Georeferencing has become a major discipline with the implementation of new technologies, from the use of geographic compasses and theodolites to today’s complex measurement tools and computerized calculations.

Traditional methods

Rudimentary method of own coordinates created to keep the volume, scale and proportion of the model. In order to create these coordinates, it is necessary to use the physical qualities of the object and space to mark different physical points that are clearly recognizable if there are no automatic recognition targets.

Once the points have been selected, we use a laser distance meter or metre. This taking of measurements will have a higher error in centimeter values than modern millimetric methodologies, but the use of common field and site tools means that it is recognized as a useful methodology that has a tolerable error in its development.

So we proceed to mark the first point, this point must be provided with X,Y,Z values, so it is necessary to measure the height of this and give value 0.000, this point1will be our point of origin, from it, we will measure the distance between the different following points we want to take, using the laser distance meter or a meter.

Georeferencing with total station

Data capture for terrestrial photogrammetry, it is possible to capture georeferenced points with a total station, this electronic device works by means of a distance meter that calculates and a microprocessor that processes the information obtained from the former in reference to an electronic theodolite.

The selection of points is preferably taken by two technicians to speed up the work and diversify the tasks, in this way, one is in charge of taking points with the mobile receiver while the other is in charge of the fixed receiver.

The station emits as a fixed element, emits a signal to the mobile element which is the circular prism to facilitate the reception of the signal and the return of this to the station itself.

Once the signal has been correctly routed, the station quantifies the time elapsed between the output and the return of this signal and quantifies the distance between both elements according to this time, quantifying and locating the exact location of the base of the total station itself, which acts as a receiver.

In order to carry out the procedure properly, different factors that may hinder the work must be taken into account, such as weather conditions, and preferably its operation by 2 operators.

GPS device

A GPS device is a device that receives a satellite signal collected by ground stations through electromagnetic signals to coordinate different values and locate its location. There are different types of GPS devices and their accuracy may vary according to their price.

The device for GPS topographic documentation has an antenna that must be placed at the point in question, this works in conjunction with a Pda or electronic notebook, the accuracy may depend on physical or climatic factors that may obstruct the transmission or reception signal as well as may also vary depending on the number of satellites that can be captured.

RTK

The RTK coordinate acquisition system, short for Real Time Kinematic, can work with the various signal navigators such as the European Galilleo or the famous GPS, providing real-time information on the state of the coordinates.

In order for the equipment to work properly and obtain the best possible results, it is necessary to connect to the location system by means of at least three satellites, in order to coordinate the distances between the different satellites. The system uses the RTK device as a base station receiver that connects with different mobile units that compare their distances and triangulate with the base station. This system uses UHF modems among other sensors to capture the information in real time.

The RTK data collection system has an error rate of less than 1%, respectively 1 centimeter

± 2 parts per million horizontally and 2 centimeters ± 2 ppm vertically.

Georeferencing with Drone

Within the aerial photogrammetric technique we must distinguish various aspects in relation to georeferencing, since by the very nature of aerial photogrammetry this discipline will be one of its main points of support for obtaining sub-centimetric results in the capture of large surfaces.

Therefore, we will pay special attention to various aspects of interest that are performed during the data capture mission, such as: The Drone model, Type of flight or the use of physical georeferenced control points or later implemented in the model as reference points.

• Drone model: As a general rule, drones can achieve good data acquisition results regardless of their model. The most modern drones are equipped with more advanced technology such as their own flight modes, collision sensors or better cameras, which is of great interest for photogrammetric science.
  Among the improvements that a modern drone can have is the georeferencing of the captured images, each image captured by the flight mission will be assigned with the coordinates in which the drone was at the time of capture, this tool that provides each image with certain coordinates, will help in carrying out the operation with the greatest possible route precision, as well as in future processes in the composition of the model in the first phases such as the alignment of photographs, ordering more effectively and quickly the images subjected to data processing according to their coordinates. In the event that the drone itself generates coordinates in its photos and GPS location, the type of coordinates must be respected, as the use of the point cloud creation and management programme may generate an error that makes the production of the three-dimensional product itself unfeasible.

• Type of flight mission: Many current drones come with predefined flight options such as points of interest or target tracking or 360º photographs, which can be useful for maintenance and control tasks, using the drone as a multipurpose work tool, it is also possible to add greater value with the use of specialized control software for missions, this type of software used in mobile terminals such as smartphones or tablets use the geolocation obtained by the control device itself that is connected to the controller, to act as a GPS signal generator for the pilot himself as well as a GPS signal receiver for the drone. This type of programme uses mapping applications to guide the routes and missions assigned to the drone, agreeing perimeters, distances and routes within the areas subject to the data capture process.

• Physical Control Points:To add more precision to the data capture for its implementation in better results in the following processes, precise coordinates can be taken by means of topographic devices and marked in the field by means of different elements such as targets. Once the first office work processes are carried out generating data, the said marks are visually searched for within the point cloud and by means of the tools provided by the programme, the different positions are selected and given the values of the coordinates previously extracted; once the corresponding coordinates have been assigned their positions, the alignment process must be repeated, to rectify the error that may have been captured during the previous point cloud sample.

• Digital Reference Points: Point cloud management programs have several tools as seen above to correct and apply accurate changes to the model with the highest possible confidence; after the point cloud is generated, the user can select or create counterpart points that have not been collected by visually marking them in all the photographs that appear, this point will be given more precise coordinates within the point cloud when the process is restarted to implement the changes and corrections.