If you are involved in any type of land surveying using drone technology, there seems to be a good chance that you know about LiDAR drones and photogrammetry. But do you know what the differences, as well as what the similarities are, between these two technologies?
In this blog, we cut through the confusion which often surrounds these two products and nail down exactly what they can offer. After reading it, you will be better placed to make the right decision when choosing between LiDAR or Photogrammetry, based on your particular use case and budget.
So how exactly do the two technologies work?
It is important to know that photogrammetry is entirely image-based, and is classed as a passive technology. It is a technology which transforms 2D images into 3D models. Photogrammetry offers depth perception which allows users to view objects in three dimensions. It relies on what the camera’s sensor can pick up through ambient light illumination.
LiDAR – an acronym for Light Detection and Ranging – is a laser technology which bases its readings on an active sensor. It actively shoots out laser beams and makes measurements of the time it takes the light to return.
Photogrammetry and LiDAR: What do you get?
Photogrammetry produces 2D and 3D models in full colour, offering the easy visualisation of terrain. Many would say these models make it easier to interpret terrain than LiDAR. The predominant outputs of photogrammetry are digital surface models, 3D points clouds and orthoimages. These outputs offer accurate visuals with a low ground sampling distance.
So what can LiDAR offer in contrast? While it might not be able to keep up in the visual stakes, without colourising the point cloud in postprocessing. LiDAR provides 3D point clouds which give a greater degree of flexibility in terms of their parameters. That means more options when it comes to different use cases such as Digital Terrain Models or power line encroachment.
This ability to alter the density of the point cloud is thanks to the LiDAR sensor characteristics, such as the repetition rate and scan frequency, which can be varied. Flight altitude and speed are other variables when it comes to making 3D point clouds with LiDAR. Colour is often used to make the monochrome dataset produced by LiDAR easier to interpret. This colour is typically added based on the classification of different ground objects.
Where LiDAR becomes invaluable is its ability to ‘cut through’ canopies and reach the terrain below, as well as modelling objects which are narrow, or harder to recognise through photogrammetry owing to poor visibility. It also offers the advantage of working in poor lighting conditions.
In general photogrammetry is not as accurate – that is in terms of both relative and absolute accuracy – as LiDAR, without extra investment being made into supplementary technology and methodology such as Ground Control Points (GCP) and RTK/PPK technology, as well as acquiring the knowledge and expertise to choose the right kit, and process the data effectively.
LiDAR is recognised as a viable and accurate technology for land surveying but needs to be paired with a GNSS receiver and inertial motion unit (IMU) in order to deal with the challenges posed by the motion of the drone. LiDAR have multiple options such as the Scanlook Revolution, Snoopy A-Series and the Snoopy V -Series.
The need for speed?
Data acquisition is quicker for LiDAR surveys, which require an overlap of 20 to 30 per cent, compared to the 60 to 90 per cent needed by photogrammetry. The multiple GCPs which are needed to increase accuracy when using photogrammetry can be another time constraint, not to mention the extra investment.
Data processing for LiDAR can take as little as a few minutes, while photogrammetry can take hours, requiring the right software on the right computers. It should be noted that for some use cases, LiDAR also requires extra data editing and additional software in order to achieve project objectives.
What’s the damage?
In addition to the type of project being undertaken, budget parameters always look likely to be a big factor when choosing between LiDAR and photogrammetry. While photogrammetry surveys typically require a drone with a camera, LiDAR operations – including sensor and selecting a drone which is most suitable to carry the equipment – can easily run into six figures.
In this regard, there is virtually no comparison in terms of price point, but LiDAR can still offer advantages which photogrammetry can’t replicate, and these can be vital according to the use case. So, the most cost-effective will depend on your objectives.
The same rule of thumb applies to data processing software. For photogrammetry, software is typically much cheaper than platforms needed for LiDAR, such as TerraScan. While photogrammetry software licences can be acquired for hundreds of pounds, licences for the LiDAR software can stretch into five figures. Though, GlobalMapper offers a cheaper alternative for lidar processing.
Of course, the cost isn’t everything, and your overall objectives will have a huge bearing on which technology you opt for.
We hope to have cleared up some misconceptions surrounding LiDAR and Photogrammetry. Happy surveying!