| GIS/Aerial Photo/Digital Ortho: Aerial Photography's Bright Future
Color aerial photography is setting Lorain County, Ohio's digital ortho-based GIS project apart from many others.
By Kevin P. Corbley Lorain County, Ohio, has never looked better.
That's what Lorain County officials expect potential developers will say when they see the full-color orthophotos and high-accuracy contour maps the county's new GIS will be capable of generating by this time next year.
Located on the western edge of the sprawling Cleveland metropolitan area, the suburban and rural county views its GIS as more than just a tool that will help its overworked employees keep pace with the recent wave of development. Lorain County plans for its orthophoto-based GIS to become one of the assets that makes it more attractive to developers and new industry.
Color aerial photography and 2-foot contour intervals are the main factors that set the Lorain digital orthophoto project apart from many others. Although these requirements present some out-of-the-ordinary mapping challenges, Aerial Services Inc., the aerial survey and digital photogrammetry firm leading the endeavor, expects to turn the project around more quickly than normal - thanks to the latest advances in digital ortho technology. Lorain in Living Color
With 288,000 acres, Lorain is the tenth largest county in Ohio. From 1990 to 1995, its population grew from 275,000 to an estimated 290,000 people, many of them crowding the county's eastern border where spillover suburbs from Cleveland are sprouting rapidly.
Lorain County has found itself in the enviable position of having predicted its growth and the need for a first-rate mapping system to manage the expansion. Five years ago, the County Auditor's Office began building a GIS development fund. Today, that money is being used along with input from the Real Estate Assessment fund to pay for the project.
The Auditor's Office plans to use the GIS for many standard county management activities: performing neighborhood real estate analysis for property appraisal, updating school district and voting precinct boundaries, developing an automated Emergency 911 mapping system, and analyzing the tax base in preparation for bond issues.
"The digital basemap will allow us to perform analysis that we simply couldn't do before," said Mark Stewart, Lorain County auditor. "One of the biggest benefits of a digital map is that we can update inexpensively and easily by having ASI refly only those areas where change is occurring."
The county opted to have the aerial photos flown with color instead of black & white after considering a tradeoff in cost and information content.
"The cost differential was relatively small compared to the benefits of color," said Stewart.
The primary benefit of color is its interpretability. The differences in color orthophotos make property lines, crop types and land surfaces much easier to distinguish with the naked eye. Color also has an aesthetic appeal that plays into Lorain's plan to use the GIS to offer superior government services that taxpayers and potential citizens will find appealing.
"If a taxpayer or a potential developer wants a vector map of a parcel, we can print off a full color picture using our deskjet printer," said Stewart. "The color map will make a much more attractive presentation."
The 2-foot contour maps aim at pulling new developers and industry into the county. Developers considering any kind of commercial or residential construction always ask for contour maps for use in costing out their projects, explained Stewart. He says that if these maps can be supplied quickly and cheaply to the developers, they will get the message that Lorain County is an appealing place to do business. The Challenge of Color Film
ASI, based in Cedar Falls, Iowa, has been flying aerial surveys for 25 years and has been involved in digital photogrammetry since 1984. ASI's president, Gary Brown, isn't going so far as to say that color orthos will be the latest trend, but he does believe color is easier to work with than many believe.
"Most GIS developers choose black & white film for two reasons - cost and quality," said Brown. "Cost must be examined in view of budget constraints, but the quality issue may be somewhat clouded by misconception."
Brown explained that B&W aerial films have traditionally offered better resolution than their color counterparts. But in the past 10 years, color film technology has increased dramatically, and B&W now enjoys only a slight quality advantage. After some study, ASI determined the color film could achieve Lorain's 1:1200 scale map requirements flown at the same altitude as B&W.
Extra care must be taken during the photo acquisition, processing and scanning to ensure that the color is always in balance or the digital correlation in the orthorectification procedure will be inaccurate, explained Walter Ertz, ASI's mapping supervisor.
During the flight, the camera operator had to constantly adjust the exposure setting to compensate for lighting variations as the sun angle changed throughout the day. If possible, ASI flight technicians loaded new film rolls only between survey lines.
ASI avoided having to compensate for film color variations induced by drastic changes in atmospheric and ground conditions in the Lorain County project thanks to a fluke of nature. Flying two aircraft almost continually on a cloud-free spring day in 1995, ASI was able to acquire both high and low altitude photos over the entire project area in a single day.
