Comparing Accuracies | Drones - Accuracy and Opportunity for Surveyors

   Our Consulting & Services Manager, Shawn Herring, wrote a great article on surveying industry insights on drones. Here's a portion of the article:

Drones, drones, drones!  Over the past few years this may be the biggest industry trend for land surveyors and civil engineers, and probably just as important of a shift as GPS data collectors.

I started a surveying firm in 2010 with a focus on ALTA surveys, general boundary and topo projects, mass volume calculations, as well as construction staking.  We then got into a little LiDar scanning and really jumped into aerial data capture about 5 years ago and have never looked back!

I love data capture, whether from the ground or from the air, and trying to keep up with the daily technology trends and advances can sometimes be overwhelming.  I thought I’d put together this article on some of the lessons we’ve learned over the past few years when it comes to utilizing a drone in our survey department.

The Accuracies – Comparing Accuracies

         Clearly the most important thing in land surveying, in my opinion anyway, is ACCURACY! I believe this to be one of the main reasons surveyors are hesitant to add this tool to the toolbox.  There are 2 different accuracies to take into account when capturing data: absolute accuracy and relative accuracy.
Absolute accuracy is the amount of error between a model and a known point on the planet earth.  Absolute accuracy must be measured against a global coordinate system, such as NAD83 or WGS84. 
Relative accuracy effectively measures the accuracy between points within a single project.  For example, if you need to measure a building footprint, it doesn’t matter if that footprint is in Utah or Arizona as long as the measurement itself correct.

Factors that contribute to accuracy issues:

•       Flight Altitude – (Careful! – Low Flight could lead to GCP issues)

•       Overlap – 75 frontlap x 75 sidelap is ideal, but could be brought down to 70x65 if needed.

•       Lighting - (Poor or Inconstant – Shadows/Bright Lights)

•       Type of Terrain - Vegetation v. Hardscape

•       Geography / Topography - (Steep (without proper GCP / Flight Planning)

•       Quantity of Ground Control (Proper Distribution of GCP)

•      Camera Quality – In my opinion, although important, this is the least important of all factors.  If other sources of error are present, then a better camera will NOT improve accuracy at all. A 20mp camera with proper workflow should yield extremely accurate data.

What is GSD? You will hear, or have heard, the term GSD when it comes to drone flights.  This is something you will want to be aware of when fully understanding accuracies.  The Ground Sample Distance is the distance between center points of each sample taken of the ground. So basically, the GSD is how big each pixel is on the ground. The GSD is determined by the geometry of the camera and its altitude above the ground.

Here are a couple of calculators I like to use when calculating GSD.

·         https://www.propelleraero.com/gsd-calculator/

·         https://support.pix4d.com/hc/en-us/articles/202560249-TOOLS-GSD-calculator

 

Below is a screen shot comparing accuracies via total station, RTK GPS and Photogrammetry.

If you'd like to learn more insights on drones in the land surveying industry, click here.