Topographic Representation from Aerial Photogrammetry

PUBLICATION 04 | TBC CYCLE | GEOCOM Engineering

 

Introduction

Undoubtedly, aerial photogrammetry has established itself as a common method for mass topographic representation in various engineering applications. In this way, the incorporation of drones has further boosted the popularity of photogrammetry as a topographic representation technique, mainly due to its significant field productivity.

However, while aerial photogrammetry has provided an evident boost in productivity for topographic activities, the management and handling of final products can be complex considering computational requirements and the large volumes of data that need to be managed. For this, TBC offers a series of tools associated with first-class photogrammetric processing, along with comprehensive management of point clouds and orthophotos, to produce a complete topographic representation that can be used by a variety of geospatial professionals.

Photogrammetric Processing

In very general terms, the photogrammetric process is divided into two parts: aerotriangulation and final product generation. Aerotriangulation aims to determine the positions and orientations for each photograph by identifying tie points (common points automatically determined in photographs by the software). This process can be supported by control points and/or, for drones equipped with GNSS, by determining the trajectory through differential processing of GNSS baselines. On the other hand, once the block of photographs is aerotriangulated, final products such as point clouds and orthophotos are obtained.

 

Figure 2. UAS data import

 

• GNSS: This option allows processing the drone's trajectory based on GNSS observations, which are processed very similarly to PPK. It is also possible to import photographs whose coordinates have been determined by RTK during the same flight operation.
  

 

• Aerotriangulation: In this operation, tie points are determined, which allows establishing the pose —position and orientation— of each photograph. Then, control points are indicated to position the block of photographs in the correct location within the associated coordinate system.

 

Figure 3. Aerotriangulation in TBC

 

Figure 4. Incorporation of control points

• Creation of deliverables: Generation of point clouds and orthophotos under pre-established criteria based on flight height and camera characteristics. The first step is to configure the quality of the results, which is related to the spatial resolution of the products, then specify the product type: cartographic means 2.5D from vertical photographs, and 3D modeling means it is obtained from oblique photographs. In this operation, tie points are determined, which allows establishing the pose - position and orientation - of each photograph. Then, control points are indicated to position the block of photographs in the correct location within the associated coordinate system.

 

Figure 5. Creation of aerial photogrammetry deliverables in TBC

Figure 6. Aerial photogrammetric products in TBC

 

Drawing in TBC

TBC has tools for point cloud processing, whether from photogrammetry or LiDAR. The Scanning module allows classifying point clouds from surveys conducted with terrestrial, aerial, mobile LiDAR, and photogrammetry. For point cloud classification, TBC offers a wide variety of classes such as terrain, buildings, vegetation, among others.

 Prior to data import, it is necessary to configure the project in terms of the coordinate system and drawing templates; however, these templates can be imported from a VCT format file.

Regarding the admissible point cloud formats in TBC, LAS, LAZ, E57, and XYZ are supported, and of course, Trimble's proprietary formats, which yield the best processing results. The import requires indicating the origin of the point cloud, for example, if it is projected (grid) or terrain, if it is georeferenced or unknown. If the point cloud has no reference, it can be georeferenced using control points.

 

Figure 8. Plan and 3D views of the imported point cloud in TBC

 

Furthermore, TBC has a series of applications that optimize point cloud management, such as classification and editing, in addition to software training through deep learning for cases where the required class does not exist.

 

 

Point cloud editing includes segmentation tools for region creation, sampling, plane cut views, bounding box, and cloud visualization as needed, such as true color, grayscale, scan, region, signal intensity, and contrast, to name a few.

 

 

Another element that TBC incorporates is associated with data extraction from the point cloud: the tool is called point, line, and geometry extraction. In this case, it has been used with a feature library that allows creating CAD points with blocks and drawing terrain features on the point cloud through 3D polylines ordered by layers, color, line type, and thickness.

Moreover, TBC in its Drafting module incorporates dimensioning, labels, creation of surface tables, coordinate tables, import and insert blocks, line types, sheet configuration, dynamic views, grid creation, among other options.

 

Figure 12. Layer Manager

 

Figure 13. CAD Drawing

 

Figure 14. Point extraction with attributes

 

Figure 15. Feature library applied to point cloud

 

An interesting TBC application is related to geometry extraction, available in the Scanning module. Initially, the shape needs to be created by drawing on the point cloud, which is then selected for TBC to identify it uniquely or multiply as appropriate. If multiple, it is necessary to define an area to be analyzed to optimize the search environment. To achieve successful extraction, the intensity and confidence thresholds must be configured.

 

Figure 16. Extract geometry

 

Moreover, the Surfaces module has a series of tools that allow incorporating geospatial data from observations made with total stations, GNSS, and importing ASCII files as points that include identifier and code. TBC also allows incorporating imported or created CAD lines from the same digital reconstruction for defining breaklines. TBC has tools to combine surfaces, limit, create islands, smooth, project CAD elements onto the surface, platforms, and many more alternatives.

 

Figure 17. Surface creation and editing

 

For its part, the Drafting module is capable of creating graphic representations from previously configured templates for automated drawing of plans, longitudinal profiles, and cross-sections. The use of Drafting allows the incorporation of vignettes, frames, and graphics associated with both horizontal and vertical scales. In this environment, the creation of sheets is very fluid and easy to manage, allowing for the creation of sheets with location plans, and the incorporation of orthophotos in different formats. The creation of dynamic views or viewports is automatic, as is the incorporation of the coordinate grid and the perfectly aligned north symbol block according to the created sheet. Figures 15 and 16 show, respectively, products with both orthophoto and CAD. Here, graphics are fundamental for their representation, as colors are essential for the incorporation of vectorization into the sheet and its contrast with it.

 

Figure 18. CAD sheet with orthophoto

 

Figure 19. CAD Sheet