Imagery Technology Helps County Make GIS Data Accessible
By J.B. Kuppe
Business Unit Manager, LizardTech, Inc.
Editor's Note: Mecklenburg County, North Carolina, one of the fastest growing counties in the United States, uses GIS in 15 departments. The Department of Engineering and Building's Mapping and GIS Services section is responsible for mapping and addressing and all GIS-related products.
GIS data collected by Mecklenburg County--except in rare cases--is considered part of the public domain due to North Carolina's Open Records law. Citizens as well as public and private agencies are encouraged to make use of GIS data for any number of applications including site planning, architectural development, real estate planning, and land brokering. It has also been the county's goal to move GIS from a specialized application to an enterprisewide decision making tool.
Developing an Accurate Basemap
Mecklenburg County began a project to create an accurate photographic basemap that would support all GIS-related projects. The County worked with Geonex Corporation, Chicago Aerial Survey Co., and Cartographic Aerial Mapping to acquire accurate digital grayscale orthophotos for all 538 square miles of the County. The digital ortho sheets were flown at a 1"=1,000' scale.
The aerial photography project for Mecklenburg County produced a typical grayscale air photo set. The 708 9" x 9" films were scanned to a one-foot resolution to one pixel, producing files of approximately 23 MB for each sheet. After orthorectification (which removes image distortion) the imagery was delivered in Tagged Image File Format (TIFF) with TIFF World (TFW) files. TFW files are the ASCII files containing information for geocoding image data so TIFF images can be used directly by GIS and CAD applications. Because the imagery is fully georeferenced and orthorectified, multiple image tiles can be pulled up in ArcView GIS, and the images fall into place seamlessly. Sixteen gigabytes of imagery, stored on 27 CD-ROMs, were delivered over the course of the 14-month project.
An End to the Juggling Act
The next challenge was to make all of this data accessible to the County's many departments and the public. Managing 16 gigabytes of georeferenced TIFF files and making that much data readily available to the public is not an easy task. Andy Goretti, project manager for the Mecklenburg County Engineering and Building Standards Department, needed a way to provide imagery to the 200 ArcView GIS users within the County as well as the outside vendors who access the imagery. During the 1998 Urban and Regional Information Systems Association (URISA) conference, Goretti got his first look at the Multiresolution Seamless Image Database (MrSID) applications from LizardTech, Inc., an Esri business partner. "I was so amazed at MrSID's ability to zoom and pan on a high-resolution image without slowing down the process," said Goretti.
MrSID, a powerful image encoder, viewer, and file format for massive images, allows instantaneous viewing and manipulation of images locally and over networks while maintaining maximum image quality. MrSID was developed at Los Alamos National Laboratory by the same team that developed the United States Federal Bureau of Investigation's standard for fingerprint image compression. Thanks to the Federal Technology Transfer Act, the exclusive license for MrSID was awarded to LizardTech in 1995. MrSID has evolved from a powerful, wavelet-based image encoding technology for scientists to a highly portable raster file image format for imaging professionals in the geospatial, medical, and publishing fields.
Making the Data Manageable
The biggest problem for Goretti's team was to find a way to provide more than a small number of tiles of imagery for an area and to do this for lots of users in a practical manner. In initial tests using MrSID, tiles were reduced to 5 percent of their original size, making an individual tile only 1.15 MB--small enough to be easily transferred to an outside vendor via FTP or on a floppy disk. After running a few initial compressions to check on quality versus file reduction, 28:1 was selected as the target ratio and MrSID was set up to run overnight on the whole data set. The source imagery was encoded at the rate of about one gigabyte per hour on a standard Pentium PC. The whole data set was compressed on a machine with 512 MB of RAM in a little less than 14 hours down to a single 608 MB MrSID Portable Image Format file. "We now have the entire data set available on a single CD. It's unbelievable," said Goretti. "We no longer have to shuffle through a pile of CDs to find the right tiles. Even better, MrSID's multiresolution feature automatically generated multiple image resolutions, eliminating the need for re-sampling. Now I can go from looking at the whole image set to an area of the source one-foot resolution with one click inside ArcView GIS."
Imagery on the Internet
Mecklenburg County knew that having all its imagery available on a single CDROM was only the first step in its goal of enabling public access to the GIS data. Plans to make GIS accessible to all Mecklenburg County residents are closer to reality now that MrSID has been used to successfully organize the County's large collection of imagery. Mecklenburg County GIS considers information one of the County's greatest resources and the goal is to distribute information in a manner that saves both time and money. The Internet has quickly become the most popular medium for the electronic dissemination of information including distributing GIS data to the public. The GIS team at Mecklenburg County recognizes that before too long, access to the Internet will be readily available for all County residents and that the demand for immediate access to information will continue to grow.
In response, the Mecklenburg County GIS team has developed a -powered Web site that lets its residents access various layers of GIS data such as roads, tax parcels, Federal Emergency Management Agency (FEMA) floodplains, building footprints, schools, churches, zoning, and tax real estate information. The Web pages include both maps and tables containing information specific to a user's request. The Web site uses the County-wide cadastral maps that were recompiled on a planimetric land base from approximately 3,800 tax maps showing 245,000 parcels. The planimetric base supports an absolute positional accuracy of plus or minus four feet.
The County has already begun to record the Mecklenburg County using color aerial photography at one foot to one pixel. "After MrSID solved our image management issue," said Goretti. "We knew color was the way to go since it gives the most accurate representation of the as-built environment. I am confident that with the systems we have put in place and with the compelling new technologies and products available today, like MrSID, we will only continue to improve public access to information."
For more information please contact
Engineering and Building Standards Department
700 North Tryon Street
Charlotte, North Carolina 28202
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1008 Western Ave., 2nd Floor
Seattle, Washington 98104