Light Detection and Ranging (LiDAR) has been in common use since the 1970s before the 1980s saw the arrival of the technology to provide measurements paired with Global Positions Systems (GPS) data. It’s only since the race to create lightweight LiDAR units for autonomous cars that the UAV mounted LiDAR market has emerged. More accurate positioning capabilities through improvements in GPS and Inertial Measurement Units (IMU) were key to this development.
Now there are a host of options for those looking to gain the benefits of LiDAR scanning from drones. There have been various claims in the market about UAV LiDAR performance, some of which may have been erroneous and led to confusion. We have put together a list of the key things to look for when adding LiDAR scanning to your UAV toolkit.
What Should You Think About When Buying UAV LiDAR?
The six mainareas for consideration are
- UAV LiDAR accuracy
- Weight and flight time
UAV LiDAR accuracy
When a LiDAR unit shoots a laser there is always going to be some level of inaccuracy in the measurement. The levels of accuracy claimed can vary between units, but generally, it sits at around +/- 3cm for most units – this can be improved by investing more money in your choice of sensor.
Accuracy is measured when the unit is in a static position. Once you attach your LiDAR to a UAV and fly you start to get a data set that looks very close to a random collection of points. Wind and other environmental factors can impact accuracy too.
The trick that all UAV mounted LiDAR units pull is to pair the dataset with the data from an IMU. Therefore every time the UAV moves the IMU captures this movement and can provide this as a calibration to the dataset. Despite this technique, there is also always inaccuracy in the IMU data. You can add this inaccuracy to your dataset.
Some manufacturers report their accuracy measurements as the combination of these two inaccuracies. However, this is not how accuracy is measured. If you invest in a UAV LiDAR with the belief that you will consistently get the claimed accuracy then you may end up disappointed with your purchase.
There are only two good ways to determine that the system is going to get you the results you need.
- Ask for an independent report. A surveyor can test the dataset for absolute and relative accuracy using ground controls.
- Test it yourself. Ask for a demonstration of the product before you buy so that you can see the results for yourself.
Weight and flight time
Weight is important to any UAV LiDAR setup – the weight of the unit will help determine flight time which can affect how much work the unit can do in a day. With more time resulting in higher costs, it’s vital to make sure that you can cover as much area as possible in the shortest possible time. Weight will also affect which types of UAV your unit can be mounted on.
When considering units check the claimed weight – does the unit include the mounting equipment and batteries (if required)? Some manufacturers exclude this equipment from the claimed weight, this can be as much as 600 grams to a unit.
You should also check with the manufacturer on how the unit is calibrated. Many units need a calibration manoeuvre before and after scanning, thereby reducing scan time. Other units use GPS antennas for heading. While this adds weight, it can increase your total available flight times.
Many LiDAR manufacturers claim that their units can acheive returns from well over 100 metres above ground level (AGL) so it’s reasonable to assume that you can fly at these heights. However, this is often not the case.
The returns are based on 80% reflectivity so, in reality, flight heights above 60 metres won’t return the claimed accuracy.
A reduction in processing time is a major benefit of LiDAR over photogrammetry, though this benefit can be lost if the workflow for processing data is time-consuming or difficult.
Ask for a tour of the workflow to process your dataset to ensure that it is swift and straightforward. Many systems require you to “stitch” flight lines together in third-party software adding to the cost and complexity of the workflow.
It may come as a surprise that for such expensive units there is often only a one year warranty offered. Therefore it pays to factor the length of reasonable lifespan you expect against the unit cost – if it dies after 18 months of us will you get a return on your investment?
Finally, have a good look at pricing. As competition in this area increases so do the variations in price
Part of this is down to development costs, which can be high for manufacturers who are creating different solutions to problems that UAV mounted LiDAR poses. Some units are more expensive by multiples but do not offer increased benefits.
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