The RICS Guidance (Earth Observation and Aerial Surveys 6th Edition) recommends that photogrammetry surveys are undertaken with a minimum sun angle of 15 degrees, effectively April through to October. I contributed to that publication so what I am doing undertaking surveys in late November?
There are a few reasons, mainly driven by my clients requirements and activities on site but also taking into account the nature of the site and helped by the advance in sensors. This week I carried out a photogrammetry survey and captured aerial imagery of a 650 acre landfill and quarry operation in the Midlands. I have surveyed this site before so was already familiar with it although you always need to plan for the unforeseen.
As with any drone work and in particular photogrammetry good pre flight planning is required. You also need to be very aware that the available time to undertake the work in November is going to be limited. At this time of the year at best the sun will get to c12 degrees between 10.00 and 14.00, that is when you need to be undertaking your flights. I had planned 8 photogrammetry flights to cover the whole site with the longest being c 20 minutes. That gives a good safety factor and landing with (ideally) 30% battery life remaining. In addition I need to capture aerial stills and video of the whole site.
The basic parameters for the project was a GSD to 2.0cm which equates to a flight height of 75m when using a Mavic 3 Multi-spectral. Front side overlap set to 80/70. Whilst having RTK Ground Control Points/Check Points were captured. Flight "design" was influenced by restrictions on access to various parts of the site so take off points and landing points need to be carefully selected. Six of these were only accessible on foot. Site access and ground conditions make the M3M a very practical choice. In fact I could carry both the M3M and a Mavic 3 Pro to each take off point, you wouldn't want to do that with a Matrice!
Turning to the survey the weather both days was forecast CAVOK with blue skies, light winds on Day 1 and 10/12knt winds on day 2. Appreciating that overcast is better to give a flat light on this occasion there were few trees/tall structures to cause shadow issues. My broad aim was to deploy the GCP's and capture as much stills and video as possible before 10.00am and after 14.00pm and do the survey element in between. What I hadn't expected was having to undertake 3 separate site inductions (long story) which took some time on the first morning. Ground conditions were also pretty bad with recent rain, frost overnight and generally a slippery sticky mud clinging terrain. Luckily I was able to drive my 4x4 to the centre of the site so getting all my gear a bit closer.
Day 1 went pretty much to plan although starting to fly a bit later than intended. The only other issue I had was with the first couple of photogrammetry flights the RTK taking a few minutes to start. The error message was "converging" so I guess this related to the RTK/Trimble link as I had excellent satellite coverage. Ben at Drone Pilot Academy was very close to having a call - Ben can solve most things over the phone.
Day 2 brought some weather challenges. The forecast looked OK with higher winds but manageable. Having set out GCP's my first flight was capturing aerial stills and video which I was doing from 80m. At this height winds were much windier than I would expect. You know fairly quickly from the noise of the drone that its working harder and sure enough within a few minutes I did get a high wind warning. The gusts were a farily short duration but intense. Slight aside and this happened on another site with a different client - explaining to someone that I can't use ear defenders or gloves when flying a drone although it is their H&S Policy can be challenging. Often I pick places to fly from away from plant and buildings so I am not near a noise source and why would you need gloves when standing in a field!! Back to the flying. The work around was flying and adapting to the weather so decreasing height in the gusty periods. At 50m I wasn't having any issues.
During the morning the gusts died off although remained steady and with the knock on effect of the drone working harder and shorter flight times. I therefore split two of the photogrammetry flights returned home with a safety net of at least 25% battery life, swopped batteries and continued.
I completed the job with 6,500 survey images captured plus 250 aerial stills and many Gb of video. People may wonder why 32 flights? Firstly with every survey I do two flights. The first is to check the drone is all working as it should and the batteries are upto temperature. I did 10 surveys so 20 flights. The other 8 were for the aerial stills and video.
In the introduction I mention sensors. I have found that the M3M is very good at capturing good imagery when using auto settings and it varies to ISO more compensate for light. Having reviewed some images on site there all looked as I would expect.
Finally always remember the pilot. Working in these conditions can be physically demanding, heavy underfoot and at times cold. One minute you are getting pretty warm carrying kit across site and then you are standing for 20 minutes cooling down. Make sure you have the right layers to compliment your PPE kit. Take plenty of snacks and hot drinks. I don't tend to stop so work through the whole day but it is manageable.
I am processing the data in PiX4D. A large data set but I have done a few before with good results. Always remember to switch off any "auto updates" on your computer!
