| Digital Orthophotos Help Map Nashville's Rapid Urban Development
By Mary Jo Wagner By the time you finish reading this story, about 35 people will have called Nashville their new home. Multiply that by 30 and you get the idea of approximately how many people move to the Tennessee capital each month. That's roughly 12,600 new Nashville residents a year.
Nashville is characterized by rolling hills, resembling a child's rendition of green humps cascading down a dragon's back. Dispersed throughout the valley of these hills lies the city, its landscape conforming neatly to the inclines and declines dictated by the mounds of green.
Since the late 1980s, Davidson County, which includes Nashville, has experienced substantial population growth - an influx in excess of 25 percent according to the Chamber of Commerce. And within the last five years, those numbers have steadily increased, making the county one of the fastest growing areas in the country.
Explosive population growth is a phenomenon affecting several medium-sized U.S. cities nationwide and they've started to bulge at the sides. But Nashville is a particular case because it has faced a rather daunting problem: it's 1996, and the Planning Commission has a ten-year-old GIS base map containing 30-year-old aerial photos with which to handle the rapid urban development.
Last November, the Metropolitan Planning Commission of Nashville and Davidson County (Metro) decided a database update of the most radical kind was in order, delving into new technology never before experienced in their facility: digital orthophotos. Metro called for new aerial photography of the entire Davidson County and an update of their existing basemaps produced from digital orthophotos. Merrick, a GIS mapping and surveying company in Denver, Colo. was given the task.
Kim McDonough, planner III at Metro and project manager for this program, says that allowing 30 years to go by was far too long, but it's not that they haven't tried. They've been diligently trying to update their GIS for the past ten years with no success. Good Intentions
The first set of aerial photographs for Davidson County were acquired in 1961. Using this photography, 1,040 property maps were painstakingly produced, based on Kelsh sheets, providing complete coverage. However, to the original photogrammetrists' credit, the property maps created were very accurate, says McDonough, considering that they were initially intended for taxation purposes.
For the next 30 years, those ink and mylar sheets were maintained in a manual form. Any changes that needed to be made were done by hand.
In the early '80s, Metro attempted to automate its mapping operations. But it failed. A second attempt was made in 1986 with the idea to create a central database that could be used by different departments to exchange geographic data. McDonough says the base map was dated at this point, particularly in areas of high development activity. A consortium was formed in 1987 and the project began.
The first task was to produce planimetric, or physical feature maps and topographic maps of the area. New aerial photos were acquired in 1987, but the production of the planimetric and topographic data wasn't completed until 1990. And by then, the program had dissolved.
So Metro found itself again without the results it had hoped for. But even though the project did not reach its goals, it paved the way for today's comprehensive updating project. Nashville's New and Improved Look
The photogrammetry/digital orthophoto project, as it's called, officially began last March. And McDonough and his colleagues may conclude that waiting this long might have had its benefits. Had they begun this project a few years ago, they might not have had the opportunity to use a new image processing system called PHODIS, manufactured by Carl Zeiss Inc. in Englewood, Colo.
With PHODIS, work on the city of Nashville's new database was enhanced and expedited using the full range of photogrammetric image processing tools offered by this system. The system includes a digital stereoplotter for 3-D data collection and editing, as well as software for creating digital orthophotos. Automatic DTM and Triangulation using digital image correlation are features also included in the package.
By the end of March, GRW Aerial Surveys in Lexington, Ky. had collected aerial photography for the entire county, which is 535 square miles. There were only three clear days the entire month allowing GRW a very tight window with which to acquire the data. The contact prints have been sent to Metro.
"The photography appears to be of very high quality," says McDonough. "I'm very pleased."
GPS surveys, performed by Cherry Land Surveying of Nashville, have also been carried out which will allow Merrick to create the digital orthophotos with more precision. Survey points marked on the ground were taken and matched with points marked on the photography for a true control over the entire project area in a process called aerial triangulation.
In preparation for the most challenging part of the project, merging the old and new data sets, Merrick is implementing its procedures on a prototype. In other words, they'll be having a test run on a smaller data set using the new Zeiss software system and stereoplotters to be confident that the process will work efficiently. PHODIS
The Photogrammetric Image Processing System from Zeiss, is a product that Merrick has been working with for over one year. More commonly known as softcopy, because the system uses softcopy stereoplotters, the product is a significant breakthrough, says Brian Raber, a certified mapping scientist and vice president of GIS at Merrick.
