Introduction
The digital orthophoto has a variable scale and until a hard copy is made or it is displayed on the GIS it has no scale. This variable scale of digital orthophotos opens the door for cost sharing the development of a common photobase. This ability to share costs between public and private entities:
• Reduces the cost for all parties involved,
• Frees up funds for more frequent photobase updates,
• For taxing entities it makes more efficient use of tax money,
• Facilities maps created using the common photobase may easily be shared and
• Pooled resources allow purchasing higher quality digital orthophotos.
      This paper has three sections-Finding a Common Base Map, Putting the Group Together, and A Digital OrthoPhoto Case Study featuring the Alamo Resource Conservation and Development Area, Inc. (RC&D.)

Finding a Common Photobase
How was it done in the past
The key of a photobase is to see and interpret features on the ground. These may range in size from a water shed to a large ranch to a fire hydrant.
      Differing needs for photo interpretation require different scales. For instance the Soil Conservation Service uses 1"=660' and 1"=1000' for its printed material, regional planners commonly use 1"=2000' and the Appraisal Districts commonly select 1"=400'.
      To acquire the photobase with these scales each entity has chosen to take aerial photography at the best height to create the hard copy for their photo interpretation needs. To illustrate the differences of acquiring data at these scales, consider the following table (The aerial extent is 1000 square miles - a typical county in Texas):
      Clearly the aerial photography used by the Soil Conservation Service does not satisfy the needs of the Appraisal District. Conversely the photos needed by the Appraisal District can be too large scale for the SCS.

The cost of imagery the old way
Cost is the key factor for any photobase acquisition. If the final photobases are of similar quality the lowest price will be used.
      The following discuses the cost to produce aerial photography for several entities at their desired photo scale. The assumptions used are those of the previous section and that the area of coverage is a square of 1000 square miles.
      Expanding on the previous table we have the following table:
      The range in cost is $10,000 to $37,000 for the same geographical coverage at different scales.
      In a typical county there are many taxing agencies and private businesses that need to produce photobases similar to the above. The scale and map production needs will be discussed in more detail later.

A new method for acquiring common imagery
The digital orthophoto photobase allows data to be placed in the GIS at the scale the user chooses. The vector data is geocoded allowing the GIS to register maps (and other feature data) over the photobase. The Appraisal District can see the number of cars in the parking lot and the swimming pool that has been added. The Soil Conservation Service can see the farms, establish the boundaries of estuaries, and study soil types. The Regional Planning Commission can see major roads and locate impediments (geographical features, man made things, etc.) to city expansion. In essence the user of the GIS can view the raster imagery, point and click and get the coordinates of the feature (your house and the new pool). These can all be tied to a data base that is capable of calls based on geodetic coordinates.
      Two terms are very important Hardcopy and Softcopy, Hardcopy is the printed map material, Softcopy is the imagery (or maps for that matter) that is displayed by the GIS. Softcopy is digital files that will be delivered on magnetic tape or CD-ROM. Hardcopy can also refer to the aerial photography that is created by photo enlargement as well as the printed picture that comes from the GIS.
      The USGS 7.5 minute Quad sheet is currently the base map standard for mapping in the United States. The USGS has defined a new product that is called a Digital Ortho Quarter Quad (DOQQ) and it will become as widely used as the 7.5 minute Quad. It will be produced in Softcopy, it will have a ground pixel size of 1 meter, and it will have an accuracy of National Map Accuracy Standards for a map scale of 1:12,000. The USGS program to produce these across the U.S. is just getting started and like the 7.5 minute Quad sheets (primarily because of budget considerations) will take some time to attain full coverage across the country. Estimates range to the turn of the century. Because the DOQQ will be produced by the USGS and will be available for public use the cost will become very inexpensive. The USGS is currently looking for joint sponsors to share part of the cost and they will underwrite some of the cost. This leads to a shared base map concept.
      DOQQs have some specific implication for use in GIS. DOQQs will be produced on quarter quad boundaries so a naming and tiling convention already exists. They will be produced with the same coordinate system leading to consistency in mapping. The registration, mapping, geodetic inventory and other GIS activities are done on the computer system and the scale of work is selected by the operator. A DOQQ can support 1"=400' scale very easily as well as any smaller scale, for example 1"=660' and 1"=2000'. The final printed material will probably not include the raster imagery but will be vector maps. This can be done at the scale needed by the user organization.
      In the previous section the cost was discussed to acquire a county photographic backdrop GIS. For a similar county of 1000 square miles the cost including new photography is near $50,000 for the DOQQ product described above. The cost to each entity in the common photobase is greatly reduced (depends on the number of entities sharing cost).

