Donna Wendt
Stan Grochowski
Abstract:
Pierce County, Washington, lead a cooperative ortho photo project also involving funding and project participation by the City of Tacoma and the City of Lakewood. Nies Mapping Group, Inc., used aerial photography photo scales of 1"= 600’ and 1"=1500’ for the urban 6" pixel data and the rural 1’ pixel data respectively. Deliverables included 2’ or 5’ contours.
This paper will feature the steps in managing the project from the creation of the RFP through funding, QC, data storage, and user applications for county and city government departments. The technical discussion will cover use of Lizardtech’s MrSID compressed image files and Imagine software by Erdas, as well as servers, networks, and deployment to Arc/Info, ArcView, and the Internet.
The authors of this paper are senior GIS analysts from the City of Tacoma and Pierce County. This paper was presented on June 28, 2000.
Introduction
The project management aspects of the ortho photo project will be described in the first half of this paper by Donna Wendt from the City of Tacoma, and Stan Grochowski from Pierce County, WA, will detail some of the technical considerations in the second half of the paper. This was a large project worked from 1997 through 2000, so this paper will cover some project highlights but not every detail. Both authors can be contacted with either management or technical questions. The project lead of the Pierce County ortho photo project was Linda Gerull, Pierce County GIS Manager. The vendor for the Pierce County ortho photos was Nies Mapping Group, Inc., now part of Triathlon.
Managing the Project
Put simply, an ortho photo is an image, such as an aerial photograph, stretched to fit a GIS map. There are many flavors of images, various kinds of colors or black and white, typically with pixel sizes from three inches up to thirty meters or more. The Pierce County project used 24-bit color tiff images from aerial photography. In the urban areas, these maps use 6" pixels from 1"=600’ photography with a +/- 2.5 feet horizontal accuracy. The rural areas have 1’ pixels from 1"=1500’ photography with a +/- 5 foot horizontal accuracy. Contours for the urban and rural non-incorporated areas were provided at 2’ and 5’ intervals respectively. The project covered Pierce County, Washington, excluding the military base and Mt. Rainier national forest.
The multi-agency Pierce County ortho photo project, led by Pierce County GIS, involved Pierce County, the City of Tacoma, and the City of Lakewood. About a dozen members from various county and city departments made up a project team that met regularly to review the RFP, evaluate vendor responses, and approve the photography. This paper will not go into detail about the contents of the RFP and responses, except that they were all at least a quarter inch thick. Agencies setting out to do an ortho photo project should get examples of these documents from agencies who have done it before, and be very specific about dates, flight details, sun angle, film, accuracy, quality, deliverables, the QC process, etc. Plan on having this phase of the project taking a few months and involving lawyers from your agency and the vendor. If time allows, do a pilot test. The more agencies involved in the project, the longer this will take, but the cost sharing benefits are worth the effort.
In planning the ortho photo project, a server with adequate capacity and speed is needed for the data. The City of Tacoma and Pierce County first experienced the use of ortho photos in their GISes with a 1996 color ortho database with 1 meter pixels. This database covered the more urban portion of the county, had an accuracy of +/- 30 feet, and took 4GB to store. It could be accommodated on existing servers, and it was a good introduction to discovering the applications of ortho photos in a GIS. It was useful to build support for the 1998 ortho photo project with the 6 inch pixels and +/- 3 foot accuracy. The new project, however, required 170GB of disk storage. Each 3,000 foot square urban tile is 105MB, and the same size area in the rural part of the county with the 1 foot pixels is 26MB. The price of the data is roughly proportional to the required disk storage.
With databases this large, plan on having resampled or compressed images available for quicker access to large map areas. Put the maximum amount of memory on plotters and upgrade the computer network if necessary. Pierce County and Tacoma use HP 1055 and 2500 plotters supplied with maximum disk and memory. Pierce County’s network at 100-Base-T works well with the orthos. If possible, get sample data from the vendor to test plotting, viewing, and network performance. The test data are also very helpful in showing first hand the need for smaller pixel sizes.
The following map samples show a comparison between the 6 inch pixel and 1 meter pixel orthos.
Notice the street edges, sidewalks, and poles drawn on top of the ortho in this QC view of the 1998 6 inch pixel ortho. Those themes from 1990 aerial photography are accurate to within 1.25 feet.
This view shows that the one-meter pixels do not work well when zoomed in to a one-block area.
One-meter pixels zoomed out to a 2-block area are still a bit marginal.
The same area, shown with 6" pixels, can show parking spaces, road markings, and building details.
In the City of Tacoma, five departments contributed towards the ortho photos. To build support for the project, ortho demonstration and brainstorming meetings were held with each department. In these meetings, potential applications for the use of ortho photos were entered into a spread sheet, and the top two or three applications were prioritized. Each application was tied to one or more of the four city strategic plan goals, which were economic development, public safety, neighborhood development, and government performance. How well the application worked using a 6-inch, one-foot, or 3.2-foot pixel ortho photo was also rated. This chart became a valuable tool for building project support from the technician level on up through management and the approving boards and councils.
