Aerial maps: manned vs unmanned
Aerial maps: manned vs unmanned
As with any new technology, comparisons to traditional techniques need to be made to understand the benefits and drawbacks. Until recently, “on demand” maps were collected using manned planes or satellites. Often now, drone based mapping is considered, largely because of an expected cost saving from using electric robots compared to jet fuelled commercial aircraft. What are the actual advantages and disadvantages of drone based mapping compared to manned aerial mapping?
Coverage per day is an obvious win for manned surveys, however coverage per dollar is a much more interesting number: what is the crossover point when manned aerial surveys become more cost effective than unmanned?
The above data is based solely on our experience in the industry over the last 18 months. There are numerous other factors at play here unaccounted for, but serves as indicative pricing. Also note the manned survey pricing here is for a capture window of up to 1 month. Manned survey providers try and bundle surveys in similar areas to bring their costs down per survey. To contract an on demand manned survey, expect to spend at least 2.5 times the above rate.
At about 1200Ha the price is roughly equal, and sites larger than this are generally considered the territory of manned aviation. There are, of course, new UAVs that will fly ~1000Ha in a single flight, however in Australia and the other markets with legalised, commercial drone operations, line of site is required at all times reducing the effective coverage of each flight. UAVs are the clear winner for smaller sites, and as the hardware improves and the regulations slowly allow longer flights, we will see the price/Ha continue to lower. Compare this to manned planes that have maximised efficiencies for some time now; with the price of AvGas continuing to rise it is unlikely we will see any step improvements in manned survey pricing.
Whilst next day data might not be critical yet, there are very few customers of aerial mapping prepared to wait weeks for the outputs. Although we have never contracted manned aerial surveys and experienced real world turnaround times, our customers have recounted numerous times manned aerial data outputs taking weeks to realise. What we are seeing is a uniform improvement in delivery times across both UAV service providers and manned providers thanks to increasing customer expectations and pressure from low cost cloud processing.
We believe as tools emerge which are designed to accept and analyse high frequency, wide area mapping, customers will realise the value and enforce this ease of use and speed across the board, whether contracting satellites, manned surveys or UAV surveys.
At Propeller, we turn around all surveys in no more than 24hrs thanks to our auto-scaling Amazon infrastructure, and because of the streamed nature of our maps and models all the data is available immediately, rather than requiring a USB key/HDD/SD card in the mail or lengthy downloads.
UAVs fly much lower than manned aircraft and so the ground sample distance (effective resolution) is much better. This provides imagery with more detail than manned aircraft. Flying low also means often underneath any cloud cover, allowing UAVs to collect clear maps even on overcast or cloudy days.
Low altitude mapping also has disadvantages though. With a wide angle lens and 400ft/120m flying altitude, the center of the captured photos is of land directly beneath, however the edge of the frame is capturing oblique information, walls, sides of buildings, etc. When the time comes to orthorectify the imagery in the photogrammetry software, the software attempts to correct all these oblique angles into “top down” mapping data. What you often get as a result is artefacts along the edges of rooflines or gutters, where smooth edges in reality turn out like a poorly reconstructed jigsaw puzzle.
Manned mapping data does not suffer from this issue. Often whole sites are captured in single photos, it is still important to capture multiple times to allow photogrammetry to build depth however orthorectification is much simpler as the high flight altitude means all of the light is travelling vertically upwards by the time the sensor captures the photo, very few vertical faces are captured.
The quality of the orthophoto is especially important if the imagery is used for any visually technical uses like digitising gutters, runoff drains, road line markings, etc. If the survey is primarily designed for terrain model capture, the increased resolution of UAVs and smaller, more precise ground control markings provide the edge here.