Proper ground control for aerial drone mapping flights is essential to ensure accuracy. Ground Control Points (GCPs) can ensure the accuracy needed for drone mapping and have become an essential tool for many professionals.
In 1972 the U.S. Geological Survey (USGS) joined forces with NASA to create Landsat. This joint mission was to provide a series of data points, satellite images, of the Earth’s surface known as Landsat Ground Control Points.
Historically, Landsat Ground Control Points, or GCPs, have been in continuous use since their inception in 1972, although their use and sophistication has increased dramatically since that time. These original Landsat GCPs were once used to provide land managers and government policy makers with an uninterrupted flow of data on the Earth’s natural resources and environment.
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Today, drone mapping utilizes a more modern form of this technology for aerial mapping because it ensures pinpoint accuracy. Drone flights already make very sophisticated use of the Global Position System (GPS), but the accuracy of GPS alone is not adequate in many use cases. GPS by itself does not always translate to the pinpoint accuracy on the ground required for complex land development projects, construction sites, or general surveys. This is where a current day use of GCPs is essential.
What are Ground Control Points (GCPs)?
Ground Control Points, as defined by the USGS, are “points on the surface of the earth of known locations” used to reference Landsat data. These points are continually improved upon, and these data points can be downloaded and used as reference data for any number of applications. Relating to drones, GCPs are defined with a specific application in mind.
In its most simplified form, GCPs for drone mapping are large, marked targets placed on the ground and spaced strategically around your area of interest.
"In drone surveying, ground control points (or GCPs) are points on the ground with known coordinates that a surveyor can precisely pinpoint... down to the centimeter."
These “known coordinates” on the ground can come from a variety of sources such as the Global Positioning System (GPS), ground surveys, geocoded images, vectors, Geographic Information Systems (GIS), topographic maps, chip databases, or by using the photogrammetric process to extend the number of GCPs in your images.
Those are some of the more complex sources for GCPs, but GCPs can be anything a drone can easily recognize in the image data captured by the drone. More commonly, this can be something like a black and white checkerboard square, which tend to make very precise “points,” or it could be a slash of orange paint on the ground in the form of an X. The hand painted GCPs tend to sacrifice some accuracy for speed, but they will work for certain applications.
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Regardless of the type of device used, what makes a good GCP for drone applications is the ability to record the marker clearly and accurately in the image data for later post-processing by the software.
Permanent "Aerial Target" Spray Painted on Asphalt
How do GCPs Work?
A Ground Control Point determines the relationship between the raw image taken by the drone and the point on the ground. This calculation is achieved by associating the pixel and line of the image coordinates to the x, y (horizontal axis), and z (vertical axis) coordinates on the ground.
An easy way to think about GCPs for drone mapping is to think of each point as a “thumbtack” on a map (placed on the ground). These thumbtacks are laid out in a grid or specific pattern required for the survey. When the drone flies over these points, specialized software can then use these thumbtacks as points for processing.
To make this work, once the GCPs are placed on the survey site, the GCP target locations need to be measured by the drone operation company or survey crew. The exact center of each GCP must be exactly measured, typically by using precision survey equipment, like a GNSS Receiver, placed in the very center of the GCP. These measurements are taken using specific standards, usually to a specified European Petroleum Survey Group (EPSG) code and should be accurate within two centimeters.
Video Credit: Propeller
Once the flight has taken place, the GCP measurements data and the image data captured by the drone are processed using the specific application software chosen for the project. The exact process, workflow, and software used would vary depending on the drone company or requirements of the project.
When to Use GCPs During Drone Mapping
Every drone mapping project is unique and requires thought and planning to ensure success. The time needed, cost involved, accuracy requirements, the terrain being surveyed, and the capabilities of the drone equipment, would all need to be taken into consideration when trying to determine whether to use GCPs for any given project.
“In cases where the aerial survey data is not corrected by real-time kinematic (RTK) or post-process kinematic (PPK) methods, GCPs need to exist at frequent intervals around your surveying area so that the map result is more accurate than standalone GPS-grade accuracy (a few meters).”
Not all drones and cameras are built the same, and they don’t all produce the same results. Drones with RTK or PPK payloads have improved significantly over the past several years, but still involve a significant investment by the drone user. If the equipment is reliable, it will greatly reduce the number of GCPs required for accurate measurements or eliminate the need for GCPs altogether.