ASI performs nearly all phases of its mapping and orthorectification work in house, but knows when special services are needed. It sent the color film to HAS Images in Dayton, Ohio, for processing. Instead of processing the rolls as a batch, HAS developed test strips for every roll to ensure maximum color balance. In the past, ASI would have ordered color diapositives from HAS, but new digital analytical aerotriangulation capabilities obviates the need for them, so negatives are used instead.
In 1994, ASI became one of the first aerial mapping firms to purchase an automatic softcopy stereoplotter for production of digital terrain models and digital orthophotographs. ASI uses the SoftPlotter softcopy photogrammetry system developed by Vision International of Alexandria, Va. The SoftPlotter system includes a Vexcel 3000 scanner that creates digital files from the color negatives.
"The key to color scanning is to make sure you don't have saturation at the end of the histogram," said John Schlimmer, ASI's Analytics Technician.
The Vexcel scanner allows the operator to adjust brightness and contrast for red, green and blue. Schlimmer said the brightness should be adjusted first and then the contrast should be stretched. He recommends using as much of the 0-255 range as possible. Autocorrelation and Color
The Lorain County project was the first color survey in which ASI used SoftPlotter. There was some concern that since the system automatically picks and correlates elements in the overlapping images for generation of X, Y and Z values, it may not be able to recognize common ground features due to slight hue variations from frame to frame. However, that problem did not arise, and autocorrelation has proceeded smoothly.
Vision's vice president, Craig Molander, explained SoftPlotter has been programmed to handle color photos using the red band as the default. "Autocorrelation runs best on the red channel because it is the sharpest and least susceptible to saturation and atmospheric haze," he said.
Schlimmer noted that any time lost in balancing the histograms of each color in preparation for scanning is recouped during the TIN editing process, which is conducted manually after the autocorrelation.
"Color makes it easier for the technician to see the ground elevation when editing points," said Schlimmer. "The technician can add breaklines more quickly at roads and water boundaries as a result." Getting the Contours
Many counties like Lorain that have a mix of suburban and rural land will have the rural land mapped at a larger scale than the suburban areas because the parcels are usually much bigger in the rural region. Gary Brown noted that in its decision to map the entire county at 1:1200 scale with 2-foot contours, Lorain was looking to the future when much of the rural land will have turned suburban, and the smaller scale maps would be needed.
The request for the contours was a late addition to the project description and required additional ground control. Original plans called for a high to low altitude drop with the photos taken at 12,000' and 3,000'. Ground control points (GCP) would be picked in the high altitude photos and transferred to the low altitude survey for densification.
A total of 100 GCPs were picked for the original survey network, but an additional 792 control points had to be surveyed with GPS to achieve the 2-foot contour interval. That averaged out to one additional point for every three low altitude frames. ASI used Trimble 4000 SSI and Ashtech Z12 dual frequency GPS receivers to conduct the ground survey. Automating the Scanning Process
Two years ago, ASI streamlined its orthorectification process with the addition of softcopy digital photogrammetry. Now the mapping firm has added a new scanning tool that is already reducing scan time by 20 percent. The new product is the Integrated Digital Aerotriangulation System (IDAS) developed by Vexcel Imaging Inc. of Boulder, Colo., for use with its line of scanners.
IDAS has completely altered the way ASI conducts its aerotriangulation. The new system, which includes software that runs with the Vexcel 3000, eliminates the need for a pug drilling device and removes the analytical plotter from the aerotriangulation procedure.
Using IDAS, ASI scans each film plate twice. The first pass is at 150 micron resolution for selection of fiducials and tie points. These can be picked manually or automatically. Once they have been chosen, the IDAS automatic film drive moves the film through the scanner for the second pass where only those patches containing fiducials or points are scanned at 8.5 microns.
"The selective second pass keeps the file size down to about 8.5 megabytes per plate," said Schlimmer. "And the auto film drive lets us run the second pass over night." Delivering the Orthos
ASI is using IDAS for the first time in an operational mode and expects to reduce scanning time even more as the project progresses. The goal is to scan an entire color plate in less than one hour for aerotriangulation and digital ortho production. ASI expects to begin delivering completed orthophotos to Lorain early this spring. They will be processed and delivered progressively over the next 12-18 months.
Lorain County will be building the GIS over a period of several years involving additional city and county agencies. The Auditor's Office has budgeted the mapping project so that funds will continually be available for long-term updating and maintenance of the maps.
"The implementation of this program will greatly impact how the taxpayers will view Lorain County now and in the future," said Stewart. "Lorain County is moving in the right direction." About the Author:
Kevin Corbley is a freelance writer specializing in remote sensing, GIS and GPS. He is located in Denver, Colo., and may be reached at 303-987-3979, or by email at [email protected]. Back |
