Photogrammetric flight

How to plan and realise photogrammetric flights with a drone?

Rapid development of unmanned technologies made photogrammetry and its application with drones very popular. But are all projects made with UAVs high quality full value surveying products? In this article, we describe the first step of every assignment – the photogrammetric flight which is the base for orthophotos, digital terrain models and other products, applicable in different sectors – road construction, forestry, precise farming, or monitoring of open cast mines.

The issue is described at the example of Mission Manager application, developed by FlyTech UAV, Polish producer of unmanned solutions.

Everything starts with planning of the photogrammetric flight. Usually, we begin with the area described by the customer. Then we choose the right type of UAV for the job. Usually the size of the area is the key factor but also the availability of places to land. In case of small objects (buildings, wind turbines, single stockpiles), the multirotor will be the right choice. When the area is bigger, it makes more sense to use a fixed wind plane. There are exceptions – big industrial zones, where it’s difficult to find the right spot to land a fixed wing plane safely, will require the use of multirotor. On the other hand, in small areas, where the fall of a multirotor could cause serious damage, we choose fixed wing plane.

When the drone is selected, we can find potential landing fields. The maps available in the app, showing the satellite view, make this step easier. It is important to find possibly many landing spots. The reason why we do that is that we can never be sure of the weather conditions (mostly wind direction).

planowanie nalotu fotogrametrycznego
The area of assignment – linear object (up) and Surface structure (down)

Also, something that in the photo looks like a meadow can in reality turn up to be a corn field. Another factor to consider is the shape of the area. Particularly in mountain areas some fields cannot be used as landings, as they are too steep. The height difference has an influence on wind direction, can cause downdrafts, updrafts and turbulences. The landing direction should always be against the wind and uphill (if it’s not flat). Having selected multiple landings, we can configure single missions.

planowanie lotu UAV
The planning of UAV flight parameters: ground pixel, flight height, photos overlap

The next step is the selection of flight parameters (flight height and distance between the rows) They depend on the end effect we want to achieve and the type of the surface. Those parameters are influenced by the focal length of the lens and the longitudinal and lateral overlap of the photos. As long as non-metric cameras are used in drones, this step has to be done carefully, so that the final product can be equally good as the end product of assignments made with manned aircraft, in typical photogrammetric procedure.

First, we need to know the application of the end product. If only a general orthophoto is necessary, we can fly with lower photo overlap (e.g 70%/40%). If an exact orthophoto is required or so called true orthophoto in combination with digital terrain model, the overlap must be increased up to 85% / 85%. That’s because photos are the source of the model and the precision of the model is going down with the decrease of the overlap. How the orthophoto and DTM are created, you can read here. The overlap is also dependent on the height differences on the surface. The bigger the differences or the denser the settlements, the higher should be the overlap.

dobór parametrów lotu UAV
Selection of UAV flight parameters and programming of the flight path

At the same time, we select the size of GSD (Ground Sampling Distance, in other words the size of the pixel). The smaller the pixel the more exact is our orthophoto and DTM. The GSD combined with the focal length of the lens determine the flight height.

When the pixel size and photo overlap are selected, we must decide what lens to choose. And again, the main factor to decide is the required precision of the end product and height differences of the surface. A wide lens (“short”, focal length – 20mm or less) allows us to lower the flight height and cover bigger area in one photo but also increases the errors due to distortion. It is not recommended where height differences are significant because it doesn’t show correctly the scale of the photo. When more precise data is required, we use narrow lenses (f=40mm or more). At this stage, we have to focus on any obstruction in the flight path and restrictions of the airspace. High constructions (chimneys, masts, wind turbines etc.) can be an obstacle of the low flying drone, whereas flight zones can limit the upper range of the flight level (e.g. around airports, where flights are allowed only up to 100 m above ground level).

As you can see, the planning of photogrammetric flights is a chain of very important, interdependent actions. Some parameters influence others, so that all of them must be considered.

Wykres wysokości lotu i przeszkód terenowych
A graph allowing the detection of terrain obstacles on the flight path

Photogrammetric control network

fotopunkt colidrone UAV
An example of a photo control point

If we want our project to be high value surveying product, we need to set up the so called photogrammetric control network. It is a particular type of surveying control network designed for photogrammetric flights, consisting of photo control points or ground control points. The difference between the photogrammetric control network and the surveying control network is in the way of stabilising and signalling the points. It must be visible in the aerial photo above all. That is why high contrast mats or special spray paint is applied. The shape of the signalling point must allow clear identification of the centre (place of measurement) in the photo. In years of business practice some forms were described in legal regulations. Sometimes other clearly identifiable objects are used, like manhole or central points of a roundabout. All depends on the scale of the project.