In most use cases though, drone mapping with GCPs can greatly reduce costs and increase accuracy for companies who in the past once relied on traditional land survey methods or fixed wing aerial surveys.
"Landpoint, a surveying company based in Louisiana, uses ground control points when creating drone maps used for topographical surveying. Using GCPs on an 85-acre map, their team conducted an accurate aerial [drone] survey, saving over 80-man hours compared to traditional land survey methods."
Each project has its own very specific requirements. As with most things, every project should have an adequate cost-benefit analysis done to determine if using GCPs would provide an overall benefit for the project in question.
Are GCPs a Requirement When Drone Mapping?
Drone mapping starts with photogrammetry, the science of collecting physical information from two-dimensional drone imagery. Software using multiple images, angles, distance, and elevation information, creates a three-dimensional map with GPS coordinates and accurate measurements.
In the past, GCPs have been a traditional part of drone mapping projects, and a required element of successful mapping projects. With the advancement of RTK enabled drones and AutoGCPs, this “requirement” is starting to change. Testing has shown that drone mapping without using GCPs may produce acceptable results, but it depends.
“Skipping ground control points may yield perfectly fine results, but your reconstruction might not have the correct scale, orientation, or absolute position information. GCPs or RTK geotags can help you verify the accuracy of the reconstruction.”
If the project is being completed without RTK capable drones, then yes, GCPs would be necessary to produce accurate results. Even if the drone is RTK capable, the improved accuracy of using a combination of RTK and GCPs still might be beneficial. Ultimately, the details of each project, professionally evaluated by the drone or survey company, should decide what setup works best for each situation.
How Many GCP’s Do I Need?
It can be very time consuming to place GCPs over the area being surveyed. Trying to decide how many GCPs to use for a given project may not be as difficult as you might think. It is recommended that you collect more than the minimum to ensure accuracy, but more isn’t always better.
Studies have shown that collecting over 20 GCPs per image does not significantly improve the accuracy for most models. However, the quality of the GCPs used does tend to impact the number of GCPs needed to ensure accuracy.
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"Independent tests show you need fewer GCPs than you think for accurate results. The Nevada Department of Transportation found that placing more than 5-10 GCPs had very little benefit."
If the requirements of the project are complex and need extremely accurate results to correctly determine the number of GCPs needed, a series of tests can be run and evaluated. A professional drone company would be able to make this determination.
Where do I place GCP’s?
Just as important as how many GCPs should be used, where you place the GCPs will impact the overall accuracy of the results. If the survey site was a simple square, you would place a GCP at each corner and one in the middle. But most sites are rarely a perfect square.
To improve the accuracy, the drone company will want to collect GCPs evenly throughout the area at a variety of elevations and in places where images overlap. The GCPs always need to be highly visible, free from obstructions such as vegetation and other obstacles which might obscure a clear view of the GCP.
In most cases you will want to place the GCPs at least 50 feet from the boundary map area and away from high traffic areas, especially on active construction sites. This will better ensure the GCPs don’t get moved, run over, or destroyed. If high traffic areas can’t be avoided the best alternative would be to use cones, survey stakes, and neon ribbon, placed nearby. This will reduce potential conflicts with heavy equipment and construction operations.
High visibility cones placed nearby a GCP
When Placing GCPs Keep in Mind
- Keep them low to the ground for consistency
- Evenly distribute GCPs across the flight area
- Place GCPs at the corners or sharp changes in direction
- Place GCPs at least 50 feet from the boundary of the map area
- Avoid high traffic areas and active construction sites
- Use caution with natural reference points such as buildings
- If using tarps be mindful of changing wind conditions to prevent doming (wind raising the center point of your marker)
- Carefully evaluate changes in elevation
- Place GCPs in high, low, and middle elevation ranges
- Don’t place more GCPs than you need for the flight
How to Make DIY Ground Control Points
Creating your own GCPs with commonly sourced materials is relatively easy to do, and most will provide perfectly accurate results. A few examples of items you can use would be things like:
- 5 Gallon bucket lids with taped black and white patterns
- Plastic or vinyl squares with black and white grids
- Spray painted surfaces with contrasting colors (circle with X or a square with crossed lines)
- tarps with taped or painted markings
- black and white checkered flags (often with numbers to identify which target you are viewing)
Source: PIX4D Community
With any of these materials there are several things to keep in mind to ensure accuracy. First, you need to know the resolution of your drone’s camera. What size sensor are you using? The higher the resolution, the greater the detail visible in the flight, and in turn the smaller the target needs to be to be identifiable. Second, how high are you going to fly? The higher the flight, the larger the target needs to be to maintain the same accuracy.
