Los Angeles - How a City is Using Digital Orthophotography to Expand Its GIS Data Base for Multi-Agency Applications
by Joseph M. Bartorelli
Georeferenced orthophotos form the foundation of any city's GIS database. Large metropolitan city governments face special challenges to keep their mapping programs up-to-date due to their sheer mass.
The City of Los Angeles has contracted HJW Geospatial (HJW), Inc., an aerial photography and digital mapping firm to provide digital orthophotographic imagery of the entire 550-square mile city. The orthophotos will provide the base map for the massive citywide GIS database and will be available to numerous agencies within the City.
This presentation will focus on the process of initiating, implementing and successfully completing a large metropolitan citywide digital orthophoto project. I will discuss the value of using Esri software for quality control throughout the project. Lastly, I will illustrate specific applications and benefits of orthophotography for a multi-agency organization.
Over the last ten years, digital orthophotography has become a valuable tool in the GIS world. No longer used as just a colorful backdrop for vector data, digital orthophotography is becoming the backbone of today's GIS data environment. In the past, agencies have had to rely on topographic maps, usually created using different technology, methods and on different dates, put together piece by piece to form a sort of overall map database. This results not only in problems with data management, but major problems with overall accuracy. Even more inaccurate if the individual maps exist on different coordinate systems.
Today, most agencies are finding that digital orthophotography is the starting point for an accurate, seamless GIS. If the pixel resolution is sub-foot, digital orthophotos provide a wealth of information: lane striping, vegetation, utilities, curb-lines, gutter-lines, building footprints, pole locations and so on. They are scalable and horizontal measurements can be derived directly from any software with a measuring tool. The surface model used to create orthos can be used for water flow studies, slope analysis and contouring. Truly, the applications for digital orthophotography have grown tremendously.
When starting a large mapping project, budget is always a large concern. How much is available? Where is it coming from? What type of data, and how accurate, can you get with available funds?
The most beneficial way to go about this is with a jointly funded project between agencies that can all utilize, and benefit, from the new land information. For the City of Los Angeles Digital Orthophoto Imagery (DOI) project, the funding was gathered from participants both within and without. Within the City, departments such as Engineering, Water & Power, Police and Fire contributed funds. Outside private entities contributed, as well, such as a large Los Angeles telecommunications company.
An RFP was released requesting 4 different options with various map scales and pixel resolutions. Proposals were received from some of the best-known mapping firms in the industry. In the end, HJW was selected to provide 1"=100' Black & White (B/W) digital orthophotography, with .5' (6 inch) pixels for 550 square miles covering the City of Los Angeles and sections of the San Fernando Valley. In addition, compressed imagery and computer-generated contours would also be delivered.
Ground control for the Los Angeles DOI project was provided by the City of Los Angeles, Survey Division. 39 ground control points (panel points) were required to control the aerial photography. As a QA/QC measure, the City placed 25 additional points that were used as checkpoints during the aerotriangulation process. The coordinate system for this project was California State Plane, Zone 5 (NAD '83 and NAVD '88).
Airborne GPS was also utilized on this project, which resulted in huge cost savings towards the City during the Ground Control stage. If Airborne GPS had not been used, the project would have required approximately 900 additional ground control points. In this technology, a GPS receiver on-board the aircraft is linked directly to the camera and several satellites. Each time the camera is triggered for an exposure, a pulse is sent to the GPS receiver that records the time of the exposure and satellite information. The exact coordinates of the center of each photograph are captured and recorded, which later were incorporated in the aerotriangulation calculations. During the flight, a GPS receiver on the ground, which was tied into a local benchmark, recorded all of the information send from the GPS receiver on-board the airplane.
The Los Angeles DOI project was flown using HJW's state-of-the-art Zeiss RMK Top 15 aerial camera. Since all products are derived from the aerial photography, this is the most important phase of any mapping project. The flight layout for the DOI project consisted of 64 flightlines and approximately 2,200 B/W exposures. The scale of the photography was 1"=600', or 1:7,200, ensuring that NMAS and ASPRS accuracies would be met for the resulting 1"=100' orthophoto map scale.
The majority of the photography was flown with 60% forwardlap and 30% sidelap. In doing this, HJW had complete stereo coverage of the entire project area to produce the digital terrain model (DTM) without compromising accuracies. However, since Los Angeles has areas with extremely high buildings, some lines of the photography were flown with 80% forward and sidelap to allow for maximum photo coverage. In addition, the downtown area and other selected sites were "spot flown" to allow certain higher structures to be centered in an exposure to diminish displacement. The extra imagery allowed HJW to make hundreds of orthophoto patches for multi-level overpasses and high buildings, which were then combined into a high-quality seamless orthophoto mosaic for the downtown areas of Los Angeles.