The control network must be measured according to surveying rules. Only then can we ensure centimetre precision of end products.

When all the flight parameters are planned, the control network must be designed. The size and shape of the project should be considered. Because the final precision results from geometric dependencies, the photo points should be regularly spread. In case of linear objects, they should be more densely located. Some of the points will be used as control points, i.e. they will not be applied in the final calculation. That’s why more points should be marked.

fotopunkty nalot fotogrametryczny UAV
Regular distribution of photo points in the flight area

Nowadays, the drones are fitted with additional GPS receivers and it allows to minimise that job. The photo control points should be stabilised and measured possibly short time before the flight, because of their fragility. Of course, their location can be changed after valuation of the local specificity (mud, high slopes), but the congruence of the whole system needs to be maintained. The control network measurement is a mid-stage between planning and the flight itself.

Photogrammetric flight

The date of flight is determined by weather conditions. Actually, we only can plan three days ahead, considering the probability of weather forecasts. When the weather is good, we can go the next step and finally fly. On the day before, the aircraft must be controlled and prepared for the flight. Every drone has its specificity, here the issue is described at the example of the Photogrammetric System Phoenix made by FlyTech UAV.

bezzałogowy system fenix ze stacją naziemną
Unmanned Photogrammetric System Phoenix FlyTech UAV with the ground station

We arrive at the preselected landing place. In the app we make the necessary corrections adding the selected landing spot, wind direction, flight range, control points distribution and height differences between landing points and the area covered during the flight.

We connect the computer with the plane wirelessly and we upload the flight mission. Next, we go through the start procedure, controlling all the mechanisms. The steering mechanisms are checked, general state of the fuselage and wings, stabilisation of the camera, functionality of the sensors, the engine, telemetric indications and the connection quality with GNSS (GPS+GLONASS). When we are sure that everything works correctly, we can take off.

UAV flies completely autonomously but the flight parameters must be controlled all the time. At this stage, safety has the absolute priority. The regulations set by the aviation law must be observed at all times.

MIsja fotogrametryczna Colidrone
During the flight the telemetric indications must be monitored

In almost all cases (due to legal regulations) the flights are conducted within the visual line of sight (VLOS). That means that we must observe the drone all the time and have the possibility to react in case of danger. The flight regulations and rules for conducting unmanned flights is a vast topic. To make it easier for the need of this article – we need to remember that the UAV operating in the airspace must give way to all manned aircraft as well as parachutes etc. A necessary separation from settlements and people must be observed as well. That’s why it’s better to supervise the flight in two men team – the operator and the observer.

The best wind direction is perpendicular to the flight direction (it enables fixed ground speed) and the best take-off and landing direction is against the wind. The local limitations can require a change from that patter (in case of a linear object like a railway line or a road we fly along it if the wind is not very strong).

During the flight, the most important parameters are the battery and the airspeed. If any of the two parameters varies from the typical values, it means that something unplanned is happening to the drone. Then we should consult other parameters, like energy consumption, connection with the satellites for navigation or deviation of the plane in three axes. Every untypical situation must be addressed immediately. To minimise the risk, the UAVs are equipped with many safety features. There are different steering modes and the automatic “return to land” mode.

The mission finishes with the landing. Phoenix can land automatically but local conditions, like trees, power lines make it sometimes impossible. The reason is the big size of the circle the plane makes before landing. It can’t be smaller because it’s another safety feature for the case of strong wind. The operator can correct the automatic approach and usually can land on a more limited space than in the automatic mode.

After the flight, we control the accuracy of the data and depending on the situation we move to the next take-off spot or we prepare the drone for the next mission.

Podejście do lądowania dron UAV
UAVs’ landing approach

Considering the Polish legal regulations and the range of the plane, we can do 4-8 missions a day, what results in up to a dozen square kilometres of area covered. All depends on the shape of the object and the parameters of the flight. A single mission takes about 50 min and the distance covered by the aircraft is about 40-50 km.

As you see, the photogrammetric flights with unmanned aerial vehicles consist of many precise procedures, from planning, control network measurements to the flight itself. Although the operator stands on the ground, we cannot forget that the drone operates in the airspace, which sometimes must be shared with other objects. That’s why procedures are so important for the safety. In photogrammetry, following procedures ensures also the correctness of the data gathered.