Photogrammetric mapping customers have become savvy to the flexibility and accuracy of digital orthophotographs, and many have begun using advanced design software that requires three-dimensional map data. To meet customers' increasingly sophisticated mapping needs, Aerial Services Inc. (ASI) has become one of the first aerial mapping firms to add softcopy digital photogrammetry to its list of in-house services.
      "This is the direction of the future in photogrammetry," said Walter Ertz, ASI's mapping supervisor. "Softcopy capability gives the customer room to grow."
      ASI's recent purchase of high-end, fully automatic softcopy stereoplotters allows it to provide customers with digital terrain models and digital orthophotos made in-house, directly from aerial photographs. The automatic stereoplotters now perform all the work that previously required a point marking device, conventional analytical stereoplotter and orthophoto machine.
      "Most importantly, it creates a digital orthophotograph in a fraction of the time it used to take us to make a hardcopy ortho manually," said Ertz.
      The Cedar Falls, Iowa, firm has been involved in aerial surveying and mapping for 25 years and in digital photogrammetry since 1984. In the past couple of years, ASI has seen its engineering, surveying and cadastral customers begin to favor digital orthophotographs because the orthos can be input directly to geographic information systems (GIS) and used to create hardcopy output in any scale that suits the requirements of a particular application.
      Ertz estimates that more than half of ASI's customers now request digital terrain model data in addition to orthophotographs for their mapping projects. "The latest engineering and design software packages utilize 'Z' data as much as 'X' and 'Y' data," he said.

ASI Chooses SoftPlotter
In mid-1994, the aerial surveying and mapping firm purchased SoftPlotter, a high-end fully digital stereoplotter that provides semi-automated aerotriangulation, stereoscopic exploitation, automatic terrain model extraction, orthorectification and feature extraction. SoftPlotter was developed by Vision International of Alexandria, Va.
      SoftPlotter and OrthoMAX, a terrain mapping system developed by Vision for the ERDAS IMAGINE software package, both evolved out of technology created for terminal area mission planning for aircraft and missile systems. The digital techniques used for this purpose included automatic terrain model extraction, and rectification and enhancement of digital images. SoftPlotter utilizes improvements in this technology and applies it to the more demanding requirements of commercial mapping.
      ASI was introduced to the Autometric product line through an association with Kork Systems Inc. of Bangor, Maine, a leading developer of PC-based photogrammetric products. ASI has used the Kork Digital Mapping System (KDMS) software for 10 years to make vector maps from aerial photos. Autometric purchased Kork in 1993 and created Vision International to market commercial photogrammetry products developed by both companies.
      "The two companies have created products that complement each other," said Ertz. "SoftPlotter interfaces with KDMS so that vector collection can be drawn on the digital orthophoto and transferred to a CAD environment."
      "Vision is responding to the market demands for user-oriented digital photogrammetric and GIS products," said Tom Reed, Vision vice president. "We are able to offer products at a price and performance level that makes it possible to integrate sophisticated softcopy digital photogrammetric products into existing map production environments."

First Tests in Iowa
Two of ASI's first tests of the automatic stereoplotter system have been conducted in mapping projects in nearby Wapello County and in ASI's home county of Blackhawk, Iowa. ASI has completed many county-wide surveys in support of cadastral mapping projects for counties and is familiar with the problems they present for photogrammetrists.
      "It's very difficult for a county to decide on the specifications they need for their maps because there are so many offices within the county that have individual needs," said Gary Brown, ASI's president. "For instance, Blackhawk wanted their scales in feet for most applications, but some offices need metric units."
      Digital photogrammetry appealed to Blackhawk for several reasons. First, the county had not commissioned a cadastral survey since 1980. The County Engineer's Office liked the idea of snapping the property and boundary lines to a digital orthophoto on a GIS rather than experience the aggravation of having the lines mismatched on a non-orthorectified image.
      Craig Molander, Vision's vice-president in charge of product development, has worked closely with ASI on the Blackhawk project. "Unlimited vector graphics capability is one of the critical advantages digital photogrammetry is providing ASI," he said. "Checking accuracy of the orthophotos, overlaying vectors on the photos, and editing the vectors interactively are all possible with softcopy."
      Digital photogrammetry also coincided with the county's recently adopted approach to mapping. The County Engineer has arranged for the digital map to be provided to all county offices, urban districts and utilities to use as basemaps within their existing GISs. In return, each of those participants would populate the basemap database with its own particular information and return it to the County Engineer's Office for inclusion in Blackhawk's county-wide GIS.
      "The county will gain a complete GIS of city and utility information," said Brown. "When the county decides to build a new road, it will know where utility gas lines are buried, and when the county tax office makes assessments, it can take into account land value determined by the city."
      One of Blackhawk County's major employers, Deere and Company, the well-known manufacturer of tractors and farm equipment, also will participate in the mapping project and will put the digital map data to interesting new uses. Deere is experimenting with intelligent farming techniques at its plant near Cedar Falls.
      Deere is equipping some tractors with GIS-based vehicle navigation systems (VNS) that will use GPS to guide the vehicles in their application of pesticides and fertilizer. The company will create land-use models based on the orthophotos to determine how much pesticide should be applied to each field. The land-use information and digital basemap will be loaded into the vehicle's GIS to govern its navigation and pesticide application.
      Blackhawk County requested ASI generate two sets of hardcopy orthophotos in addition to the digital database. To cover the 576 square mile county, the County Engineer asked for 288 map sheets at a scale of one inch equals 400 feet (1"=400'), and 650 sheets at one inch equals 100 feet (1"=100').
      The county has found the orthophotos offer a certain public recognition quality that vector maps don't have. Citizens relate better to projects that are diagrammed on photographic images instead of on line maps, which helps the county promote new development plans in the community.