Several departments have developed first applications of ortho photos in Pierce County. In Planning and Land Services, they are being used for public community planning, priority habitat studies, and as an editing backdrop for basemaps. The Pierce County Assessor department prints maps with orthos for assessors to use in the field, and they also use the ortho as an editing backdrop for maps. Public Works is doing a very large watershed plan project in which the orthos and contours are essential. The Sheriff’s Department is using ortho photos as part of their crime scene management and pre-incident planning information.
At the Permit Counter, ortho photos are commonly used for map backgrounds and customer prints. Personnel there said that "they can’t live without orthos now." Pierce County GIS has added the ortho theme to their CountyView application, an enterprise-wide GIS using ArcView.
www.TacomaSpace.com, a commercial space multiple listing site
The City of Tacoma has included the downtown area ortho photos on their interactive mapping web site, www.TacomaSpace.com. That application, driven by ArcView IMS, uses a MrSid file with 1:20 compression. In a letter from a developer, CDC (Commercial Development & Consulting) states: "The aerials and maps you recently furnished are incredible and without a doubt will become very, very valuable in all of the marketing efforts and presentations on the Pacific Steps project overall."
The City of Lakewood is currently using the ortho photos for pre-engineering, right-of-way information, a sidewalk study, and park planning. All of the above are initial applications of the orthos; more applications will be implemented in time.
Project funding was done differently by agency. Pierce County has established an ortho fund which cannot be used for any other purpose. Contributing departments have use of the ortho photos. The fund covers the 1998 project and future reflights. In the City of Tacoma five departments, Power, Water, Public Works, Fire, and Economic Development, funded the orthos. Full access is provided for all city departments, but future orthos need to be budgeted. The City of Lakewood is concerned with funding future ortho photos due to the recent budget-limiting Initiative 695 passed in the state of Washington.
In the area of project management, several tools and techniques were useful. The vendor established a weekly phone conference meeting which fostered good communication and addressed problems promptly. On-site meetings were held as needed in Tacoma and at Nies and Triathlon. ArcView was used to track the progress of tile deliveries and Quality Control. On the index map, tile locations and numbers were displayed, as well as original flight lines and photo centers. QC dates, technician names, status, and problem area shapes were tracked. An image catalog was needed to track the locations of the tiles on multiple servers. All email communications and documents were stored in a project folder.
Future project tasks include implementing compressed images, image classifications, expanded uses, and planning for future reflights. A pilot test of some "true ortho" techniques will be done on a mile of Pacific Avenue, and some 3D visualization tests will be done.
Pierce County in conjunction with the City of Tacoma and the City of Lakewood leased orthophotos from Nies Mapping Group Inc. for a large portion of Pierce County (Figure 1.). This project besides its interesting history and management considerations had many and varied technical related challenges. The first but not least of these challenges stems from the fact that the orthophoto files in their original Tiff format are very large. There are a total of 1579 tiles in Pierce County and each of those tiles whether they are 6" pixel tiles or 1’ pixel tiles are approximately 108 megabytes in size, for a total of 170 gigabytes. See Figure 2 below for a summary of the sizes of the various files involved in the project. Additionally there are two other file types generated from this project, MrSid files which total 8 gigabytes and contour and DTM data in DXF format which total 5.6 gigabytes. With all of this data it is wise to plan ahead and have reliable, fast access disk space available. After having designated an optical disk drive for this purpose we quickly found out that an optical drive is neither reliable nor fast. After much negative user feedback we decided to purchase a Clarion mass storage device that did have the fast and reliable attributes.
As mentioned above two different pixel size orthophoto tiles were received. The smaller 6" pixel was done on the portion of Pierce County designated as the Urban Growth Area. The smaller pixel size results in better resolution in the urban areas where greater accuracy and more detail is generally required. Figure 3 and Figure 4 below show the difference in clarity, detail and resolution at the boundary between the different pixel size orthophotos. At the 1:1200 scale, a scale at which the tax assessment maps are captured there is little loss of detail or clarity and either orthophoto can be used equally well for this purpose. However at the 1:600 scale the maps become noticeably different in detail and clarity. This type of scale is better suited for Public Works types of applications. Besides the differences in pixel size there is also a difference in flight height between the two orthophoto tiles. The 6" tiles were flown at a scale of 1:600 and the 1’ tiles were flown at a scale of 1:1500.
As the orthophotos were delivered quality control measures had to be put in place to insure we were getting the desired results. In order to take care of some the initial problems we needed to understand how the world files that accompanied each orthophoto tile was structured. We found that there were rounding errors being generated in some of the tiles which caused narrow black lines between some of the tiles when displayed. The tables below show the structure of these files and where the problems were occurring.