Enabling Technologies
The world of computers is changing very fast. Faster, more powerful computers provide the enabling technologies for the creation of economical digital orthophotos. Three computer related units are involved - computing speed, random access memory and disc storage. There are some software elements that are necessary (namely GIS) to utilize these elements to their full potential.
      The author (Sedgwick) has for many years tracked the three elements that make computers fast. For constant prices all three are changing by a square factor. The author still remembers paying $3,500 for 10 Megabytes of Disc storage, currently that much money will purchase 10 Gigabytes. That is a 1000 to 1 increase in 15 years with no change in price. The computing speed and memory size has taken similar leaps.
      The increases in capability have allowed the storage of large raster image files. This has also provided the signal processing capability necessary to create digital orthophotos and use them as a backdrop for vector drawings.

You'll never get away from YOUR vectors
Raster Imagery works well for the base layer in a GIS environment but when the time comes to show the boundary lines of a piece of property there is no substitute for a vector drawing.
     

For the Appraisal District the raster imagery serves a number of purposes:
• Find taxable structures
• Help delineate property boundaries
• Arbitration hearings to show like properties
• Vegetation, crops and Forest.
     

For 9-1-1 mapping the raster imagery serves to:
• Find houses
• Map roads
• Set up the Emergency Service Number area.
     

For the Soil Conservation Service the raster imagery serves to:
• Show cultivated areas
• Illustrate different crops

PUTTING THE GROUP TOGETHER
Is it appropriate to share costs
YES!!! In fact it is inappropriate to not share costs for a common photobase. As previously discussed, if data is shared the individual project participants can have better data at less cost.
     Governmental agencies and private industry have the need for a high quality current photobase. From the previous example the cost for a county is approximately $50,000. If 15 participants share in the acquisition of the data the cost to an individual participant is only $3000.
      This need for a good photobase is shared at all levels - County, Regional, State and National. Many private organizations have similar needs.

Those that can share
The natural breakdown of organizations that can share in a photobase project is done best by the geographical area as discussed above - County, Regional, State and National. The breakdown works well for both public and private concerns.
      Taxing, Law Enforcement and Transportation all have organizations at all levels but most have enough autonomy that they will make purchasing decisions separately. The County Engineer working on county roads will have a different budget than the Texas Department of Transportation.
      A Cellular Telephone operator will have regional needs but might purchase the single county where the next instillation is planned. This company might also participate on a regional level.

