Photogrammetry is the estimation of the geometric and semantic properties of objects based on images or observations from similar sensors. What this translates to is that photogrammetry is the process of taking measurements of distances, areas, and volumes of an environment based on photographs taken from various vantage points. These measurements can then be used to produce an orthophoto map (Orthomosaic), Digital Surface Model, or 3D point cloud.
Legislative changes and advancements in UAV technology, size and weight reduction in sensors, and advances in compact data storage/transmission has seen significant growth in UAV aerial photogrammetry. Aerial photogrammetry is expanding into new applications and end uses that were previously too costly or impractical. Professional photogrammetry can produce outputs with horizontal (x,y) accuracies in the range of 1 cm (0.4 in) and elevation (z) accuracies of 2 to 3 cm (0.8 to 1.2 in), which enables precise volumetric analysis. In this section we are going to focus on photogrammetry and the advantages of using a GNSS/INS to provide more efficient and cost effective aerial mapping.
Figure 1: Photogrammetric Digital Terrain Model
Georeferencing (Georectification) is the process of applying a coordinate system to an image so that it can be accurately located on a map. In order to georectify an image the camera orientation and position, or Exterior Orientation Parameters (EOP), need to be known for every image in order to create a planimetrically correct image that can be used for Geographic Information System (GIS) analysis. EOPs include 6 parameters, three rotational angles:
and three position coordinates for X, Y and Z.
Aerial Triangulation (AT) is the process used to determine EOPs from captured images. In order to achieve successful AT the mission plan (intended flight path), needs to ensure that there will be sufficient overlap of adjacent images. Typically a 60-80% forward lap and 30-60% side lap is required to ensure sufficient data is collected to generate a suitably accurate surface model.
AT is the process of image block adjustments with images that contain information from well distributed and accurately known ground control points (GCPs) and tie points.
Bundle Adjustment is the final step in generating 3D reconstructed models. Using the Tie Points and GCPs the bundle adjustment computations calculate the EOPs for each image. First images are oriented and combined to form an image block using the tie points, such as one of the strips in Figure 2. This block is then adjusted to fit to the ground using the GCPs. This is a simultaneous process of refining image coordinates, camera parameters (Interior Orientation Parameters) and EOPs using collinearity equations to find the best statistical fit.
Modern software tools such as Pix4D and ArcGIS automate the complex AT process and can take advantage of the processing power of cloud based servers to complete the heavy computational load relatively quickly. Once the bundle adjustment is complete a photogrammetric point cloud, or density cloud, can be created which is then meshed (filled between points) to form a Digital Elevation Model (DEM).
Direct Georeferencing (DG) on the other hand is the direct observations of the exterior orientation (EO) of the imaging sensor using GNSS receivers and inertial measurement sensors. The GNSS receivers provide position (x, y, z) information and inertial sensors orientation (ω, φ, κ) data. Some of the advantages of DG is that it reduces the number of Ground Control Points required (possibly eliminating their need completely), speeds the selection of tie points and reduces the time to configure the project. In summary, DG offers efficiency improvements throughout the mapping workflow, from the field to data processing.
In order to determine which sensor would be most suitable for DG we need to determine some important performance and operating criteria based on the mapping requirements.
In order to achieve survey-grade photogrammetry results it is necessary to use GNSS Receivers capable of either Real-Time Kinematic (RTK) or Post-Processed Kinematic (PPK) corrections to determine image coordinates to less than 1cm.
RTK positioning requires a robust communication link between the GNSS Base Station and the imaging sensor GNSS in order to transmit real-time correction data to the camera UAV. Additionally, the GNSS Base Station must be correctly configured and surveyed-in for the most accurate result. The requirement for an active link poses operational challenges for long range mapping applications and especially when Line of Sight (LOS) cannot be maintained.
To learn more about Aerial Photogrammetry, the advantages and limitations of photogrammetry, the practical limitations of using photogrammetry for surveying and the impact of GNSS/INS on photogrammetry accuracy. Please download our application note.
Figure 6: RTK Diagram
Figure 7: PPK Diagram
Is DG necessary for survey grade photogrammetry? No, but DG does offer users several advantages that can be used in conjunction with Indirect Georeferencing or even without:
Advances in computing power, software, and digital cameras are attracting stakeholders in measurement centered applications to adopt photogrammetry. Precise photogrammetry relies on cameras integrated with high precision GNSS/INS in combination with post-processing software to create an optimised SWaP solution that can digitize vast areas.
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