A land surveyor measuring a ground control point with a GNSS receiver

Unlock the Benefits of Ground Control Points: What You Need to Know

Imagine soaring above the ground, capturing detailed aerial images that can be transformed into accurate maps and models. Ground Control Points (GCPs) play a critical role in achieving this level of accuracy for UAV mapping projects. In this blog post, we’ll guide you through the ins and outs of GCPs, from understanding their basics to leveraging cutting-edge technology for GCP placement and surveying. Let’s unlock the benefits of Ground Control Points together!

Short Summary

Understanding Ground Control Points: The Basics

Using the correct number and location for Ground Control Points and check points is important for an accurate survey & quality control

Using the correct number and location for Ground Control Points and checkpoints is important for an accurate survey & quality control

GCPs are essential elements in accurate UAV mapping projects, as they provide reference points with known coordinates to ensure the correct positioning of objects in a surveyed area. These control points, typically square-shaped and measuring around 60cm x 60cm (2′ x 2′), are placed on the ground and used as a basis for georeferencing aerial images. By incorporating GCPs into your projects, you can achieve high levels of absolute accuracy and precision, improving the quality of your digital aerial maps and aerial survey data.

However, it’s important to distinguish between control points and manual tie points. While both play a role in aerial mapping, they serve different purposes and impact accuracy differently. Let’s delve into their differences in the following sections.

Control Points vs. Manual Tie Points

Control points are used for absolute accuracy, ensuring that aerial images align with real-world coordinates and are precisely positioned in a project. In contrast, manual tie points only enhance relative accuracy, which refers to the consistency of measurements within a project without necessarily being tied to real-world coordinates. These manual tie points are employed to assemble images and generate a comprehensive photogrammetric map, but their use alone does not guarantee the precise positioning of objects in the surveyed area.

Understanding the difference between control points and manual tie points is crucial for drone mapping projects, as it allows you to select the appropriate method to achieve the desired level of accuracy. For projects requiring high-precision absolute positioning, control points are indispensable, while manual tie points can be used to improve overall consistency and quality within a project.

Traditional Surveying Methods and GCPs

Traditional surveying methods, such as the use of theodolites or total stations, involve measuring angles and distances between points on the ground, often requiring line-of-sight between the instrument and a roving surveyor or ground access across the surveyed area. While these methods offer high precision and cost-efficiency, they may also present challenges in terms of accessibility and visibility.

GCPs can be employed with traditional surveying methods to enhance precision and minimize the time necessary to complete a survey. By providing supplementary points of reference, GCPs can be leveraged to validate the accuracy of a survey conducted using traditional methods, ensuring a comprehensive and precise representation of the surveyed area.

This combination of techniques allows for greater accuracy and efficiency in aerial mapping projects.

Types of Ground Control Points: Temporary and Permanent

Improved accuracy in Drone Mapping with Ground Control Points. In this example a temporary foldable GCP

Temporary GCP made of a foldable material for easy transport and storage

There are two main types of GCPs: temporary and permanent. Temporary GCPs can be made from any object with high-contrast colours, making them easy to spot and detect from aerial images. On the other hand, permanent GCPs are a cost-effective option for projects that require multiple visits, as they are made from durable materials such as vinyl tiles affixed to patio blocks, nails or other permanent ground markers. Both temporary and permanent GCPs play a vital role in UAV mapping projects, offering flexibility and adaptability depending on the aerial survey project requirements.

Choosing the right type of GCP for your aerial mapping project is essential, as it can impact the overall accuracy and efficiency of your mapping efforts. Let’s take a closer look at how to create your own GCPs and explore commercial GCP solutions.

Creating Your Own GCPs

If you prefer a more hands-on approach, you can create your own GCPs by carefully considering factors such as size, shape, and colour. By establishing your GCPs with these factors in mind, you ensure that they are easily detectable in aerial images and can be accurately georeferenced in your mapping project.

Creating your own GCPs can be a cost-effective and customizable solution for your aerial mapping projects, allowing you to tailor them to your specific requirements. However, if you’re looking for a more streamlined and efficient approach, commercial GCP solutions are also available.

Commercial GCP Solutions

Commercial GCP solutions offer a range of options for those seeking a more efficient and standardized approach to GCP placement. Smart targets, for example, are pre-made GCPs that are easy to deploy and detect, saving you time and effort in the field. Additionally, GCP-as-a-service providers like Civil Tracker offer the necessary equipment and processing of GCPs, with the customer responsible for installing the GCPs and utilizing a one-button system to capture data.

Propellor AeroPoints is a popular quick solution for temporary Ground Control Points. These rigid smart GCPs include GPS data with a single click. While they can be placed on permanent markers they aren’t permanent GCPs in their own right.