Next, with these parameters in mind, you will need to calculate your Ground Sampling Distance (or GSD). This calculation will give you a starting point for creating your own GCP that is the proper size for your project. Using the proper size GCP will make the marker easier to identify and record.
The Best GCP’s For Drone Mapping
While you can use a variety of sources for GCPs, the quality of your GCP will directly impact the accuracy of your results. As noted above, it is best to choose GCPs that are easily identifiable at the base raw resolution of your image.
Something else to look for would be to choose GCPs that lay flat on the ground or can be staked down. Due to “perspective distortion,” and to a lesser extent, doming, targets that rise above the ground may appear to lean in the image, changing the accuracy of the reading. Therefore, using buildings does not always produce the best results.
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Some Great Examples of GCPs on the Market Today
Propeller AeroPoints 2
The AeroPoints 2 are the second generation GCP from Propeller. They designed these GCPs to be easy for beginners but feature rich for any veteran drone company. Each GCP has its own GPS data with wireless upload to your user portal, and they built mobile app management into the AeroPoints.
Since Propeller integrated the data transmission as part of the GCP it eliminates the need for taking your own readings, saving a huge amount of time. You just lay down the GCP, press a button, and move on.
- Pros – Durable, lightweight design, one-button operation, wireless upload, mobile app-based management.
- Price – $8,000 (10 Pack)
- Where to Buy – Vertigo Drones, Baseline Equipment, Multicopter Warehouse
Hoodman Ground Control Point Kit
The Hoodman GCP Kit comes with five three-foot diameter GCPs. They are designed and built to be rugged, with highly visible orange and black panels. Each GCP has a steel perimeter cable that snaps into place when expanded. They are made from long lasting, easy to wash polyester, and have large sequential numbers printed for easy identification in post-processing.
The GCPs themselves are weighted to eliminate the need for staking during short projects, and each GCP has a center grommet for surveyor staff placement for pinpoint accuracy of GPS coordinates. The entire kit comes with a carrying bag and weighs around 22 pounds.
- Pros – Lightweight, folds to 1/3rd its size, high visibility, relatively inexpensive, different sizes and shapes for different projects
- Price – $800 (10 Pack)
- Where to Buy – Amazon, Adorama, B&H, Blue Skies Drones, Vertigo Drones
Sky High Bull's-Eye XL (48"x48") UAV GCPs
The Sky High Bull’s-Eye GCPs are made and designed by fellow drone pilots to be lightweight, portable, and durable. As a small business startup, they set out to solve some of the issues they found with “home-made” GCPs and created an inexpensive yet high quality product.
Sky High Bull’s-Eye GCPs are made from non-reflective material to avoid glare even on the brightest flights, and have two options for center point staking, a center eyelet or a passthrough cutout.
- Pros – Small business owner, lightweight foldable design, large easy to identify, long lasting mesh material, affordable on almost any budget
- Price – $120 (6 Pack)
- Where to Buy – Amazon, Sky High Bull’s Eye
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A List of Red Flags to Avoid When Hiring Drone Service Providers.
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SECO Drone Ground Control (Survey) Targets
SECO is known to be on the cutting edge in designing and manufacturing tools for surveying, construction, and other industries that utilize GPS/GNSS technology. Their Drone Control Targets are compatible with any drone software that recognizes GCP targets for drone mapping.
Each Drone Survey Target is an 18″ square, made from rigid weather resistant Aluminum Composite Material (ACM), has a 1/4″ center hole, and each is numbered for high visibility identification. Each GCP weighs 1.65 pounds and the set with a carrying case weighs close to 20 pounds.
