How the solar industry uses mapping
Where solar plant developer wants to build a
solar plant on a proposed area of land, an important step is to accurately
assess the suitability of the terrain. During the planning stage, solar design
engineers require precise 3D models of the land to gain an understanding of its
surface and key features. This data is important to identify risk and ensure
optimal designs.
A traditional on-site survey of an
area of 100 hectares could take weeks to be completed. As solar plants continue
to grow in size this method becomes hugely time-consuming and costly. There is
no question about the value of drone-based topographical mapping. However, there is a discussion about when it
is more appropriate to use LIDAR for topographic mapping over the most common
technology - Photogrammetry.
LiDAR
Recent developments in technology have seen drones, rather than planes, being used to carry the equipment, making LiDAR more accessible. LiDAR uses millions of individual geo-tagged laser returns to generate a point cloud of the area being scanned, with GPS aligned points. Those millions of location-known laser points combine to form a 3D map. LiDAR data is often confused with a more generic dataset of ‘Point Clouds’ because it is the main output generated by LiDAR. LiDAR has many different uses and applications, including assisting autonomous vehicles, surveying power lines, coastlines and helping the Building Information Modelling sector.
Nearly all LiDAR systems now also have a dual sensor with a visual camera, allowing them collect co-register LiDAR data and photogrammetry RGB imagery at the same time to make it easier to interpret, and create coloured 3D models as well.
It is very important to understand that LiDAR equipment varies in quality and this has a major impact on the quality of the data collected and processed. Here are some of examples of LiDAR equipment:
- High-end – Riegl Vux / YellowScan Ultra
- Mid-range – YellowScan Mapper
- Entry level – DJI L1
Entry-level LiDAR equipment is not recommended as they generally have poor quality Inertial Measurement Units (IMU), which leads to a bad alignment of the LiDAR and photogrammetry point clouds. Choosing the right LiDAR equipment for a topographical study of a future solar farm is key, as this will have a major impact on the quality of the data, especially when it comes to capturing data in vegetation and forested areas.
Impact of vegetation on Topographical Mapping
The debate about selecting which technology to use for your topographical study centers on the vegetation present either on the site or adjacent to the site.
We have listed some common scenarios and whether LiDAR can improve the deliverables.
LiDAR does not offer any significant advantage over Photogrammetry in these scenarios:
- Capture of near shading objects
If the adjacent/border vegetation (trees, bushes etc.) will remain, the most important information is the dimension of the vegetation, to understand the impact it will have on the plant design and yield estimation (shading impact).
- Individual trees
If there are individual trees (often to be removed) located across the site, with clear space between – see Image 1. LiDAR does not offer any advantage as photogrammetry will be able to capture the true ground elevation around and at the base of those trees.
- Arable crop in fields
and areas with dense non-arboreal vegetation
The site has crop growing or dense non-arboreal vegetation during the topographical study. From our experience, LiDAR and Photogrammetry data has a minimum error of 30cm, which can increase dramatically for certain types of crops.
LiDAR offers significant advantages over Photogrammetry in these scenarios:
- The site has areas of forest / trees plantation (arboreal vegetation) that will be removed (see Image 3), and true ground elevation is to be captured.
- The site has areas of low-density non-arboreal vegetation that will be removed (see Image 4) and true ground elevation is to be captured.
- Gullies & Streams covered by low-density vegetation where and true ground elevation is to be captured, such as width and depth.
Practical and Commercial Considerations
LiDAR sensors have in the past generally been used with airplanes as they are heavy pieces of equipment. They have gotten lighter as the technology has advanced, but they are still heavier than a standard RGB camera (1.5-4kg without batteries) and can therefore only be carried by a limited number of drones. The types of drones used typically have a shorter battery duration (due to the weight of the equipment) which means they need to be changed frequently, which is not ideal for mapping large areas. These factors mean more time spent on-site and an increased project cost, particularly when covering larger areas.
Here are some numbers showing the difference:
Flight duration:
- LiDAR ~ 10-12 mins
- Photogrammetry ~ 20-25mins
Area covered (daily):
- LiDAR ~ 50-100 hectares
- Photogrammetry ~ 100-500 hectares (depending on the equipment)
Drone-based LiDAR sensors are expensive in comparison to photogrammetry equipment. The GCPs needed for LiDAR data collection are heavier, more expensive and laborious than those needed for photogrammetry.
Additionally, preparing Topographical Mapping deliverables is more time-consuming and costly when using LiDAR as the data needs to be processed, classified and the two datasets (LiDAR and photogrammetry) compared.
Conclusion
Generally, photogrammetry will be the best option for projects that require visual and engineering data over larger scales as it provides high quality data at a reasonable price. Drone-based LiDAR has advantages for surveying narrow structures such as power lines or telecom towers, and in cases where it is important to get good ground elevation data below certain vegetation types (see previous section). Given the additional costs of capturing and processing LIDAR data, one is well advised to only deploy where absolutely necessary and to consider it as a complementary methodology to photogrammetry, not a substitutive one.