuracy GPS Positioning, it all means the same thing right?

Actually, they don’t. Accuracy can be defined as a way to measure how correct something is. If you have an accurate length measurement, it means that the measurement is very close to the actual length of the object that you’re measuring. 

Precision is defined as how consistently something can be measured accurately. To put that into context, a precise measurement is one that is the same every time. It doesn’t have to be accurate to be described as precise. Because precise measurements are repeatable, it doesn’t necessarily need to be accurate. If the accurate length of an object is 380mm, but your measuring tool is saying 385mm, and you measure 385mm 4 different times, that is a precise measurement because the repeatability of your measurement is spot on.

So how does this apply to GPS on our jobsite? We need to look at accuracy as the first order of business on our site. Which means getting the site set up on solid control, and a 3D Surface Model that is accurate to the grading design. These 2 things are the basis of accuracy on your site. If you can be sure that your GPS is set up accurately by using good site control, and your 3D model is accurate to the grading design, you can have confidence in your operation. 

Here’s an example of horizontal accuracy and precision. You measure a point with your GPS rover along the edge of pavement on your road. It measures, in relation to your edge of pavement line, 0.02m, or 2cm, off of the line. You can be confident that the accuracy of your measurement is good, because you set up your gps on solid control, which calibrates the linework map to the real world. We have an accurate relationship with the real world.

Now to apply precision to our job site, if we return the next day and measure the exact same place on our edge of pavement, and it gives us the same offset from the edge of pavement line on our map, 0.02m, that is precision. The fact that our GPS measured the same offset as the day before, we have good confidence in the repeatability of our measurements.

Another application of precision on our jobsite is 3D Machine Control grading. It’s pretty important that our dozer is precise, because if we grade a section of road on Monday, then come back through on Thursday, the grade on that dozer blade had better be the same as it was on Monday, or we’re in trouble. This is where precision on our grading machines plays a big role. If our machine won’t push at the same grade consistently all day, every day, how would we ever get something finished?

A good practice that we all should be doing is checking our precision on a daily basis. We should have a checkpoint or control point that we set on site the day that we do the site calibration. We know that it is an accurate point, because after we did the site calibration using the solid control points, and it measured in nicely, we set this point. Every morning when we turn our rover on, we head over to our control point and measure it. The GPS will measure your location, and it will check if the measured coordinates are the same as the control point’s coordinates. When the measured GPS coordinates match the control point coordinates, every day, day after day, we know our GPS precision is dialed in.