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Which Drone Sensor is Right for Your Business?

Tuesday, November 6, 2018   (0 Comments)
Posted by: Logan Campbell and Daniel Katz, Aerotas
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Article originally published by Logan Campbell and Daniel Katz, Aerotas. Republished in the Summer 2018 issue of the ISPLS Hoosier Surveyor.

 

Our goal at Aerotas is to provide the best drone solutions for land surveyors. To achieve this goal, we constantly stay up to date on the latest developments in survey drone technology. We are not attached to any specific providers of drone-related technology; remaining independent so that when better tools are available, we can provide them to our customers. When analyzing technology, we focus on cost-benefit analysis: what delivers the most benefit to the average surveyor for the lowest cost.

An effective drone program involves dozens of components, and we regularly test the options available for each. The three specific technology components we get the most questions about are drone airframes, sensors, and georeferencing options.

This article begins a three-part series covering those three technologies. These articles summarize our current analysis of the options for each of these components based on our continual R&D and our work with hundreds of surveyors nationwide.

It is important to understand that the drone is only one part of a successful drone program. Even the best drone will not deliver the survey or business results needed unless it is paired with the right field Standard Operating Procedures (SOPs) and data processing workflow to get final linework.

Types of Drone Sensors for Surveyors

There is an incredible variety of sensors made for drones, for the incredible variety of applications drones are used for. The drone sensors most surveyors are likely to consider fall into four general categories: built-in cameras, small independent cameras, high-end independent cameras, and LiDAR. The first three options are different types of standard cameras, used as photogrammetry tools, while a LiDAR sensor is effectively a laser scanner mounted to a drone. Each of these sensors has it merits and drawbacks. All four are viable to be used in an effective drone survey program with the right SOPs and processing workflow, though they differ in their real-world applications.

 

 

Built-in camera

Small independent camera

High-end independent camera

LiDAR

Airframe requirement

Small multi-rotor, which camera is built into

Fixed wing, or custom small or medium multirotor

Large multi-rotor

Large multi-rotor

Common price, with likely airframe

$1,500

$2,000-$40,000

$20,000+

$150,000+

Real-world accuracy

0.1’

0.1’-0.3’

0.1’

0.2’-0.3’

 

Built-In Camera

Built-in cameras are designed and built specifically for use in the drone they are attached to. They are completely integrated into the drone airframe and cannot be removed nor replaced without significant manual modification. While only a small number of drone manufacturers build first-party cameras, they have proven to be so dominant as to deserve their own category. DJI is the largest manufacturer of these drones, with the Phantom series being the one most commonly used in survey applications (note: the DJI X4S camera carried by the DJI Inspire and M200 series is functionally identical to the Phantom 4 Professional or Advanced camera). Built-in cameras often have as large as 20 megapixel sensors with global shutters (as opposed to rolling shutters, which can cause image distortion), and are most often carried on small multirotor airframes. 

The primary benefit of this type of sensor is its high accuracy capability relative to its low cost and high reliability. With the right field SOPs and data processing, built-in cameras can reliably produce survey data at better than 0.1’ accuracy. They are also extremely inexpensive, with the DJI Phantom series costing $1,500, including the sensor. Given that they are built specifically for use in these aircrafts, these sensors are extremely simple and reliable. This means minimal maintenance, calibration, and downtime, and maximum return on investment.

Since they are built into the aircraft, however, these sensors are less flexible, since they can’t be easily swapped out for alternate sensors. Because the image sensors are smaller than some alternatives, they need to be flown fairly low (~100’) to attain the 0.1’ accuracy, meaning their range is somewhat limited, to approximately 25 acres/hour.

With their low cost, high reliability, and high accuracy, built-in sensors are usually the best bet for most surveyors focused on topographic and planimetric mapping on projects less than 250 acres.

Small Independent Camera

These are third-party sensors mounted onto airframes either by the drone manufacturer or aftermarket modification. They are often ~20 megapixel image sensors with global shutters. These are the most common option for fixed-wing aircraft, which have more weight limitations, as well as some custom-built small multi-rotors. Due to the mechanics of fixed-wings and the complexity of custom integration on small multi-rotors, these cameras are often not mounted on a gimbal — a device that allows the camera to move independently of the airframe.

A main benefit of these cameras is their ability to be used on fixed-wing airframes, which have longer ranges than multi-rotors. If used on a multi-rotor with a gimbal, they can produce equivalent accuracy to a built-in sensor. Since they are not integrated into the airframe, they can be swapped easier than a built-in sensor. 

Because they are not built into the airframe, however, they often require more complex work, calibration, and maintenance than a built-in sensor. Specifically, in our experience, the camera shutter-trigger mechanism can be particularly challenging, resulting in unpredictable data-collection failures. Whenever a camera is used without a gimbal, as on nearly all fixed-wings, there are additional data quality issues. Without a gimbal, whenever the drone vibrates, turns, or banks to fight wind gusts, images will be blurred, resulting in lower-accuracy data.

The best use-case for a small independent camera is on a fixed-wing drone, when large acreages need to be covered at lower accuracy. If a large project only requires spot elevations sufficient for one-foot contours, this type of sensor is an excellent choice.

