What is Survey Grade Accuracy?
Anyone who works in land surveying knows that accuracy is of critical importance. A survey that isn’t accurate is not just worthless, but actually dangerous. After all, if a building was constructed using bad survey data, it can cost millions of dollars to fix the mistakes caused by the bad survey.
Because of this, good surveyors, engineers, and drone pilots frequently request “survey grade” accuracy on their final deliverables. But, there is a lot of confusion about what survey grade means, and how to achieve it with a drone. In this article, we will go into detail about what truly makes a project survey grade.
Key Points:
Survey grade means different things to different people
Survey grade can be defined differently based on the goals of the project
Accuracy can only be truly measured using independent checkpoints
Accuracy estimates from software programs can be misleading
Accuracy should be tailored to each individual project
There is no single definition of survey grade accuracy
The first thing to know is that there is no universally accepted definition of what survey grade actually means. There have been numerous methodologies for how to measure accuracy over the years, from the National Map Accuracy Standards, to the more modern ASPRS Positional Accuracy Standards. While we at Aerotas prefer using the ASPRS positional accuracy standards, it is important to be aware that when one person talks about accuracy, there is a good chance that the other person doesn’t necessarily have the same definition of accuracy in their mind. So when talking about survey grade accuracy, it is important to define your measurement as well. For the remainder of this article, we will be referring to accuracy as measured by the ASPRS positional accuracy standards.
Requirements for survey grade accuracy vary by project
The second point to be aware of is that survey grade might mean different things on different projects. For example, a small survey involving wheelchair ramps may require an accuracy of 0.05’, while the federal government making a topographic map of 10,000 acres of raw land might only require an accuracy of 2’. Even though the larger survey has an accuracy that is 40x worse than the small wheelchair ramps, both can be considered survey grade.
Even within single projects, not all measurements are created equal. As previously mentioned, wheelchair ramps, along with other points such as building corners or railroad tracks often require very high levels of accuracy. Parking lot paint stripes or topographic points on the other hand, often require a lower level of accuracy. Calling a project survey grade would require understanding the needed accuracy for every point, and making sure that the map holds to those requirements.
True accuracy is measured with independent checkpoints
The only way to truly measure accuracy is to use independent checkpoints. The ASPRS positional accuracy standards contain the most complete description of how to properly measure checkpoints, but the most common mistake we see is people that try to use a single point as both a ground control point and a checkpoint at the same time. If a point has been used as a ground control point, it cannot be considered an independent checkpoint. All this means that, without independent checkpoints, it is impossible to measure a project’s true accuracy, all you can do is create an estimate. However:
Estimating accuracy can be dangerous
Estimating accuracy can be dangerous, and should only be done by experienced photogrammetrists. Many different programs or hardware configurations purport to estimate accuracy on their projects, however, this information should be used with extreme caution. Accuracy estimates, without any independent checkpoints, introduce an enormous amount of risk into a project. There are thousands of possible things that can go wrong with a photogrammetry project, and only a very experienced photogrammetrist should be relied upon to make an accuracy estimate on a given project.
Accuracy should be tailored to each individual project
Aerotas provides “survey grade” accuracy because we make sure that the accuracy is tailored to each individual project. Aerotas is proud to have a human-in-the-loop workflow, where our expert photogrammetrists review every project to ensure that it has survey grade accuracy for the project at hand. Yes, that means that some projects have higher accuracy than others, but our professionals make sure that projects are delivered with the accuracy needed for the stated goal, and the supporting information required to ensure that your surveys are always able to be considered truly survey grade.
At Aerotas, we hold ourselves to the highest photogrammetry industry standards. In this case, our guiding document is the ASPRS Positional Accuracy Standards for Digital Geospatial Data. These are the exact same standards that are used for manned aerial photogrammetry with larger, metrically calibrated cameras. However, we strongly recommend that surveyors hold back check shots on every single project and verify the accuracy of the deliverable themselves.
The accuracy and quality of your deliverables should be measured independently with checkpoints that were not used in the photogrammetry process, and those checkpoints should be evenly distributed throughout the project.
If you would like to discuss ways to improve your drone program Aerotas is here to help.
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