Digital orthophotography: the foundation of GIS

Municipal and county governments are taking advantage of technology that helps them to better plan for and address the growing needs of residents. GIS, a method of capturing recognizable geographic details of a land area from an aerial view and displaying them for various purposes, is no longer an exclusive tool of technologically adventurous or financially well-off communities. It is ideal for situations in which city planners and engineers or public works officials need visual access to a large area of land at a glance.

GIS applications can include trash routing and tax collection; viewing street centerlines, meandering driveways and buildings for emergency dispatch; locating pavement edges, fences and walls for parcel location as well as poles, manholes and catchbasins for utility management. Quantum leaps in cost efficiency and user friendliness have made the technology available to many city and county governments.

Several key advancements in GIS technology have resulted in its cost effectiveness. Among them are:

* advancements in computing technology that have reduced the cost of designing an effective GIS. Improvements in computer hardware, software and storage capabilities allow users to deal with the large volume of data created by the ortho process at a lower cost;

* more powerful and multi-tasking GIS software at economical prices that lowers map land base costs; and

* airborne Global Positioning System (GPS) surveys that have reduced the cost of geodetic control surveys (the mathematical foundation of GIS). Now, GPS surveys are capable of more accurate collection of geographic features.

Acquiring a map land base and converting the data into a workable medium are vital to the construction of a GIS for any city or municipality. Orthophotography in combination with intelligent vector mapping is the foundation of the land base.

An orthophoto shows natural and man-made details on the earth’s surface. It is divided into a grid with units of measurement called picture elements or pixels, which are associated with a precise geodetic coordinate. The size of each pixel is indirectly proportionate to the clarity or resolution of the photo. The smaller the pixel, the more defined the resolution. As pixel size decreases, however, the cost of the process increases.

Technological changes in recent years have made orthophotography more cost efficient and precise. Computer software has combined with traditional ortho information-gathering techniques to create digital orthophotography.

Digital orthophotography differs from conventional orthophotography in that it is much more accurate and can be produced in a fraction of the time required for the conventional orthophoto. Both are scanned raster or screen images, usually acquired from an aerial photograph, that are used to construct a computer-generated illustration of the city’s layout. The image is accurately rectified, or closely compared to match image details from the coordinates of the photo to the scanned image, with geodetic surveying (surface point location) and photogrammetry.

Although the same principle of rectification is used to produce both types of orthophotos, the method of application is different. According to Photo Science, a Maryland-based orthophotography firm, conventional orthophotos are scanned in small strips, and rectification is done at the seams of each strip but not within each strip. The width of the strip determines the accuracy of the orthophoto, and what results is an image with precise accuracy in some areas but not in others.

Digital orthophotos allow the user to insert ortho-corrected imagery into a computer system with GIS capabilities. To create a digital orthophoto, a photographic image in print or transparency form is scanned using a high resolution scanner with a capacity of up to 2000 lines per inch. Each pixel is corrected for displacement or tip and tilt using mathematical calculations that realign pixels to their original geographic location.

Displacement and tip and tilt refer to the angle at which the aerial photos wer taken. The photo must be adjusted so that the images do not appear angled. Tilt displacements exist in any photo if the plane is tilted during the exact moment of exposure. “Looking at an aerial photo, everything towards the center of the photo seems to be straight up and down, says Phil Kern, photogrammetry marketing manager of Intergraph in Huntsville, Ala. “But as you start moving out towards the center of that photo, buildings and trees are leaning away from the center of the land, and it is a result of the camera lens. A hill will cause the effect to be more pronounced, and the displacement has to be corrected.” Differential rectification eliminates the effects of tilt and produces an equivalent vertical photo.

After rectification, each corrected pixel is then adjusted to surrounding pixels by relating photogrammetrically-captured digital terrain data coordinates to the pixel coordinates. Transformation of the pixels is then done in a high speed image processor. Therefore, rectification occurs within and between each pixel, which results in much greater accuracy than with traditional orthophotography rectification.

The result is a spatially accurate image with planimetric features represented in their true geographic positions. The size of each pixel is also important to the clarity of the digital orthophoto. The GIS applications of a county would more than likely be satisfied by a pixel size of two to three feet on the ground.

In contrast, most municipal or city GIS applications require a smaller size, such as a six inch to one foot pixel resolution. Pixel size is largely determined by the dimensions of the area in question and the degree of resolution needed for surface details.

The amount of geographical or ground features on the city layout that must be clearly identified is also vital to the digital orthophoto. All the geographical features of a city need not be identified to create a land base map, so feature collection is application-driven.

By collecting select geographic features, multiple layers of data can be created and manipulated to fit the particular application. Then, GIS software works along with the identified features on the digital orthophotoo provide a viable picture of a city’s landscape.

Depending on the application, users can select the ground features necessary to the visual layout, display them and let the digital ortho backdrop provide the balance of the geography. City planners and engineers should determine the ground features necessary to the particular application and the degree of magnification that will best serve their needs.

Surface features in the digital orthophoto are easily recognizable by GIS users and the lay population. The interpretation that was needed for lines on a traditional GIS land base is significantly reduced. And, because digital orthophotos are easy to understand and translate, they can be successfully used in a variety of applications and by users who are not trained in GIS development.

Simplifications in software programs have contributed also to the user-friendliness of GIS. GIS software programs now offer options to the user such as color classification, synthetic color addition, image enhancement, interactive text and graphics and other features that make the finished product more understandable. ESRI, a software company based in Redlands, Calif. offers ArcView Version 2, which allows users to automatically import features as shapes and tables and control color, line styles, font and size.

Digital orthophotos are cost-effective, readily available and should be included in a well-implemented GIS program. It is important to determine when building digital orthophotos whether to use black and white or color photographs, or whether to simply purchase the digital orthophotos and appropriate pixel size.

Color digital orthophotos can be interpreted more easily than black and white orthophotos but they are costly to purchase and store. Color aerial photography, 25 percent more expensive than black and white must be used, and the photo must be scanned individually in red, blue and green, tripling the scanning cost. Tone matching between adjacent images and flight lines also increases image processing time. Finally, color digital orthophotos require three times more disk storage space than black and wite photos of equal resolution. In all, the difference between the cost of color orthophotos and black and white is about 60 to 70 percent.

Availability of storage space is also critical to GIS formulation. “The importance of file compression should not be underestimated since digital raster files, especially color, can take up a large amount of space,” Kern says. “Without some type of file compression, the entire process becomes complicated with excessive data management.”

File size affects almost every step of the photogrammetric workflow — from scanning to feature collecting and editing to data output. File size is critical in desktop environments where data storage space may be limited. Some compresion methods can significantly reduce file size without compromising image quality. Typical reductions of this type are one-third to one-fourth the size of uncompressed images.

Another option in GIS implementation is purchasing orthophotos. through the digital orthophoto quarter quad (DOQQ) program of the U.S. Geological Survey (USGS). Through the National Aerial Photography Program (NAPP), the USGS is currently compiling one meter resolution digital orthophotos produced from 1:40,000 NAPP aerial photos. The USGS plans to complete national coverage by the end of the decade, making DOQQs available on hard and soft copy.

The DOQQs, however, will be only as current as the photos from which they were created, and the positional accuracy will be within a range of plus or minus 50 feet. The DOQQ grid must be converted because its standard measuring unit is the one meter pixel.

The digital orthophoto alone is not a complete GIS land base solution. It is, however, an economical means of collecting and displaying a significant amount of geography. The combination of multiple technologies (digital orthophotography, photogrammetry, GPS) will result in a GIS land map base that is affordable, application driven and cost-effective.