High-End Independent Cameras

These sensors are larger cameras developed for uses other than drone mapping (e.g., digital SLR cameras), which must be carried on large multi-rotor airframes that are designed to carry large sensors. These cameras can often have up to 40-megapixel sensors.

The primary benefit of these sensors is the very high image resolution they can produce, which translates to lower (better) ground sampling distance in aerial imagery. This allows them to consistently produce survey data accurate to 0.1’ when used with appropriate field SOPs and data processing. Because of their higher resolution, they can achieve this accuracy at higher flight altitudes than built-in cameras, meaning they can cover slightly more ground and clear tall obstacles without sacrificing accuracy.

The main drawback of these sensors on a drone is complexity. They almost always require custom integration, thus being more prone to faults and requiring regular significant maintenance and calibration. The shutter-trigger integration is often particularly fault-prone, and managing autopilot settings to ensure consistently optimal overlap is challenging. Despite the higher resolution of the camera, there is no benefit to accuracy since they must be flown higher to avoid warping and artifacting in data-processing. Despite the larger image sensor, range benefits are minimal due to the heavier camera and airframe. Finally, high-end cameras and the airframes they require are quite expensive, making it harder for a business to get a return on the investment.

Our analysis is that a built-in camera is usually preferable to a high-end independent camera, due to getting equivalent accuracy cheaper and more reliably. The best use-case for a high-end camera is if very high-resolution orthophotos are required as base-maps on special projects, or if an airframe with swappable payloads is required (e.g., to swap for a thermal sensor for roof inspections) – though often it will be cheaper and more reliable to just have separate dedicated drones for other sensors. Regardless, high-end cameras are only recommended for very experienced custom drone technicians.

LiDAR Sensor

Several companies are starting to make drone-specific LiDAR sensors. These sensors produce point-cloud data, which require laser scanner software to manage and reduce into usable survey deliverables.

The primary benefit of LiDAR sensors over cameras is that they can penetrate some ground cover. On projects which have some sparse tree, bush, or grass cover, LiDAR can return some true ground elevations beneath the cover, reducing the amount of supplemental ground data collection needed.

The primary drawbacks of LiDAR come down to complexity and cost. LiDAR integrations are highly technical and complex, so very prone to faults, and requiring a great deal of time-consuming calibration and maintenance. LiDAR data management is also very complicated. Whereas drafting linework from photogrammetric orthophotos and 3D mesh surface models is somewhat straightforward, LiDAR requires working in point clouds. This requires very high-powered computers and a time-consuming process of selectively reducing point clouds down to only the points needed to create the surface. Despite this added complexity, LiDAR sensors are substantially less accurate than cameras – though LiDAR lasers are very precise, their ground-tested accuracy is usually around 0.3’. Finally, LiDAR is very expensive, making for a challenging business investment.

Our current analysis shows that LiDAR is still maturing. With its high cost to accuracy ratio, it is not a good investment for many survey firms today. However, for firms that frequently work on sites with moderate to sparse ground cover and have a great deal of experience with custom drone technology, LiDAR sensors, and point cloud management, these sensors could make a profitable option.

 
Choosing the right tool for the job
 

 

Built-in camera

Small independent camera

High-end independent camera

LiDAR

Benefits

· Simple & reliable

· Best-in class accuracy: 0.1’

· Inexpensive

· Capable of greater range on fixed-wing

· Capable of 0.1’ accuracy on multi-rotor

· Can be interchangeable

· Very high resolution

· Best-in class accuracy: 0.1’

· Can achieve 0.1’ accuracy at higher flight altitude

· Slight range benefit over built-in camera

· Interchangeable

· Can penetrate some sparse ground cover

Drawbacks

· Inflexible: can’t interchange sensor

· Limited range (~25 acres/hr at 0.1’ accuracy)

· Integration complexity can lead to unreliability

· Data quality issues when used without gimbal

· Integration complexity often leads to aircraft and data unreliability

· Substantial maintenance needs

· Expensive

· Integration complexity often leads to unreliability

· Substantial maintenance needs

· Data management is very time-intensive

· Low accuracy

· Very expensive

Best for

· Most standard survey work <250 acres

 

· Use on fixed-wings for regular very large projects with low accuracy needs

· Very high-resolution imagery

· Very drone-experienced team

· Frequent work in sparsely-vegetated areas

· Very drone-experienced team

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

There is no one right choice that applies to every company. For most firms focused on small to medium sized topographic, ALTA, or similar projects, a drone carrying a built-in camera is usually the best option. For firms focused on large projects with lower accuracy requirements, a small independent camera mounted on a fixed-wing aircraft can be a great choice. For firms with substantial drone experience that want to differentiate themselves via very high-resolution imagery, a high-end independent camera can work well. And for firms that regularly work on sites with some ground cover, have large budgets, and have ample experience with point cloud management, a LiDAR sensor will work well.

Logan Campbell and Daniel Katz are Co-Founders of Aerotas, where they enable land surveyors to use drones to get survey linework and contours with industry-best accuracy. Learn more at www.aerotas.com.


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