Scanning was performed on a Vexcel VX4000 roll-feed, precision scanner. The negatives for the entire project were scanned immediately after processing to reduce the risk of dust, scratches or other foreign objects being present in the scanned imagery. Production of contact prints and diapositives are also performed after scanning for this same reason.
On a project of this size, there are always nominal variations in radiometry throughout the initial scans. Flightlines are flown on different days and during different conditions, so it is virtually unavoidable. The best way to reduce this is to maintain flight times between 10 AM and 2 PM when the sun is near it's zenith and shadows are minimal. To balance radiometry in the scans even further, sophisticated auto-dodging software was used during scanning to match and balance histograms.
The next process is the addition of supplemental control to the targeted ground control. Since all of the photography was scanned for orthophotography, HJW opted for Softcopy Aerotriangulation. Softcopy Aerotriangulation, (Soft AT), is a routine FAAT photogrammetric procedure for establishing additional control points without the need for additional ground surveys. It densifies the targeted ground control and acts as a check on the ground control measurements. The digital images and the GPS ground control data are loaded onto a digital photogrammetric workstation. The individual images are brought up to measure the fiducials and the interior orientation is computed. The system automatically selects and measures all of the passpoints and tiepoints on all of the images upon which they appear. This step can be performed in a semiautomatic mode where the operator interactively assists in the selection process; under some conditions, this is the more efficient method. The relative orientation is computed and will show all residuals on the passpoints and tiepoints. Any residual that exceeds the predetermined limit will be automatically "flagged" and will be revisited and manually corrected.
During the aerotriangulation process, the system will read the GPS ground control data, which resides on the system. This data, along with the results of the relative orientation and the ground control data, will be used to compute the final bundle adjustment for a given block of images. The final output from the bundle adjustment will include all the parameters necessary to set up an analytical or softcopy stereo instrument. The analytical instruments have special software that enables them to be set up automatically from the output parameters since the diapositives are not being "pugged."
The project area was sectioned out into a total of five blocks for the AT process. This way, DTM generation could be started immediately after AT was completed for each block and schedules could be achieved. In addition, the additional control placed by the City was withheld from the AT solution, then used as checkpoints to verify accuracy.
A digital terrain model (DTM) was also collected from the stereo photography and used for ortho generation and contouring. The DTM is comprised of break lines which delineate various terrain defining features, such as ravines, ridges, tops and toes of slopes, as well as mass points which are used in flatter areas. In addition, HJW collected DTM "patches" over bridges and overpasses. HJW's OrthoView software has a unique module that removes distortion that often occurs with elevated structures.
The DTM for the Los Angeles DOI project was collected at a map scale of 1"=100' and has a horizontal accuracy of +/-2.5'. As an added value, 5' contours were also delivered for the entire project area.
The last step of production was to create the digital orthophotography. Control points from the aerotriangulation and ground survey were used to tie the digital images to real world coordinates. The DTM collected from the stereo photography was then used to adjust each image pixel into its correct position. For the Los Angeles DOI project, HJW used a cubic convolution algorithm to perform the ortho processing. This technique provides a much more accurate solution than more widely used nearest neighbor methods.
As I mentioned in the Aerial Photography section, often radiometric differences can occur between adjoining images due to varying camera angle, sun angle, and weather conditions between adjoining photographs. To help reduce this effect, both the DTM data and the scanned images overlap adjoining images when orthorectification takes place. This was done to further reduce all geometric edge matching problems and radiometric differences were minimal to nonexistent. To remove any remaining radiometric differences, HJW used special software to perform this task. The software uses large blocks of reduced resolution orthophotos to take radiometric samples, approximately 1,000,000 per frame, to create histograms and compute the radiometric corrections for the entire block of orthophotos. The software then applies the computed corrections to each orthophoto frame as it is stitched into the overall mosaic. This is a unique method of balancing in as much as it looks at the entire block of images rather than just the adjacent image to determine the corrections, thus creating a more uniform overall product. In addition, digital orthophotos were joined, with operator assistance, by inserting seam-lines, or mosaic-lines, at ground level. In this way, it is virtually impossible to detect edge lines in the overall mosaic or between adjacent tiles.
The result was a combined seamless, radiometrically correct orthophoto for the entire project area and tiles (~1800) were "cookie-cut" out into the City's existing tiling scheme.
For the City's QA/QC process, HJW loaned the Survey Division a high-end NT workstation loaded with Esri's ArcVIEW software. For ease of delivery, the NT was fitted with an 80 GB removable drive. Deliveries were made every other week on this removable media with images in .TIF, .SID and .ECW formats. This allowed the City to load the orthos and have the data present immediately on their system without having to copy the orthos from CD-ROM or tape.
