Capturing planimetric and topographic data for your project
Photogrammetric mapping is a methodology by which precision flown aerial photographs, accurate camera and ground control data are combined with digital data capture to produce planimetric, and topographic maps. The steps in this methodology are:
- Adjustment of the raw control data
- Analytical Aerial Triangulation Control Processing
- Photogrammetric Data Capture
- Photogrammetric Data Quality Control
- Photogrammetric Data Translation
At Bradstreet Consultants, our team of photogrammetrists and map technicians use ALBANY block adjustment software, fully analytical aerial triangulation, first order analytical stereoplotters, Kork Digital Mapping System (KDMS) software, and Autometric's Softplotter NT softcopy photogrametry system to examine, rigorous test and collect the 3-D data necessary to produce your planimetric and topographic maps to exacting mapping standards.
Adjustment of the Raw Control Data
Accuracy of the camera station position at the time of shutter exposure is the foundation of your digital map data. Bradstreet Consultants verifies and determines these coordinates precisely through the use of an independent expert. The photos are prepared, measured and derived coordinates are established for at least six points in each stereomodel. This enforces a "double check" on the foundation of the project, eliminating any costly re-work.
Analytical Aerial Triangulation Control Processing
Analytical aerial triangulation is a process which requires a specific order of procedures and rigorous mathematical computations.
Airborne GPS (A-GPS) may be used to precisely locate the center of an aerial camera's lens at exact time of photo exposure within 5 cm in XY&Z. A-GPS coupled with an Inertial Measuring Unit (IMU), which precisely measures the rotation angles (attitude) of the camera at time of exposure within 0.5 arcsecond, allows a photogrammetrist the option to forgo the costs of ground control and aerotriangulation, normally 15% of the total project cost.
Photogrammetric Data Capture
Stereo Compilation Techniques - At the beginning of a new compilation project, the project manager reviews the scope of work with the compilation supervisor and the computer systems manager. Any changes from standard collection procedures are discussed to insure that all aspects of the project are fully examined prior to beginning full production. After the review process, production support materials such as model layouts and sheet layouts are generated with Microsoft Project, Microstation, KDMS or AutoCad. This creates a centralized location for all personnel to review the status of a project.
Stereo Model Orientation - Using only first-order analytical stereoplotters, Bradstreet Consultants orients each stereoplotter using detailed orientation process to mathematically solve for and adjust the photogrammetric restitution of each stereo model.
Collection Process - Bradstreet Consultants utilizes Kork Digital Mapping System (KDMS) software to collect 2D and 3D geographic data. KORK is a collection of programs designed to rapidly and accurately collect, process, plot, and translate 2D and 3D geographic data. Combined with Spectra-Precision Terramodel, the program is the world's leading data capture software tool.
Collection of data begins with planimetric features. The amount of detail to collect is dictated by the final map scale to be produced and what is requested by the client to meet their individual project needs. Data collection conforms to the standards set forth in the "Surveying and Mapping Manual" developed by the Aerial Surveys Group, Design Division, FHA in November of 1985.
The next phase of collection is the extraction of topographic data for the creation of a Digital Terrain Model (DTM). All photogrammetric data captured in 3D is used. This data includes the collection of breakline features supplemented with random mass points to maintain mapping accuracies. Breaklines are a special class of lines which indicate instantaneous slope changes in the terrain. Mass points are spot elevation data randomly collected in varying density to effectively and accurately define the terrain. The breaklines and masspoints are used to create a Triangulated Irregular Network (TIN) surface. From this surface, index and intermediate contours are created.
Photogrammetric Data Quality Control
After the final phase of the collection process, a quality control check of the stereo model is performed. This is completed by the compilation supervisor, who reviews the model setup as well as the accuracy and completeness of the digital data. Two types of editing programs are used in the quality control process: automatic batch programs and the interactive programs.
Automatic Batch Editing involves the use of user-defined support files that define the editing processes, therefore special operator intervention is not required. Bradstreet Consultants uses automatic batch processes to prepare sheets to their final formats.
Interactive Editing allows the operator to perform editing operations at any time during collection or digitizing. This enables the stereoplotter operator to correct his own work, plus allows other digitizing workstations to be used to perform the edits on a per sheet basis.
Prior to submitting the digitized mapping data in electronic form, a draft paper manuscript will be provided for review and evaluation.
Photogrammetric Data Translation
The KORK digital mapping files can easily be translated into a variety of digital formats such as DLG-3, AutoCad, ArcInfo, and Intergraph system formats. A translation table is specially prepared to translate the KORK files to the client's exact specifications. The output file will conform to established standards and be delivered electronically on the media of the customer's choice.
