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Doing The Impossible with LiDAR

Doing the impossible tp creating a top of bank survey.

Across the country, housing demand has far surpassed supply. With that, every part of the construction and development sector has been hit with new challenges. Survey companies are no exception. In some places in Texas, the wait time for a property survey is more than 3 months. There are simply not enough surveyors to do all the work.

Fortunately, drones offer many solutions to speed up the process. A topographic survey that would take months to do can be done with more detail and more precision in a fraction of that time. However, drones are not the magic wand some believe they are. The skill and technique that some areas of drone operations take cannot be engineered away by the big-name drone companies. Take for instance the DJI L1. DJI promised it would be the LiDAR system to end all LiDAR systems. Instead, it turned out to be a very imprecise LiDAR that is best used for crude planning purposes only.

Given the technique and experience needed, a drone and an adequate LiDAR system can be an extremely powerful tool. Just like other pieces of highly specialized technology, it isn’t something an operator can pickup and do with ease. Once mastered, the capabilities are truly amazing.

Recently, Lone Star Drone was contacted by a survey company that was thinking outside the box. Instead of subjecting their operators to many days of summer heat over 100°, they called us and asked if it was possible to get a detailed enough LiDAR scan to accurately judge a top of bank to top of bank on a creek. The property line was being redrawn and this was the terrain feature that set the line. LiDAR, of course, is exceptionally adept at getting through tree canopy to scan the ground surface below. What most DON’T realize, is that it is equal parts equipment and equal parts technique to get a proper scan. Moreover, not just any LiDAR can get a detailed enough scan to produce a complete terrain model. There will inevitably be holes in that data. This becomes a problem when a surveyor’s professional credentials are on the line and the area being scanned needs to be absolutely complete.

Top of bank to Top of bank:

Property lines are often defined by key terrain features similar to country borders defined by rivers, mountains, and oceans. In both cases, issues occur when those lines shift or change. Erosion, receding water tables, or rerouted rivers and creeks pose a problem when property lines need to be defined. In most cases, a survey company will use older property records to reestablish the property lines and define them based on Latitude/Longitude, or through other terrain features. When the creek or river still exists, surveyors will usually reestablish the lines based on what is called a top of bank line.

Top of bank

Top of the bank simply means that where the creek or river transitions to a hard break to the area of normal water flow. Accuracy is key when it comes to creating a top of bank survey. Because the property line depends completely on where the highest point is on both sides of the creek or river, if the accuracy of a topographic survey is not completely spot on, the property line may be drawn along the wrong path. This is why a LiDAR mission that cannot capture the area bank lines completely is of little use to a surveyor. Guess work cannot be used when creating property lines, and a top of bank survey is no exception. Without every inch of the top of bank mapped, the lines cannot be created.

When we agreed to do the job, we had to make it expressly clear that, despite being the best in the industry at LiDAR, there was a possibility that the scan wouldn’t work. It is our belief that you should never over promise and under deliver and to manage expectations is always a good thing. When we pulled up the satellite image of the area it was clear that we had our work cut out for us. The canopy what completely solid and there was no break in the trees.

The canopy over this creek was completely solid.
Tree cover of the creek

When we arrived on site, the tree cover was as we had seen with very little daylight making it through. We set to work, though, and began to scan the area. Our technique was completely different than we regularly use on most properties. We set GCPs along the side of the creek that had the most open field beside it. With the several we managed to place on the opposite side of the creek, the data truthing would be more than enough. In this case, there was no ortho taken, so our GCPs were there only to make sure there was no IMU screw occurring.

If the job was to capture the tops of the trees, mission accomplished.
LiDAR model of the tree canopy

Arriving back at base we began to process the data and classify the trees. When we saw the bottom of the pointcloud we could rest a bit easier. It was absolutely flawless and well within our quoted delta of accuracy at just 2cm. When we delivered the data to the client we asked if the value add justified the LiDAR expense since the survey company didn’t bill the client for LiDAR. “Not having to have my field guys out there in this heat and breaking brush for days on end, yeah, it was absolutely worth it.”

The final model was crystal clear and covered every inch of the top of bank on both sides.
Final TIN model

These are the situations that make us feel good. Allowing a survey company to mitigate the risk to their field crew is worth a lot to us. Utilizing high end LiDAR and techniques to accomplish an impossible mission prove that our folks at Lone Star Drone are truly the best!

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