Whether you opt for a do-it-yourself approach or a commercial GCP solution, the key is to ensure that your GCPs are accurately placed and easily detectable in your aerial images, resulting in precise and reliable mapping data.

Optimal Placement of Ground Control Points

On sites with varying elevations it is important to make sure GCPs are located in the extreme position including the elevation

On sites with varying elevations, it is important to make sure GCPs are located in the extreme position including the elevation

Strategically placing your GCPs is crucial for accurate mapping of large areas. GCPs should be placed around the perimeter and throughout the middle of the worksite, ensuring that the entire area is covered. However, configurations may vary depending on the shape of the site, and it’s essential to avoid uneven distribution of GCPs, which can hinder the process of accurately correlating aerial photos with the surveyed area.

Achieving optimal GCP placement is a delicate balance of various factors and considerations. Let’s explore some of the factors that influence GCP placement and how they impact vertical accuracy in UAV mapping projects.

Factors Influencing GCP Placement

An aerial image with a clearly visible ground control point is used for accurate mapping and surveying. In this case, used to reference photogrammetry but a similar approach can be taken to LiDAR surveys. This establishes common reference points between surveys

An aerial image with a clearly visible ground control point is used for accurate mapping and surveying. In this case, used to reference photogrammetry but a similar approach can be taken to LiDAR surveys. This establishes common reference points between surveys

Several factors can influence GCP placement, such as terrain, obstructions, and vertical accuracy. Uneven terrain can make it difficult to measure GCPs accurately, while obstructions such as trees, buildings, and other objects can impede their visibility and subsequently impact the accuracy of your mapping project. For an example drone mapping project have a look at this drone survey for a planning application

Vertical accuracy is another critical factor when positioning GCPs, as it directly influences the accuracy of the measurements taken. If GCPs are not positioned at the correct elevation, the measurements will be inaccurate, affecting the overall quality of your mapping data.

Let’s delve deeper into the relationship between vertical accuracy and GCPs.

Vertical Accuracy and GCPs

Vertical accuracy refers to the measure of positional accuracy with respect to a specified vertical datum at a specified confidence level. GCPs can enhance vertical accuracy by providing reference points for elevation data and using more GCPs placed at different elevations can improve the accuracy of this data.

For optimal vertical accuracy, GCPs should be placed in areas where elevation data is likely to be less precise, such as those with steep slopes, sharp changes in elevation, and high levels of vegetation. The number of GCPs required to achieve optimal vertical accuracy is contingent on the scope and complexity of the project, with smaller projects requiring fewer GCPs and larger projects necessitating more.

Leveraging Technology for Ground Control Point Placement and Surveying

Aerial view of a land surveyor placing a Ground Control Point

Aerial view of a land surveyor placing a Ground Control Point

In addition to traditional Ground Control Point placement methods, technological advancements have given rise to innovative approaches for Ground Control Point placement and surveying. Drone-based Ground Control Point placement and machine learning with AutoGCPs are examples of cutting-edge solutions that can enhance the accuracy and efficiency of your aerial mapping projects.

Let’s explore these technological advancements in more detail, gaining insights into how they can revolutionize GCP placement and surveying in aerial mapping projects.

Drone-Based GCP Placement

Utilizing drones for GCP placement offers several advantages, such as precision, cost-efficiency, and the ability to access hard-to-reach areas. By flying drones over the survey area and capturing images of strategically placed GCPs with known coordinates, you can ensure the accurate positioning of objects in the surveyed area.

Before flying a drone with GCPs, it’s crucial to ensure that weather conditions are favourable and that GCPs are evenly distributed throughout the area of interest, guaranteeing their visibility from the sky. This approach can save time and effort in the field while maintaining high levels of accuracy in your UAV mapping projects.

Machine Learning and AutoGCPs

AutoGCPs are an exclusive module of Pix4Dengine, utilizing machine learning and computer vision to automatically detect GCPs in a project. This innovative approach can achieve better than pixel precision when used with RTK-enabled drones, enhancing the accuracy of your mapping data. DJI Terra and other photogrammetry applications have similar detection routines.

By leveraging the power of machine learning and AutoGCPs, you can streamline GCP placement and surveying processes, ensuring accurate and efficient UAV mapping projects with minimal manual intervention. This use of machine learning and AI and its use, in general, is only going to increase in the future.

How Many Ground Control Points Do You Need?

Integrated surveying kit comprising of a total station and a GNSS receiver used to measure relative accuracy or absolute accuracy respectively

Integrated surveying kit comprising of a total station and a GNSS receiver used to measure relative accuracy or absolute accuracy respectively

The number of GCPs needed for a project depends on its size and complexity. For small-scale projects, a minimum of 1-3 GCPs is recommended to ensure accurate mapping. However, larger projects may require a more substantial number of GCPs to achieve the desired level of accuracy and precision.