- Pros – numbered targets, rigid weather-resistant ACM panels, 10 targets include per bag, includes carrying case
- Price – $700 (10 Pack)
- Where to Buy – Baseline Equipment, Survey Supply
Additional Drone Mapping Equipment You Should Have
A professional drone mapping company will need a variety of equipment. This list is a starting point for some of the equipment needed, not a complete list. It is important to remember that in addition to the equipment listed below, training and experience in using this equipment are skills a professional drone company acquires over a long period of time. Don’t expect to be an expert in each of these areas immediately, perfecting each one takes time.
RTK Equipped Drone
An RTK equipped drone carries an onboard GNSS RTK receiver that gathers data from satellites and stationary ground stations to more accurately correct flight data in real time. This is essential when carrying out complex drone mapping projects. When combined with GCPs the results are highly accurate.
DJI Phantom 4 RTK Drone
HD Camera with Mechanical Shutter
Camera resolution is vital to drone mapping and accurate GCP readings. Drones may or may not come with their own camera setup. Check the specifications of each camera setup carefully. One with a resolution of at least 1920 by 1080 (or 1080, HD) and a mechanical shutter is highly recommended. A mechanical shutter is preferred because they still outperform electronic shutters in action/motion capture.
Base Station
A base station is a highly precise GNSS receiver that supports all major global satellite navigation systems and provides real-time differential corrections. This generates centimeter-level positioning data for your flight and greatly improves accuracy. It is a great tool for drone mapping, but depending on the GCPs in use, it may be a required piece of equipment to complete the project.
GNSS Base Station
Processing Software
This software will turn the real-world landscape into a digital asset your client requires, and as such, the correct processing software for drone mapping is essential. The processing software takes all the raw data from the drone flight, plus the GCP data, and creates the assets and information requested by the client.
Flight Planning Software
Flight planning software is more than software to control a drone’s physical location during a flight. Flight planning is as much about flight data management and setting flight parameters and is an important part of drone mapping.
The software allows pilots to create custom flight zones for the drone to follow, set waypoints, define actions for those waypoints, and set the end-mission action items.
Screenshot of Dronedeploy Flight Planning App
Tablet for Navigation
Tablets with large screens are not necessarily a requirement for flying, but they are highly recommended. The larger the screen size the more detail available and visible to the pilot. This becomes more important as flights become more complex. Tablets with screens at least 9.5″ in diameter are a plus, the newer iPad Air tablets strike a good balance between power, screen size, and cost.
Landing Pad
A high visibility landing pad is something that is very inexpensive and provides a great deal of value. They are easy to set up and will keep your camera lens clear of debris during take-off and landing.
Heavy Duty Drone Landing Pad
Extra Batteries
Mapping missions can result in hours of flight time, especially with higher overlap, extra batteries are a must. Before you purchase any third-party batteries be sure to read up on the battery capacity specifications. Even though OEM batteries tend to be more expensive they often provide a much higher overall value. High quality batteries will provide more consistency in flight and less frustration for the pilot.
SD Cards
Choose high quality SD cards. SD cards are the backbone of your flight, so if an SD card gets corrupted mid-flight or in post-processing, it can be very time consuming and costly to correct. Different cards write at different speeds, and a high write speed is essential. As with batteries, buy high quality SD cards like the SanDisk Extreme Pro.
Getting Started: Hiring a Professional Drone Mapping Company
There is more to successful survey mapping than the drone flight itself. Drone mapping can be a very complex and highly skilled craft to perfect. Understanding the workflow involved from start to finish, from the client’s requirements, to delivering the finished product, are all vital skills for a professional drone mapping company.
When looking to hire a drone mapping company, they should be able to evaluate the needs of your project and design a customized plan specific to the outcomes you desire. A highly experienced professional will be able to process and analyze all the data collected and give you the tools you need to make the best decisions possible for your company. This is where hiring licensed professionals can save you time, money, and frustration.
The Drone Life has extensive experience with drone mapping from evaluating and planning, through delivery of the finished product. We work with owners, developers, general contractors and more to create unique plans specific to your project. We have an expert team of highly qualified licensed professionals ready to document your very own project. You can schedule a free consultation with a drone expert today.
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What's included in YOUR GUIDE...
A Proven Roadmap of Success for Implementing Drones in Construction.
A List of Red Flags to Avoid When Hiring Drone Service Providers.
How to Persuade Your Leadership Team to Leverage Drones on Every Jobsite.
