Georeferencing is the process of attaching geographic coordinates to a photograph / an image acquired by aerial or satellite imaging. In Remote Sensing world, it is conventionally done by establishing a math model between image and Ground Control Points (GCPs). While the process is simple, it calls for collecting many GCPs and the models are not robust enough tohandle complex terrains. Hence, Universal Sensor Models are now being employed to overcome these limitations.
In Photogrammetry, this is done somewhat differently. The photographs are stitched by common points lying in overlapping areas for relative stability and then augmenting by well-distributed GCPs to determine the camera position and attitude (Exterior Orientation). This is popularly known as Aerial Triangulation. This process is critical, highly procedural, time-taking and often less-appreciated.
What is direct georeferencing?
The research efforts to reduce the demand on GCPs resulted in a novel and revolutionary technique called Direct Georeferencing (DG). The camera on-board is integrated with devices for determination of position and orientation angles. GPS, a popular technique by now, gets the position. An Inertial Measurement Unit, commonly known as an IMU, is an electronic device that measures the orientation, velocity, besides gravitational forces through the use of accelerometers and gyroscopes.
GPS/IMU technology ushered in a new paradigm in digital photogrammetry. These 6 EO parameters (XYZfrom GPS and omega-phi-kappa of the sensor) are critically needed in photogrammetry. If you are using open source tools, use gdalinfo to print the ground coverage of a DG photograph.
The method of recording EO directly from GPS/IMU is called Direct Georeferencing.Based on DG ability, theoretically, it does not require GCPs and it can obviate the complex AT process. In other words, the role of Control Points is limited to performing QC checks. This means, getting EOs is faster, cheaper and hence workflow becomes simple. Without AT, you can directly produce ortho photos. It is like half of the job is already done!
How reliable is DG?
Are we so lucky every time that we don't need GCPs when DG is adopted? Well, it depends. There are various makes of GPS/IMU that come with associated accuracies. (Leica IPAS20, IGI AEROcontrol, Applanix POS/AV are some of the popular GPS/IMU units used for direct georeferencing.) And of course, it also matters if the EOs are refined through post-processing.
But yes, photogrammetrists reported DG-derived EOs are comparable with conventional AT results. That is quite a promising observation. As a mandatory QC check, it is a good practice to overlay the GCPs on the stereo models and check for the compliance. And also watch for the menacing Y-parallax, as it disturbs the stereo-compilation process if present. It is difficult for the operator to determine the elevation of the feature (such as building height) from parallax-affected stereo-models. To overcome the residual Y-parallax, AT is done using GCPs, albeit in much lesser number.
Finally, how is geocoding different from georeferencing?
The process of associating an address or a placename with coordinates on the map is called Geocoding.Georeferencing is the process of assigning a coordinate system and coordinates to an image or a vector coverage by translating, transforming, and warping it into a position relative to the reference spatial data, such as survey locations, or road intersections.
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