The following sections delve into the specific requirements for GCPs in both small-scale and large-scale UAV mapping projects, providing guidance on determining the appropriate number of GCPs for your project.

Small-Scale Projects

For small-scale photogrammetry projects, it is recommended to use at least three GCPs for accuracy and precision. Utilizing five or more GCPs can further enhance accuracy, ensuring that your project is as precise as possible.

Factors such as the size and topology of the survey site can also impact the number of GCPs required, with larger and more intricate topologies potentially necessitating an increased number of GCPs. By carefully considering the size and complexity of your small-scale project, you can determine the appropriate number of GCPs to achieve optimal accuracy and precision in your UAV mapping efforts.

Large-Scale Projects

For large-scale projects, a general rule of thumb is to use a minimum of 4-5 GCPs per square mile (2 km2). However, the specific number of GCPs required can vary depending on the desired level of accuracy and project complexity, with some studies suggesting that 9-12 GCPs per 100 hectares (1 km2) are generally necessary.

As a simple example, a 20-acre landfill cell would require a minimum of 5 GCPs, with 7 recommended for greater accuracy.

By understanding the unique requirements of your large-scale project, you can ensure that the appropriate number of GCPs are used to achieve optimal accuracy and precision in your UAV mapping efforts.

Common Mistakes to Avoid When Using Ground Control Points

App showing the current arrangement of GNSS constellations

App showing GNSS constellations

Most GNSS field systems show the sky as a 180-degree dome from horizon to horizon with the viewer at the centre. Each concentric ring represents the verticle elevation of the dome. The outer bold ring is the horizon with 90-degree above the viewer in the centre. The hatched area is the elevation mask which is used to filter out satellites close to the ground where errors are more likely due to atmospherics or ground-based objects. Dilution of Precision (DOP) is also shown as well as the number of satellites in the sky at that point in time.

When working with GCPs, it’s important to be mindful of potential errors, such as improper placement, inadequate quantity, uneven distribution, inaccurate recording, and not using high-contrast colours for GCPs. Inadequate placement can result in difficulty accurately measuring the area, while an insufficient number of GCPs can impact the overall accuracy of your mapping data. For GCPs coordinated with GNSS receivers, it is important to consider the satellite constellations, the number of satellites, their relative distribution in the sky, and whether they are obscured by trees or buildings. Also, you need to be aware of reflected satellite signals from building facades known as ‘multi-path’ signals, which can also dilute the positional accuracy. Dilution of Precision (DOP) is reported on GNSS systems and is a good guide to the quality of the data you are receiving. Often called pDOP or split into horizontal (hDOP) and verticle (vDOP) components.

If you are surrounded by tall trees or buildings adjust your elevation mask to filter out satellites and their signals which may be hurting your positional accuracy.

Site survey with a drone: A challenging survey area in a deep river valley covered in trees, with no mobile/cell coverage

A challenging survey area in a deep river valley covered in trees, with no mobile/cell coverage

If you are processing your GCPs using Post Processed Kinematic (PPK) then as the name implies your control data isn’t processed until you are back at the office. Any mistakes captured in the field will be discovered too late and may mean a return to the site. If your GCPs were temporary then this could you give you even bigger problems. If you can survey your GCPs using Real-Time Kinematic(RTK) as positional accuracy is reported in real-time. When using RTK make sure your system is properly initialised and positions are locked. Failure to do that will mean that you will be recording uncorrected positions. If you lose ‘lock’ it is important to regain the lock or initialisation before taking measurements. Modern receivers are much better at maintaining lock in tricky spots in and around trees and buildings.

Make sure you take measurements for the correct number of epochs on either PPK or RTK for control points, as defined by the GNSS receiver manufacturer.

By being aware of these common mistakes and taking steps to avoid them, you can ensure the highest levels of accuracy and precision in your UAV mapping projects, achieving reliable and consistent results.

Summary

In conclusion, Ground Control Points play a vital role in accurate drone surveying projects, providing a solid foundation for precise georeferencing and positioning. By understanding the basics, choosing the right GCP type, optimally placing GCPs, and leveraging cutting-edge technology, you can unlock the full potential of GCPs for your mapping projects. As you embark on your next drone surveying endeavour, remember the importance of GCPs and strive to achieve the highest levels of accuracy and precision. The sky’s the limit!

A land surveyor measuring a ground control point with a GNSS receiver

A land surveyor measuring a ground control point with a GNSS receiver

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David Walker McInstCES AssocRICS avatar