Surveying Dangerous Structures: Follow-up Q & A to our crumbling dam project

The Microdrones team was recently featured in the sUAS Guide for helping a government conservation agency successfully inspect a crumbling dam that was too dangerous for surveyors to access using traditional methods.

Since this article was released, we’ve received some great questions on the project that we thought we’d share since the answers could be helpful to other surveyors, civil engineers, inspectors, etc. Some of these questions are below.

What was the intended use of the data being collected?

The government agency needed extremely precise measurements so it could create an accurate orthoimage/ digital surface model as well as a 3D model.

Their goal was to issue a formal request for proposals to restore the dam. Microdrones was able to deliver the required accuracy using our md4-1000 and just two ground control points.


What was the accuracy of this project’s results?

The accuracy of the orthoimage/digital surface model was:

  • with local base station, the accuracy was 1.4 cm (X,Y) and 2.2 cm (Z)
  • with Smart Base Station (public base stations known), the accuracy was 2.7cm (X,Y) and 4.4cm (Z)*
  • with Primary GNSS (RealTime), the accuracy was 21 cm (X,Y) and 23.4 cm (Z)

*This was the perfect solution for the client because it required no base station on site, so they could just fly the system.


What kinds of challenges did this project present?

Surveying this dam was difficult for a few reasons, the obvious one being the risk to human lives posed by the condition of the dam.

Another challenge was the fact that the dam (of course) was over water. Thankfully, the md4-1000 is great in turbulent wind. Its carbon fibre housing, specially developed motors, efficient batteries, and resilient structural parts and electronics allow the drone to be used in strong winds, rain, snow, and very high temperatures up to 130° C.

During this mission, we also learned some cool things about how light reflected from water impacts our images. At the time, the effects of this light made post-processing a challenge, but these lessons will help us be more efficient in that phase of future projects like this one.

Last, the sheer size of the dam was a challenge. The dam is 350 meters x 11 meters x 16 meters.


Woah! That’s a big dam! How long did the mission take?

Believe it or not, we did it in just one day! It took 11 flights total: 1 NADIR (APX-15), 4 north of the dam, 4 south of the dam, and 3 over the dam.

The md4-1000’s super long flight times helped us to survey this huge structure efficiently. md4-1000 can stay in the air from 45 minutes to an hour and a half, depending on payload. We’re proud of the fact that this is the longest flight time in the industry, but projects don’t always require this extra-long flight time. This one did, since the drone needed to fly back and forth over the dam for extended periods of time in rough winds.


Exactly what equipment was used for this project?

To survey this dam, we used:

  • md4-1000 drone
  • 2 Axis Pro Gimbal
  • SONY A7R with 35 mm lens
  • Applanix APX-15 UAV


Can drones be used in any situation where measurements are needed in a location that is too dangerous for people to be?

Pretty much. UAS is often the way to go in cases where information is needed about a structure or location where human access is life-threatening. There are, of course, exceptions, but when the terrain is dangerous, drones tend to be a more efficient, accurate, and safe method than surveying by foot or using traditional aircraft.


Why is it a big deal to use only two ground control points (GCPs)?

Time and hassle. Setting up ground control points correctly can be excessively time-consuming. Eliminating the need for GCPs while maintaining exceptional accuracy makes a project much more efficient. In many cases, our drones can meet advanced accuracy requirements without using any GCPs at all.


How exactly did they accomplish that accuracy in one day with only two GCPs?

Two words: direct georeferencing (DG). Surveyors, mappers, civil engineers, and inspectors that require serious accuracy should strongly consider the benefits of DG when making decisions about equipment – but that’s the topic of another article!


Have a surveying or mapping question?

There’s never been a better time to ask it! We love helping people in the industry to learn about better ways to achieve their goals.

And, more importantly, for a limited time Microdrones is celebrating the passage of Part 107 with a high-value offer:ask us a relevant surveying or mapping question, and we’ll give you 60 free acres (or 24 hectares) of post-processing. No purchase is necessary – even if you’re using a competitor’s drone.

Why? We want to demonstrate our expertise to the world and help others learn more about using UAS for surveying, mapping, and inspection.

We hope to hear from you!