Using the Automatic Stereoplotter
ASI acquires its own aerial photography using a Piper Navajo Twin and a Cessna 206 aircraft. Each plane is equipped with a Jena LMK 1000 six-inch focal length camera system. Both cameras have forward motion compensation devices and gyrostabilized mounts to ensure image sharpness. The mission is planned on the ground and programmed into a CCNS-4 photo flight management system to ensure accurate photography.
      Choosing the proper altitude is still extremely important in digital photogrammetry, explained Walter Ertz. "Since you can output digital imagery at any scale, there is a temptation to save money by flying at a higher altitude, but that's a false economy. The quality of the original aerial images still dictates the quality of the digital output."
      For the 1"=400' images, ASI flew at 12,000' above ground level (AGL), and chose a 3,000' AGL altitude for the 1"=100' scale photos.
      Once Blackhawk County was flown and photographed, ASI created diapositives of each aerial image which were then digitally captured on the Vexcel 3000 scanner included in the SoftPlotter package. ASI created one digital file per diapositive.
      "The scanning phase is relatively straight forward," said John Schlimmer, ASI's analytical technician involved in the Blackhawk project. "We experimented with brightness and contrast on the Vexcel to get maximum scanning quality. It is crucial for gray scale values to be consistent from image to image for accurate digital correlation."
      ASI gathered ground control points using Trimble 4000 SSE dual frequency GPS receivers to determine X, Y and Z values. To reduce ground surveying time and cost, ASI has provided conventional aerotriangulation services to its clients for the last seven years. SoftPlotter now does almost the entire aerotriangulation automatically, eliminating the need for a point marking device.
      For the interior orientation, ASI technicians input the camera focal length and location of the fiducials into SoftPlotter's mathematical model. ASI then used Erio Technology's ALBANY software on a PC to compute triangulation measurements. ALBANY data was fed directly into the SoftPlotter software to match ground points with image data to determine exterior orientation and set up the stereo pair.
      "Extracting elevation points from the stereophotos is where the automatic stereoplotter really saves us time," said Schlimmer. Essentially all the user has to do is choose the ground spacing of the point collection. The stereoplotter then automatically correlates elements on the two images to generate the X, Y and Z locations of the points for the digital terrain model.
      "Using the analytical stereoplotter would have taken us about three hours to generate the grid points at 100 foot spacing in a single section [8,000' x 3,600'] of the Blackhawk County project," said Walter Ertz. "It took about 10 minutes with the automatic stereoplotter."
      ASI technicians manually added breaklines and corrected point locations obscured by cultural and terrain features. SoftPlotter then took the digital terrain model data to automatically rectify the images horizontally and vertically. The result is digital orthophotographs ready for input into a GIS or hardcopy output device.
      "We used to have orthophotos made by a subcontractor," said Coral Schneberger, ASI's assistant mapping supervisor. "That took extra time, and we weren't able to control the consistency of the product."
      Schneberger estimates ASI is turning around digital orthophoto products for the Blackhawk County project in 25 to 50 percent less time than it took them with manual analytical stereoplotters and outside orthophoto vendors.
      "Not only has softcopy photogrammetry saved us time and given us complete quality control, but it also allows us to offer digital orthophotos to customers," she said. "Before this, we could only provide our customers with hardcopy orthophotos made by an outside vendor. The problem with hardcopy is that what you get is all you've got."
      For Blackhawk County, ASI is creating the digital ortho tiles and the hardcopy orthophoto sheets in-house using ERDAS Imagine. The ERDAS software mosaics the orthophotos and feathers them together to create the seamless map sheets. Most will be delivered as single section orthophotos or quarter section photos.

Extra Costs Pay Off
"Digital ortho-photos cost slightly more to create than hardcopy orthophotos because of the technology involved, but the softcopy data will pay for itself by allowing customers to do so many things they couldn't before," said ASI's Gary Brown. "For instance, the digital data lets the customer make an unlimited number of hardcopy orthophotos, each at whatever scale they need, and every one will be first-print quality. In general, the results are more accurate."
      Customers will experience most benefits from digital photogrammetry in the future, said Vision's Craig Molander. When it comes time to update Blackhawk County, ASI will not have to re-fly the entire county. It can photograph only those areas that have undergone sudden growth or natural change, and the data can be added digitally to the database, in some cases without creating a new digital terrain model. Imagery from any satellite system, regardless of sensor type or pixel resolution, can also be integrated into the database later with little problem.
      "The softcopy digital photogrammetry gives us the ability to meet our clients' current and future needs," said Brown.

About the Author:
Kevin Corbley is the principal in Corbley Communications of Denver, Colo., which provides public relations and marketing services to remote sensing, GIS and GPS firms.

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