9 Key Differences Between Photogrammetry and LIDAR

Queensland Drones has been delivering high quality topographical survey mapping services for almost 10 years. For the first six years, we used exclusively photogrammetric mapping techniques. Over the past four years, however, we have progressively moved more and more into drone LIDAR mapping.

Over this time, we’ve put a lot of thought into how modern drone LIDAR mapping and traditional photogrammetric mapping compare, and what are the key differences between the two methods that would cause us to use one or the other.

But before we get into comparing the two methods, let’s first consider how each method works to generate topographic mapping information, and what the strengths and weaknesses are for each method.

About Photogrammetric Mapping

Drone photogrammetry is a passive remote sensing method, relying on ambient light to illuminate the scene. A drone equipped with a high-resolution camera captures numerous overlapping colour images of the survey area. High overlaps between images allow processing systems to view each point in the imagery from two different directions, creating a “stereoscopic” comparison which allows the software to estimate the distance between the camera and the points in the image. This calculation is used to estimate the changes in ground elevation and to detect structures and vegetation.

The key strength of aerial photogrammetry has always been the lower cost, due to cheaper sensing systems. Entry-level drone photogrammetry systems cost less than $10,000 to purchase, so drone photogrammetry services can be provided more cheaply and some regular users are equipping their teams with in-house drone photogrammetry capabilities.

The key weakness of aerial photogrammetry is that the camera only sees what the eye can see, so it cannot penetrate vegetation and the results can be affected by, for example, changes in ambient light conditions, cloud cover or deep shadows in the terrain. Getting the best results from drone photogrammetry requires careful planning and execution, especially in respect of weather conditions.

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About LIDAR Mapping

Drone LIDAR mapping is an active remote sensing method. It uses a “Light Detection and Ranging (LIDAR)” sensor mounted on a drone to create a 3D point cloud of the survey area. Rather than relying on colour photos, like photogrammetry, LIDAR emits hundreds of laser pulses every second and measures the time it takes for them to return to the sensor. This time-of-flight measurement, along with the sensor’s built in high-accuracy satellite positioning systems, allows the sensor to determine the precise location of each point in the terrain.

The key strength of LIDAR mapping is that the light pulses can pass through tiny gaps in vegetation to find points on the ground, even underneath grasses and bushes. Most LIDAR systems emit several pulses simultaneously, allowing measurement of multiple points in the same exact location which allows accurate measurement of heights of buildings, trees and other structures as well as accurate identification of the underlying ground elevations. Note however, that LIDAR is not magical – it cannot actually “see through” obstacles, but because pulses are emitted across a wide Field of View (FOV), pulses are fired at each point from many different directions which means the chances of a pulse finding a small gap in dense vegetation are quite high.

The key weakness of LIDAR mapping has traditionally been the cost. Until recently, LIDAR data could only be captured from systems mounted on light aircraft or helicopters which were very expensive to equip and deploy. But over the past few years, drone-mounted LIDAR systems have progressively replaced manned aircraft as the primary method for LIDAR mapping surveys. The entry point for purchasing a drone-mounted LIDAR system is now less than $50,000 which means service delivery costs for this option are becoming cheaper.

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The Major Weak Point in Photogrammetry and LIDAR mapping

Because the financial barriers to entry for photogrammetry are low and those for LIDAR mapping are getting lower each year, this has allowed increasingly inexperienced operators to enter the industry and compete for mapping services on price. Typically these lower cost operators have little or no understanding of surveying methods or the impacts of poor data on engineering outcomes, relying in large part on third party cloud platforms to design their mapping plans and process their data. This means they have little or no chance of knowing if their data is accurate, precise or even complete, which can create major headaches for their clients. This is an even bigger challenge for inexperienced operators using entry-level LIDAR systems, as the collection parameters and the processing methods required to achieve reliable, precise outcomes are very complex.