Due to the fact that the City had an existing parcel landbase that was highly accurate (survey grade), it was ideal to use as a QA/QC tool. Each image was loaded into ArcVIEW and checked against the parcel landbase for positional (horizontal) accuracy. If errors were evident, the City would mark the area and add the polygon to an "error" layer. Not only was it a great way to check the horizontal accuracy of the orthophotos, it was also a great way to check the horizontal accuracy of the City's parcel data. During this process, the City would also check each orthophoto for cosmetic errors and tiling discrepancies. As a part of the contract, the City also selected areas where they wanted to add spot shots over areas where tall building lean was blocking certain areas of interest. After each image was approved, it would then be sent over the City's LAN to the main server for distribution to the other agencies involved.
For archiving purposes, the entire orthophoto dataset (~88 GB) was written to high-capacity data tapes for storage in the City's Public Records Department.
As I stated before, the applications are virtually endless. The following are some of the specific applications or benefits that the City of Los Angeles DOI project offered:
Los Angeles Public Works will be utilizing the orthophotography on virtually every new project. Planning can be achieved more efficiently and with better accuracy. Technicians can perform pre-project field visits from within the office without driving out to locations. Engineers can use the orthophotography to decide where to put new storm drains, manholes and roads. Planners will be able to locate vacant lots, dilapidated hillsides and abandoned buildings for new construction or improvement.
The Department of Water & Power will be employing the orthophotos for many similar reasons to that of Public Works. DWP will also be utilizing the imagery to determine landscape area measurement, or what percent of a parcel is hardscape and what percent is vegetation. This will in turn help predict future billing forecasts and determine yearly water usage.
Transportation can establish what streets need to be repaired from the new orthophotography. DOT can determine where to build new transportation corridors, overpasses and public transit stations. Officials can settle on the best route to detour traffic through construction zones, as well.
Flood Control will be utilizing the surface model and contours for flow studies throughout the City. Slope analysis and volume calculations will be performed using the newly created DTM to model water run-off and flood scenarios.
The Los Angeles Police Department will use the orthophotos to help detail a crime scene during investigations. The orthophotos will be used to establish security locations and patrols at large sporting events, parades and conventions. The compressed imagery can be placed in squad cars and show police officers where alleys, fences and dead ends are to help in pursuit of suspects.
The Los Angeles Fire Department will utilize the imagery to basically fight a fire in every building in the City before it starts. Initial Attacks can be planned and saved for reference. If a fire breaks out, the Fire Department will know where to block traffic, where hydrants are located and where to park fire engines. Officials will know the best location to set up Incident Command, supply and medical stations. The Department can determine vegetation type of certain areas and what kind of fuel content is present. They also can locate alternate sources of water: swimming pools, ponds and lakes.
The World Wide Web applications for digital imagery are in their infancy, but growing on a daily basis. The Los Angeles orthophotos are slated to be added to the well-known NavigateLA program designed by Gatekeeper Systems located in Pasadena, CA. Through NavigateLA, City staff, and the general public, can navigate to a location on the map by providing a variety of search criteria - address, assessor parcel number, property owner name, street intersection, etc. All from within your standard Internet browser. The Los Angeles orthophotography will also be sold via the Web to benefit the private sector, with the proceeds returning to fund the City's planned orthophoto update program.
By establishing a multi-funded project like the City of Los Angeles, it makes digital orthophotography more economical and easily attainable. The orthophotography can be used in a multitude of different ways and within separate departments. If you are interested in establishing a clean, accurate and seamless GIS database, digital orthophotography is your ideal starting point.
As this paper clearly suggests, the applications for digital orthophotography are extremely valuable and growing on a daily basis. With technology on the rise, who knows how far orthophotography will go. With advancements in 3-D modeling and true-orthophotography, it's very likely that the GIS environment of the future will be a virtual world of imagery with upright buildings, trees and other above ground features.
Joe Bartorelli is Director of Consulting and Development Services for HJW Geospatial, Inc. He is President of the Southwest U.S. Chapter of the American Society of Photogrammetry & Remote Sensing. Mr. Bartorelli is also involved in numerous professional organizations, including the American Public Works Association (APWA), Urban Regional Information Systems Association (URISA) and Geographic Information & Technology Association (GITA), among others.
Mr. Bartorelli has nearly ten years of experience in the fields of photogrammetry and remote sensing. He has direct hands-on experience initiating and managing large-scale mapping projects, including for the Cities of Los Angeles, Oxnard and Colton as well as for Douglas County and Clark County, NV.