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HOME > ARCHIVES > 1995 > AUGUST
Digital Orthophotos For Smaller Projects? Yes You Can!
By Robert A. Fowler

With many large institutional GIS users looking to maintain their base data through the application of digital orthophoto production, many smaller users are probably wondering if this technology is really suitable for them. The short answer is yes, it is.
      The small municipality, the civil engineering design group, the transportation engineer and the planner can all use digital orthophoto products as effectively as a base map for whatever purpose they would normally use a vector map.
      The primary consideration for the prospective user will be whether their current GIS or CAD system can accept raster data and, if not, what they will need to update their systems. While most GIS systems and many CAD systems will accept raster information, there are sometimes limits. The universally found AutoCAD, for example, requires an add on package such as CAD Overlay GS to enable it to read a gray scale raster. Intergraph requires the IRAS C module to be able to use gray scale data. I made a distinctive reference there to gray scale because there are less expensive raster packages which will accept "document raster" data, which are data where the pixels are strictly black and white (i.e. the pixel is either off or on with no gray value). To use a digital orthophoto you need a package that will provide you with 256 levels of data from black through various shades of gray to white. ARC/INFO versions 6 and greater will handle raster information and Tydac SPANS being a raster system also has no problem with gray levels. While I have mentioned a few of the more popular systems, many other GIS packages now have raster capabilities and readers should consult their manual or phone their supplier if they are not sure.
      Raster files are usually relatively simple files. A raster header usually indicates the size of the file in pixels which is determined by the number of pixels across (columns) and the number of pixels down (rows). In other words a raster file could be 5,000 columns by 5,000 rows or any other combination of figures. The number of rows does not have to be the same as the number of columns, but the file will have to form a rectangle. Where a chunk of photo (gray scale) data may be missing from the file, say for instance a corner of the photograph has been cut out, the raster image file must still be a complete rectangle, and the missing photo data will be replaced by an equivalent number of pixels - they will just either be all black or all white.
      When loading the data file, the CAD system or GIS has to know several other important pieces of information. First it has to know where the raster information starts. This is where the first pixel in the first column and row is located. Again, usually it is either the top left of the file (or if you want to think in visual terms, the top left of your screen) or the bottom left. Then it has to know the geographic value of this insertion point (easting and northing of the first pixel), then the direction the raster will go from there and the size of each pixel. For example, does the pixel represent 5 feet by 5 feet of the ground or 1 foot by 1 foot, etc. (Pixels, of course, should be square.) Then the rest of the raster file is usually the gray scale record of each succeeding pixel. If any of the header information is incorrect, say the length of the row is wrongly entered, then the result will be the raster reaches the point where it was told to end the line, and immediately wraps around onto the next row, creating a garble of unintelligble pixels.
      So, much of the important bits of information are in the header and, preferably, should also be on the label of the tape cassette or CDROM in case you physically have to type in the file format information when setting up your work space in the system.
      In the case of color, of course, there are three records for each pixel: they being the gray scale value of the red, the green and the blue primary projection colors. As you can guess from this, however, color files are three times the size of black and white files. When dealing with large areas this can pose a significant problem for data storage and file manipulation. If it takes your system one minute to load a black and white raster image, it will take three minutes to load the same image in color. The upside is, in most systems once the image is called to the screen, regeneration of the image at different looks (zooms or pans) is relatively quick.
      While it is possible that many companies may not have all of the pieces that they need to be able to look at and use digital orthophoto files, in most cases the upgrades necessary do not cost very much compared to the overall savings which can accrue through the use of digital orthophotos. For example, digital orthophoto production is usually (depending on the circumstances) less expensive than vector mapping. If a digital orthophoto project is to cost $8,000, compared to the cost of a vector line map at $11,000, then it is possible to pay for the add-on software required for AutoCAD, for instance, on one project.
      The advantage of the image file is the technical staff or engineer can see what is really there on the ground, not what someone (even a well meaning and professional someone) has interpreted or simply left off the map during the mapping process. An example of this could be flower beds or other vegetation cover, which are not normally shown on a line map. While a flower bed is not a major impediment to a construction project, people have a tendency to get upset if someone appears one day with a backhoe and without so much as a by-your-leave destroys the zinnias planted in front of their property!
