Accuracy vs. Precision

Client: “I’m not getting the accuracy I’m supposed to out of my GPS.”

Representative: “What has led you to believe that?”

Client: “It is not lining up with my aerial imagery” (insert historical data, monumentation, the onsite surveyor’s readings, Google Earth, etc.)

Representative: “Really? I’m sorry to hear that. How far off is it?”

Client: “Always 3 feet north and 8 feet east.”

Hopefully an alarm is sounding off in your head. We TOTALLY get it. Generally in the mapping world, especially with high accuracy, we need our positional data to align with our photogrammetry. We make maps and the most ‘base’ layer, which is usually the aerial imagery, provides the foundation to spatially correspond with all of the other data (for cartographic purposes).  If you have experienced this problem, chances are you spent a lot more on the imagery than the hardware.

To understand this better, let’s have a quick refresher regarding accuracy and precision. For the purposes of this article, I’ll be referencing the definitions right out of Microsoft Word (where I’m typing this document):

Accuracy – the correctness or truthfulness of something
Precision – exactness or accuracy

Hmmm… still somewhat of a vague definition as it relates to the geospatial world.

To clarify, let’s define accuracy as how close any measurement is to some “True” value, and precision as how close measurements are to each other. Have a look at the following graphic:

bullseye image showing the difference between accuracy and precision

Let’s further break down precision into two components:

  1. Repeatability: The variation observed when the same operator measures the same part repeatedly with the same device.
  2. Reproducibility: The variation observed when different operators measure the same part using the same device.

Now let’s talk about accuracy in regard to our high accuracy GPS hardware. You are accurate when your precise measurements align or correspond with a standard (or datum). There are many standards. Here are a few of my favorites:

    • IGS08 epoch 2005
    • ITRF2000 epoch 1997.0
    • NAVD88

GIS professional mapping his assets

Whether you are familiar with these or they are Latin to you, it is important to understand that once you can define what standard you are trying to be accurate to, you can “shift/calibrate/offset/transform” your precise observations onto the ‘bullseye.’

Going back to the original dialog, I generally ask my clients to take a reading on the same location enough times to prove to themselves that their hardware is precise, meaning all of their positions fall within the hardware specifications. Once that is accomplished we only need to find the reason they are precise but not accurate (yet).

Below are two potential reasons for inaccuracy:

1.     Datum shifts
2.     Units (in State Plane there is a significant difference between Feet and Survey Feet).

Fortunately, you do business with CSDS and our representatives are very knowledgeable. If we can’t find the solution our Support Department is even more knowledgeable. If Support can’t find the solution we can consult the oracle at Trimble. We have your solution.

Contact CSDS for more information

If you would like to learn more about GPS, call 800-243-1414 or email You may also visit the Mapping area of our website by CLICKING HERE.

Jon Gipson, PG, GISP

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