Comparison of Drone Photogrammetry vs Drone LIDAR

Feature	Photogrammetry	LIDAR
Technology	Passive remote sensing using high-resolution cameras	Active remote sensing using laser pulses
Data Acquisition	Captures overlapping images illuminated by ambient light	Emits laser pulses and measures their return time
Accuracy	High horizontal accuracy (less than 10 cm) over hard surfaces. Lower vertical accuracy (typically 15-20 cm.	High vertical accuracy (around 3-5 cm). Similar horizontal accuracy (5-10 cm), even on “softer” surfaces.
Vegetation Penetration	Limited – struggles with dense vegetation, making it unsuitable for heavily forested areas.	Excellent – excels at penetrating dense vegetation, allowing accurate terrain mapping even in forested areas.
Lighting Conditions	Sensitive – image quality can be affected by shadows, cloud cover, and other lighting variations.	Independent – operates regardless of ambient light, enabling surveys in low-light, heavy overcast or even at night.
Data Outputs	Orthomosaics, point clouds, 3D textured meshes, DSMs, but poor identification of actual ground surfaces in vegetated or forested areas	3D point clouds, DEMs, DTMs, contour maps, planimetric features. Most LIDAR systems can also generate high quality orthomosaic images.
Visual Fidelity	High – produces photorealistic outputs with colour information.	Lower – can lack the visual detail and colour information of photogrammetry, however many LIDAR systems now have synchronised colour mapping cameras built in.
Cost	Generally more affordable than LIDAR due to lower equipment costs and less complexity in data processing methods.	Generally more expensive than photogrammetry but the cost is coming down each year as equipment becomes more affordable and delivery systems improve.
Processing	Well-established workflows and software support; processing can take several hours depending on project size.	Processing can be more complex, requiring specialised software and expertise; processing time varies from less than an hour to several days.

PROs of Photogrammetry for Construction Surveys:

• Detailed Visualisation: The high-resolution, photorealistic outputs provide a clear understanding of the site, aiding in planning, design, and communication.1 This is particularly useful for visualising existing conditions and planning site layouts.
• Cost-Effective: Photogrammetry is generally more budget-friendly, especially for smaller projects or when vegetation is not a major factor.
• Efficient Workflow: Established software solutions and service providers enable streamlined data processing.

CONs of Photogrammetry for Construction Surveys:

• Limited in Dense Vegetation: Photogrammetry struggles to penetrate thick foliage, making it unsuitable for accurately mapping terrain in heavily forested areas. For construction projects in such environments, LIDAR is necessary to obtain accurate ground data.
• Susceptible to Lighting Conditions: Shadows, cloud cover, and other lighting variations can impact image quality and accuracy. Construction surveys often require precise data, so careful planning is necessary to mitigate the impact of lighting conditions.

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PROs of LIDAR for Construction Surveys:

• Penetrates Dense Vegetation: LIDAR’s ability to “see” through vegetation makes it ideal for surveying sites with heavy tree cover or thick brush, providing accurate terrain data even in challenging environments. This is crucial for construction projects in areas where photogrammetry would be ineffective.
• High Accuracy: LIDAR offers exceptional vertical accuracy, which is vital for construction projects requiring precise elevation data for earthworks, grading, and drainage design.
• Works in Various Lighting: LIDAR is unaffected by lighting conditions, allowing surveys to be conducted at night or in low light when necessary. This can be advantageous for projects with time constraints or safety concerns during daylight operations.

CONs of LIDAR for Construction Surveys:

• Higher Cost: LIDAR systems are generally more expensive than photogrammetry, potentially impacting project budgets.
• Complex Processing: LIDAR data processing can be intricate, demanding specialised software and expertise. This may increase processing time and costs.

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When to Choose Photogrammetry:

• Construction sites with light vegetation: Photogrammetry offers a cost-effective solution for generating accurate and visually detailed maps for sites without dense vegetation.
• Projects requiring high visual fidelity: Photogrammetry is ideal for producing realistic 3D models and orthomosaics, which are valuable for presentations, marketing materials, and stakeholder communication.
• Projects with budget constraints: Photogrammetry is generally more affordable than LIDAR, making it accessible to a wider range of construction projects.

When to Choose LIDAR:

• Construction sites with dense vegetation: LIDAR is essential for obtaining accurate terrain data in areas where photogrammetry would be ineffective due to thick foliage.81819
• Projects demanding high vertical accuracy: LIDAR’s precise elevation data is critical for earthworks, grading, and drainage design in construction projects.
• Projects requiring surveys in challenging lighting conditions: LIDAR’s ability to operate regardless of ambient light makes it suitable for nighttime surveys or projects with safety concerns during daylight operations.

Conclusion

In conclusion, both photogrammetry and LIDAR are valuable tools for construction surveys. The optimal choice depends on the specific project requirements, balancing factors such as vegetation density, accuracy needs, budget, and desired visual outputs.

To discuss your specific project needs and find out more about using photogrammetry or LIDAR for your surveying requirements, give Tony a call on 1300 125 111 or complete the enquiry form below and we’ll give you a call.