      Indeed, one of the great advantages of orthophoto products is their use during public meetings. A plan showing the proposed changes to an intersection is far more understandable if the planned road limits are dropped out (white) on an ortho photograph. Humans are very visual animals and in short order can recognize objects on an aerial view and are much more easily able to appreciate proposed changes shown on actual images rather than lines on a map which tend to be confusing to the average citizen.
      Intera recently worked with the engineering firm Proctor & Redfern Ltd., in the Kingston, Ontario area, on a project which involved an infrastructure study of the Canadian Forces Base (CFB) just outside the city. The purpose of the study was to evaluate the state of services in the married quarters area of the Forces Base before transferring responsibility of providing these services to the local municipality, the township of Pittsburgh.
      Intera flew large scale (1"=250') aerial photography, and provided assistance on the ground control. Proctor & Redfern did all of the field measurements and picked up a large amount of infrastructure detail such as manholes, catch basins, street lighting fixtures etc., using a total station system. Precision surveying techniques were used and coordinates were adjusted over the network with expected accuracies to half an inch.
      In the meantime Intera completed aerial triangulation, produced a digital terrain model and, using these with the camera calibration report, rectified the imagery using their International Imaging Systems' PRI2SM digital orthophoto system. The output pixel size was 0.1 meter (4 inches) and all data were recorded on CD ROM for use with Proctor & Redfern's AutoCAD system with CAD Overlay GS.
      Proctor & Redfern downloaded the total station coordinates to the system as precise positions of the detail collected in the field and these data were laid over the image in the CAD system. Following this the infrastructure routes were constructed and placed in appropriate layers. Layers were provided for electrical, storm water drainage, sewage, and water lines. A plot file was made of various plans (water, sewer, drainage, electrical etc.) and Intera, using an IRIS plotter, made hard copy clear film mylars of the image with overlaid vectors.
      There were some strange anomalies which became apparent at random spots. A number of manholes appeared to be misplaced. The pixels on the image were displaced as much as two pixels from the field coordinates. Various checks of the processes (field and digital orthophoto production) uncovered no major errors, although from interpretation of the photography it was obvious the area had significant changes in relief and a couple of the manholes appeared higher than the surrounding ground. Finally a field check was made by Intera to verify the image data on the ground.
      In every case the problem involved a major shift in elevation. For example, two manholes, as expected, were actually elevated: sitting on concrete pipes about 1.5 feet above the surrounding ground. As they were, in both cases, near the extremities of the area of a photo used in the rectification process, the lean of the object created displacements almost equal to their height. In three other instances, the manholes were on the sides of steep slopes (1:4) which were not entirely reflected in the digital terrain model.
      As a result of the precision field coordinate survey, which created an incredibly accurate field quality check on the digital image, it was determined that the majority of the digital ortho was well within the required specifications. However, one of the things learned was that where high accuracy results are required, a very closely spaced digital terrain model is necessary. In addition, objects with sheer vertical steps (with current technology, at least) will always have some displacement unless they are close to the principal point of the photograph.
      The net result of the project, however, was that all parties were extremely pleased with the information and deliverables. Intera was pleased to have proven the accuracy of their digital orthophotos using real world ground coordinates for precision, quality control. Proctor & Redfern proved the technology to themselves and its application to engineering programs. They appreciated its abundant information in assisting their planning and product output. The ultimate client was pleased to have easily readable image files and a database for a significant area in its jurisdiction. The Department of National Defense was also pleased to have an image record of part of CFB Kingston.

About the Author:
Robert Fowler, O.L.S., C.S.T., C.E.T., has written on surveying and mapping topics before for Earth Observation Magazine, as well as for a number of other publications around the world. He is manager of marketing for Intera Information Technologies based in their Ottawa office. He may be reached at 613-226-5442. Perry Petersen, P. Eng., works for Proctor & Redfern Limited in their Kingston, Ontario office and also teaches courses in surveying and engineering. He may be reached at 613-542-3434.

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