Softcopy allows Merrick to view the ground in a 3-dimensional model. The old vector map information of Davidson County, produced in 1987, can now be draped over the 3-D model, easily indicating areas needing updating.
"Previously what the city would have to do is manually go through the old and new aerial photos," he explains. "Using information like building permits, improvement plans and local knowledge, they'd have to identify the areas that had changed for the photogrammetrist.
"Softcopy has eliminated the need for that labor-intensive and costly process," he continues. "That was a significant step that we had never been able to accomplish in previous projects that involved updating."
Merrick has been working with softcopy on a daily basis to perfect the procedures. After all, this project is the largest softcopy undertaking to date, calling for the production of over 720 tiles, or map sheets, with each tile covering nearly one square mile. The Summer Mapping Season
That's why this summer will prove to be the most challenging part of the project. That's when the mapping will begin. The first task will be to digitize, or rasterize, the new aerial photography into the PHODIS system. Merrick will use the Zeiss SCAI Scanner to scan original aerial negatives at 14 microns. Then they will begin to merge the data sets together - the old aerial photos, the digital property maps and the planimetric and topographic data. This may be a daunting task indeed if you consider this: Metro's property maps reveal right-of-ways for roads that don't even exist on the planimetric maps; and two new runways at Nashville's International Airport that appear on the 1996 aerial photography were just open fields on the '87 photos.
Raber says it all comes with the territory of updating projects.
"The biggest challenge in these projects is taking the previous data, which was produced with the best technology available at the time, and trying to normalize that to current GIS standards," Raber says. "We discover inconsistent data, features that weren't collected, polygons such as building outlines that don't close. Information that forces us to have to work harder to fix the problem."
Metro requested 22 specific features, including street centerlines, bridges, driveways, tennis courts and cemeteries that need to be identified in the final maps as well.
Using the softcopy process, Merrick will produce 800 digital orthophotos. Raber says, depending on the amount of changes that have occurred in any given area, it can take from two to 20 hours of processing time per model. But the outcome is worth it, and the information contained in each digital orthophoto is superior to the traditional aerial photos, says Jack Gains, project manager at Merrick.
"With aerial photos, you can use them and plot them, but what you need for engineering and planning purposes is to be able to also plot the road edges, the utility features, the parcel features," he says. "And the orthophotos make a very good backdrop. They are a very good, accurate mapping tool."
In addition to creating the digital orthophotos, Merrick will also record and attribute Meta Data to each data set. For this project, the Meta Data will include the scale, the date collected, who collected it and who owns it. This will supply Metro with the historical lineage specific to each data set to ensure accuracy in the future.
The existing GIS database, containing the planimetric and topographic data, will be completed first, as McDonough says that was their first priority. Merrick plans to provide Metro with the final set of digital orthophotos, at a scale of 1"=200 ft., by the end of 1997.
Even though McDonough says the digital orthophotos are new territory for him and his colleagues, he's excited about the new technology, he says. But he's even more excited about the improvements the GIS is already bringing to the workplace.
"Much of our planning and statistical work is now taking place on the GIS," he says. "In the past, it was never practical to create a county-wide zoning map because it was too complex and time consuming. Now we can make a new zoning map everyday. We can track our new developments better. And we're developing the capability to where we can look at detailed landuse on a county-wide basis.
"Before, anytime a sub-division came in, we sent out 22 copies of that to different agencies and they all updated their records accordingly," he continues. "Now we're finally on the path to where one set of maps gets updated and everybody looks at those. That's a big time and money saver."
Indeed, Metro has come a long way from the ink and mylar sheets of 30 years ago. In fact, Metro has become a bit like the suburban family who's just gotten a new car and is the envy of everyone on the block. Now the public works department has taken steps to develop a GIS, and provide data to Metro's GIS. The health department is interested in the technology and the water services department already has equipment and software on order.
The police department is also investigating using a GIS for neighborhood incident mapping. Using a GIS will allow an officer's isolated report to be fully integrated and easily tied to other information sources, possibly revealing patterns of problems in certain neighborhoods.
Both McDonough and Merrick feel a trend coming on. "Today you can produce many more maps, much quicker, using GIS, compared to having stacks of mylar sheets which could take weeks to update," says Gains. "I think there are going to be a lot of "update mapping" projects starting in the next few years." About the Author:
Mary Jo Wagner is a freelance writer/editor who writes about GIS and remote sensing. She may be reached at 415-291-8292 (phone) or e-mail: [email protected] Back |
