Photogrammetry & Its Application PDF

Photogrammetry & its Application 1  What is Photogrammetry? Photogrammetry is a sophisticated process by which information is extracted from photographs to create accurate three-dimensional maps and models. Using ultra-high resolution aerial photographs, this p...

Photogrammetry & its Application 1  What is Photogrammetry? Photogrammetry is a sophisticated process by which information is extracted from photographs to create accurate three-dimensional maps and models. Using ultra-high resolution aerial photographs, this practice combines UAV-mounted overhead sensors with powerful GIS mapping systems to create dynamic, measurable documents for a number of real-world situations and uses. Photogrammetry has its earliest origins in surveillance and reconnaissance. Pilots during the First World War combined new innovations in both photography and manned flight to gather intel from behind enemy lines. The photographs alone weren’t super valuable without context, so these pioneers used local landmarks and landscape features to determine the orientation of objects in the images. 2  What is Photogrammetry? In the decades that followed, these practices would evolve with new tools, from stratospheric U2 aircraft to advanced meteorological satellites to modern drone photogrammetry. Today’s photogrammetric maps are constructed using advanced GIS software that can generate surveyor- grade measurements of landscapes and infrastructure. These maps are detailed enough to provide valuable insight into on-the-ground environmental conditions by documenting erosion, vegetation density, water clarity, and more. And that’s just the beginning of what photogrammetry software can do. 3  Basics Terminologies of Photogrammetry Ortho Photo and Ortho Image An ortho-photo is an aerial image that’s geometrically corrected to produce a uniform perspective and scale, so it can be used to measure true differences. To produce a uniform scale, the image needs to be corrected for factors including camera tilt, lens distortion, and environmental conditions. 4  Basics Terminologies of Photogrammetry What is an orthomosaic map? Using advanced software, a selection of orthophotos can be stitched together to produce a 2D or 3D map of on-the-ground conditions. An orthomosaic map is a distortion-free, interactive display of high-resolution imagery that can be used to measure accurate distances between actual geographic features. 5  Basics Terminologies of Photogrammetry What is remote sensing? Remote sensing describes a suite of technologies that use overhead photography and sensors to create detailed maps for measurement and study. Photogrammetry is one of several tools in remote sensing, and it is used to process images collected by sensors mounted on UAVs, manned aircraft, and satellites. Other forms of remote sensing document infrared and UV radiation, point-by-point distances, and more. 6  Basics Terminologies of Photogrammetry What is Structure from Motion (SFM)? Structure from Motion is a technique that calibrates two-dimensional images into a reconstruction of a three-dimensional structure, scene, or object. Using ultra-high-resolution digital surface imagery, SFM can produce incredible point cloud-based 3D models with similar measurement quality to LiDAR. 7 Basics Terminologies of Photogrammetry What is a geographic information system (GIS)? Geographic information systems (GIS) are used to pin high-resolution imagery onto satellite positioning data for mapping purposes. Google Earth is perhaps the most ubiquitous GIS system in existence, but geographic information system data also powers meteorology, advanced surveying and mapping, navigation, and much more. 8  Basics Terminologies of Photogrammetry What is metadata? Metadata is a series of data-encoded notes collected alongside ortho-images to provide additional context for mapping and modeling software. Metadata may include: GPS coordinates Time/date Focal length Resolution settings Atmospheric conditions And more Metadata will tell users the conditions in which the data set was created and who created it, both of which offer valuable information for building a uniform scale and perspective. 9 How is resolution defined in photogrammetry? The quality of an orthophoto is centered on three forms of resolution: spatial, temporal, and spectral. What is spatial resolution? Spatial resolution describes the amount of visual data collected in each image pixel. Spatial resolution is measured in physical terms—a document with 100m resolution documents 100 meters by 100 meters worth of clear data per pixel. What is temporal resolution? Temporal resolution is a metric for describing how time elapsed between images or data sets, which impacts the analytic quality. Good temporal resolution requires data collection in regular intervals with few substantial gaps. What is spectral resolution? Spectral resolution describes the capacity of a sensor to collect information on electromagnetic wavelengths. A sensor may be well suited to documented variances in color, infrared light, or other electromagnetic energy forms. 10  Computation of Photogrammetry Scale of a Vertical Photograph  Scale is ordinarily interpreted as the ratio of a distance on a map to that same length on the ground.  On a map it is uniform throughout because a map is an orthographic projection.  The scale of a vertical photograph is the ratio of a photo distance to the corresponding ground distance.  Since a photograph is a perspective view, scale varies from point to point with variations in terrain elevation.  L is the exposure station of a vertical photograph taken at an altitude H above datum.  The camera focal length is f and o is the photographic principal point. 11 12  Points A, B, C, and D, which lie at elevations above datum of hA, hB, hC, and hD, respectively, are imaged on the photograph at a, b, c, and d.  The scale at any point can be expressed in terms of its elevation, the camera focal length, and the flying height above datum.  From similar triangles Lab and LAB, the following expression can be written:  Also from similar triangles Loa and LOA, a similar expression results:  Equating (a) and (b), recognizing that ab>AB equals photo scale at A and B, and considering AB to be infinitesimally short, the equation for the scale at A is 13 Examples 14  Scale of a Photograph The scale of a photograph can be determined if a map is available of the same area. This method does not require the focal length and flying height to be known. Rather, it is necessary only to measure the photographic distance between two well-defined points also identifiable on the map. Photo scale is then calculated from the equation: 15 16 17