Specifications that most entities can live with
Two ranges of scales often are discussed, Regional and Large Scale. The scales are 1:24,000 to 1:4800 for the regional scale and 1:2400 to 1:600 for the Large Scale. It is generally conceded that the Regional Scale is needed by almost all organizations, and that the coverage should be of the whole region of interest. The larger scale is needed by some organizations but is limited to smaller areas.
      The authors have done extensive "non formal" research to discover these scales, so they may not be perfect and the reader may disagree. Even if the reader chooses to disagree, one thing is for certain, there is a lot of entities in the range of 1"=400' to 1"=2000' for their primary mapping scales. The USGS went through a similar exercise to determine the resolution and accuracy for the Digital Ortho Quarter Quads. They chose 1 meter ground pixel size and pixel placement accuracy associated with a map scale of 1:12,000' (1"=1000'). It appears that this is a good compromise.
      A one meter pixel allows display of the raster imagery at 1"=400' without "seeing" the pixel, if inspected close enough the pixel is visible. The authors have frequently seen users view the one meter raster imagery on the GIS system at scales of up to 1"=100'. If hard copy is produced from the one meter digital imagery at scales greater that 1"=400' significant re-sampling will be required.
      Pixel placement accuracy is probably the least understood of the two basic specifications pertaining to digital orthophotos. For the USGS DOQQ program the pixel placement accuracy is specified as having the pixel within its real position by 9 meters 90% of the time. If you use a GIS and place the pointer on a feature shown on the digital raster imagery and read the coordinates they will be within 9 meters of the real position 90% of the time. The pixel placement accuracy is largely dependent on the Digital Elevation Model (DEM) used for ortho rectification. This process also depends on the accuracy of ground control, number of ground control points, aero triangulation and camera lens radial distortion.
     

The specifications can be tabulated as follows:
• Photobase must be affordable.
• The 1:12,000 NMAS are affordable and do meet most users needs.
• Black and White photography is more affordable than Color and CIR.
• One meter pixel size.
• Where higher resolution is needed it can be ordered for the individual area rather than the whole county.
• CD-ROM is the most useful media due to savings in hardware costs.
      The USGS has selected the softcopy imagery discussed above as the next base map product and as such will probably receive (in time) the same popularity as the 7.5 minute quad sheets.

A CASE STUDY
Alamo RC&D
The Alamo Resource Conservation and Development Area, Inc. (RC&D) is in the process of completing a shared cost project in 6 counties around San Antonio, Texas. The Alamo RC&D has taken on the following activities:
• Develop a set of specifications for the project.
• Select a vendor.
• Set up contracts with project participants.
• Manage the contract with the vendor.
• Distribute copies to all project participants in the format for their GIS.
• Maintain an archive set of copies.
• Provide a method for non participants to purchase coverage after the project is complete.
• Provide a path for updates in the future.
      There are 26 RC&D areas in Texas. Each is set up as a non profit corporation with one person from each county serving on its board of directors. The charter of the RC&Ds is to assist the rural areas in attaining the services that the urban areas have in place.
      The process of building a common base map from digital orthophotography has already been started in the San Antonio area (Bexar County). This project was the forerunner to the project discussed in this paper.
      In 1991 the Alamo Area Council of Governments (AACOG in San Antonio) and the Alamo Resource Conservation and Development Area, Inc. selected a vendor and, as a pilot project, built digital orthophotography for the County of Bexar. Entities in Bexar County each provided a small amount of funds to facilitate the project. The data purchased has been distributed to the participating entities and most are using it in their GIS activities. The knowledge learned on this project is being used to do a better job on this project.
      Building on the Bexar county project, Alamo RC&D moved forward with the common photobase project soliciting individual funds from organizations that have GIS needs in the 6 county area. These six counties were selected because of their proximity to the Bexar county project:
• Medina
• Bandera
• Kerr
• Hays
• Comal
• Guadalupe
      One of the main reasons for selecting these counties was that some of the entities in the Bexar project asked for additional digital orthophotos beyond the Bexar county line. The following discussion highlights how organizations outside the individual counties can be major contributors.      The steps in this project are complete and waiting the final commitment of funds. A vendor has been selected to produce the digital orthophotos. The vendor has produced six quarter quads and submitted them to the USGS for approval. As soon as USGS approval is given the vendor will begin production.

About the Authors:
Fred Sedgwick is president of Sedgwick Associates which specializes in customer driven unique solutions to the development of land information. He may be reached at 713-493-2584.
Wayne Griffin is the coordinator for the Alamo Resource Conservation and Development. There are 26 RC&D areas in Texas. Each is set up as a non profit organization with one person from each county serving on the board of directors. The charter of the RC&Ds is to assist the rural areas in attaining the services that the urban areas have in place. Wayne may be reached at 210-426-5516.

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