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DIGITAL ORTHOS
City of Napa Creates GIS Basemaps
Aerial survey technique allays concerns over disruptions
By Kevin P. Corbley The vineyards and gently rolling hills of Napa Valley conjure images of serenity and tranquillity, drawing tourists from all over the world to Northern California each year. And the city of Napa wants to make sure visitors leave the area with pleasant memories of wineries, country inns and bicycle rides instead of flooded roads and backed-up drainage basins.
To accomplish this, the water division of the Napa Public Works Department is spearheading an effort to map water utilities and build a GIS for management of water utility infrastructure, as well as for use in other city departments.
"Right at the start we decided to use digital orthophotographs as the basemaps for our GIS," said Gil Harrington, GIS coordinator in public works. "Orthophotos are the quickest way to assemble a basemap, and they provide us with much more information than digitized maps ever could."
Fortunately, flooding along the Napa River in the spring months has not been deadly. It has, however, caused major damage within the city, and in the past several years the high water has become increasingly more common and more severe. As a result, flood control is another application Napa plans for a new GIS now under development. The city contracted HJW (Hammon, Jensen, Wallen & Associates) of Oakland, California, to conduct the aerial survey, perform the orthorectification, produce contour maps, and complete a GPS ground survey of the area.
"Every aerial survey and orthorectification project presents unique challenges both in the air and on the ground," said Bill Zeman, HJW's senior project manager. "We worked closely with Napa to create minimal disruption to the tourist trade and to keep within the project budget."
HJW was able to satisfy both of these requirements by utilizing airborne GPS synchronized with its aerial camera.
"Airborne GPS is one of the latest trends in controlling digital orthorectification," said Zeman. "It keeps costs down by reducing the number of ground control points needed, and in Napa that meant we didn't have to clutter the landscape with a lot of painted markers and panels." Collecting Air Photos
Napa Public Works requested a digital ortho basemap delivered at three scales: 1"=50' for water treatment plants; 1"=100' for the city itself; and 1"=200' for the pipelines from the reservoirs to the city. The department also wanted contour intervals of 5' for the 200-scale, 2' for the 100-scale, and 1' for the 50-scale products.
To achieve the desired scales, HJW flew the survey at 1,650-feet, 3,300-feet, and 6,600 feet above mean terrain elevation to achieve the sub-foot pixel resolution needed to generate 50-, 100-, and 200-scale orthophotos, respectively. The firm used its own Cessna 310 equipped with a Zeiss RMK Top camera with forward motion compensation.
"We now typically fly each survey with two GPS systems in the aircraft-one navigates and the other records coordinates for each photo exposure," said Zeman.
Mid-winter was chosen for photo acquisition to take advantage of the leaf-off vegetative condition that creates fewer shadows that might obscure features in the orthorectification process. Prior to the flights, HJW technicians planned the precise location of each exposure over the 20-square mile project area. The coordinates for each were input into the Zeiss T-Flight mission planning software.
The mission planning software attaches to a Trimble 4000SSE dual-frequency GPS receiver in the aircraft. Together, the software and GPS guide the pilot to the survey site and actually help to line the plane up onto the correct flight paths to collect the programmed exposures. The software also fires the camera at the precise acquisition time.
HJW has integrated a Trimble 4000SSI dual-frequency airborne GPS receiver directly into the Zeiss mapping camera. It is synchronized with the camera to collect a coordinate with each exposure, usually in half-second intervals.
"We have found that airborne GPS provides the vertical accuracy we need to create orthos and contours for mapping scales of 1"=200' and above with minimal ground control," said Zeman. "But at 1"=100', we recommend having some additional vertical ground control available, especially when 2' contours are needed."
Napa already had a series of good benchmarks available for creating the 100-scale orthos. HJW placed panel markers on 40 of these benchmarks on the days of the photo acquisition.
"Napa was really pleased that we didn't have to go around painting a lot of markers on city streets and sidewalks for this project," said Zeman. "There have been incidents in other locations where these markers have inconvenienced citizens."
Zeman stressed that the vertical accuracy of the airborne GPS is not sufficient to support 1"=50' scale mapping without additional vertical ground control. HJW placed control markers around the water treatment plants where these photos were acquired. Since these were out of town, they did not interfere with traffic.
"We really felt the price for the aerial survey and the orthorectification was well worth the expenditure," said Harrington. "The main reason the price was so reasonable was because HJW used so few ground control points. The airborne GPS really worked out well in this project." Creating the Digital Orthos
HJW performed the entire sequence of photogrammetric techniques required for the Napa project in-house. They used stereo photogrammetric techniques to collect the digital terrain models (DTMs) from the digitized aerial photographs. Contours were created from the DTMs and control points using Plus III Terramodel contour interpolation software.
Next, the HJW technicians scanned the black-and-white photos with a Vexcel VX4000 scanner at 1000 dots per inch to yield the desired pixel resolution. They then input the DTM, aerotriangulation data and camera solution information into OrthoViewª, a semi-automated UNIX-based digital orthorectification package developed by HJW. With control points selected by the operator, the software performs a pixel-by-pixel orthorectification of the photos.
"It takes about 10 to 45 minutes to orthorectify one image depending on the resolution," said Zeman. "Our software allows for batch processing and therefore efficient use of our networked SUN workstations."
After the photos were orthorectified, the technicians radiometrically corrected and mosaicked them into a seamless digital basemap. The technicians were able to edge match even those photos that had been acquired at different scales.
HJW delivered the orthophoto mosaic to Napa Public Works in TIFF format with TFW world header files for georeferencing. Contours were created and shipped separately as AutoCAD files. HJW delivered all of the files in the summer of 1997. Capturing Utility Data
Because the Napa GIS will be used extensively by the water division, water utility infrastructure is an important part of the system. Public works also contracted HJW to conduct a GPS-based ground survey of utility features-manholes, fire hydrants, sewers, storm drains, and water valves.
"In 1"=100' photography, we can usually capture significant features like the manholes and storm drains, but the smaller items, such as water valves, have to be painted with symbols to show up in the photos," said Zeman.
But it was exactly that kind of painting to which the city objected because it would have detracted from the aesthetics and clean appearance Napa projects. HJW suggested a real-time GPS ground survey using differential correction.
HJW field crews hit the streets of Napa equipped with Trimble Pathfinder Pro XR GPS receivers and data collection systems. In two months, the crews gathered location information and details on more than 16,000 water-related features in and around the city.
"This system offered the kind of speed and accuracy we needed for this project," said Zeman. "It took an average of 10 seconds to occupy a feature and record its location with sub-meter accuracy."
HJW divided the utility features into appropriate GIS layers and delivered the feature file to Napa in AutoCAD format. Adding GIS Layers
Public works has built numerous layers on top of the basemap in the past year, many with flood control in mind. For instance, digital line graph flood maps have been acquired from the Federal Emergency Management Agency (FEMA) and merged with the contour intervals to model various flood scenarios.
"Ultimately, the city will be able to use this information in a major project to re-align the Napa River back to its natural state, which should reduce the flooding," said Harrington. "In the meantime, the GIS models will help us pinpoint flood-prone facilities."
Public works has also rectified parcel maps from the Assessor's Office with the basemap and added rights of way data to the GIS layers. Other city departments including fire and police have been invited to contribute data to the system. Eventually, the GIS will be tied to the city's existing SCADA system.
"Everyone is pleased that we decided to use orthophotos for the GIS basemap because it provides us with so many options to add value to the data," said Harrington. About the Author:
Kevin Corbley is a freelance writer and consultant specializing in remote sensing, GIS and GPS. He lives in Denver and may be reached at 303-722-0312 or at [email protected].
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