Pierce County also chose to receive delivery of the same data in Mr. SID format. MrSid is an acronym for Multi-resolution Seamless Image Database. To quote Lizard Tech Inc. "MrSid is a powerful wavelet based image compressor, viewer and file format for massive raster images that enables instantaneous viewing and manipulation of images locally and over networks while maintaining maximum image quality". The MrSid format was chosen in order to take advantage of the compressed file size at different scales, faster drawing speed and expected better plotting results. The first two reasons have held true but the drawing or plotting results have been less than we had hoped. Tests run on ArcView comparing MrSid and tiff images showed some favorable results for the MrSid images. Figure 6 shows those results.
Another major factor in using the orthophotos was security. Pierce County has an enterprise-wide GIS, CountyView, built around ArcView GIS and other applications using MapObjects which access the orthophotos. However the lease agreement Pierce County has with Nies Mapping Group Inc. allows for access by only those agencies which participated in the funding for the orthophotos. Also since some municipalities and other agencies outside of Pierce County government have access to CountyView some security had to be in place to allow only those who participated in the leasing of the orthophotos to view the orthophotos. The orthophotos reside in a UNIX file system and UNIX security is used to establish which users are allowed access. A single UNIX group is maintained for all orthophoto users. However the large number of users within the county poses a problem for the group security file. Entries in a group security file have a limit of 512 bytes, which was much too small to hold the login name of each user. What UNIX does allow however, is multiple groups with different names but the same group number. Multiple groups with the same group number can be created to accommodate all the user names needed. When UNIX checks the group it uses the number and not the name to establish authentication. See Figure 7 below as an example of the group security file. A second problem encountered during this process was the fact that UNIX only allows users to be a member of 15 groups concurrently. UNIX uses the first 15 groups encountered in the group security file. Care must be taken to ensure users are not members of more than 15 groups or the security will keep users out of a group for what seems to be no apparent reason.
Plotting the orthophotos has also become quite an issue in itself. The tiff files when converted to PostScript files for printing are quite large. Large geographic areas including many tiff tiles which can easily overload a printer’s memory capacity. The MrSid files though compressed for viewing are not compressed when used in PostScript and can at times produce larger plot files than the tiff images themselves. The PostScript files also have the tendency to give a greenish tint to the orthophotos. ArcPress can be used to rasterize the plot before sending them to the plotter and this does cut down the file size but ArcPress tends to create plots which are dark and also maintain the greenish tint of the PostScript files. After many attempts at adjusting the colors through ArcView parameters no good settings were ever found to produce plots that were even close to WYSIWYG. The best results we have been able to get thus far is with ghostscript, a public domain program which can convert plot files to PDF format. These plots come out with the colors looking true and about a 45% reduction in file size as compared to the PostScript files.
The Image Catalogs, which is the way in which Esri organizes image tiles for display in ArcInfo and ArcView, pose some of there own unique problems. First of all is the difference in UNIX and PC image catalogs. In most cases ArcView is smart enough to ignore the difference in slash direction in accessing files. Forward slashes are used in UNIX files systems and back slashes in PC file systems. In the case of Image Catalogs the pathname to the orthos need to have the correct direction of slashes thus two separate Image catalogs are maintained in order for the two operating systems to read the files properly. See Figures 8 & 9 below.
Other types of tiled data while not specifically supported in ArcView as image catalogs can use similar types of files to maintain access to those files. Pierce County has developed image catalogs for MrSid data and for DXF data generated from the orthophoto project. These files mimic the standard image catalogs and with minimal coding can be used in a similar manner. Creation of these files can be done by first creating a standard tiff image catalog then using substitution to change the file type to whatever type you want to create. This process only works if the tile structure is the same for both. There are also some scripts and extensions available on the Esri web site for image catalog creation. Imagecat.avx, creates a standard image catalog. Dfgicat.avx creates creates a dfg image catalog. Imagegrid.avx creates a catalog of grids.
MapObjects does not support image catalogs at all. However the same idea can be used with it to use image catalog like files to store and view file path names to the images. In this case a polygon shapefile of the tile boundaries must be created along with the path to the image. Poly in poly queries can then be executed on the shapefile using the View or Map extent to determine which tiles need to be added to the map.
ArcView Example:
theView = av.GetActiveDoc
theRect = theView.GetDisplay.ReturnVisExtent
theDXFshp = theView.FindTheme("Ortho - DXF's")
theDXFFTab = theDXFshp.GetFTab
theDXFshp.SelectByRect(theRect,#VTAB_SELTYPE_NEW
MapObjects Example:
lmo_recordset = ole_map.Object.Layers.Item('Orthos').SearchShape(ole_map.Object.Extent,moExtentOverlap,'')
Orthophotos have proven to be the single most popular theme within Pierce County's GIS. But making them available has not been a simple process. Be prepared for crossing many hurdles in order to make effective use of the orthos. It is a process well worth it as the orthophotos will nearly define GIS to many of the more casual users.