Compiled by:

John Deck, University of California, Santa Cruz GIS Lab

Members, CCJDC Imagery Special Interest Group

Mary Tsui, Land Systems Group, Monterey, California

Technical Review by:

Peter Ashley, LSIT, Hammon Jensen Wallen Associates, Oakland, California

The Purpose of This Publication:

What is Aerial Imagery and How is it Used?

Orthophotography

Flight Planning

Agencies have varying needs

Getting Ready to Fly

What You Should Get From Your Vendor

Appendix A: Definitions of Terms Frequently Used in Aerial Imaging

Appendix B: Recommendations for Photography by Agency Type

Appendix C: Metadata

More Information and Further Reading

Many agencies in the public, private and non-profit sectors have a need for aerial and satellite imagery. Since aerial imagery is an expensive and complex commodity, the intent of this publication is to define its basic terms and to discuss and explain the common issues surrounding its acquisition. In addition, minimum standards for imagery are suggested.

This activity is undertaken to provide agencies and companies with the information and knowledge required to successfully plan and negotiate for their imaging and photographic needs. With the information contained in this publication and in the references cited on the closing page, readers should be able to work more closely with imagery vendors to develop a product tailored to their needs.

While the impetus for generating imagery may vary, many agencies can use the same coverages (i.e. the photographic imagery of the same geographic area). Through cooperation, agencies in a region can share costs and follow minimum standards, enabling the acquisition of imagery of higher resolution and accuracy, and greater geographic extent.

Another intended benefit of these guidelines is to encourage regional data networks and thus prepare regions to respond more quickly to their imaging needs. In the course of emergencies and other natural disasters, it is common for a variety of agencies to express a need for immediate aerial imagery and to scramble to define the product needed and accumulate funds. For example, intense flooding creates a need for many agencies to have imagery of the affected areas to identify the scope, respond to the crisis, and ultimately plan for future mitigation or prevention. Ideally, the imagery of the flooded areas will be compared to pre-flood photography. Having a set of minimum standards and guidelines and a strong network between agencies will create planning and funding alliances that can quickly respond to needs born of such emergencies. Through this network, agencies can also ensure that adequate and frequent aerial image updates of the region can be obtained.

In general terms, aerial imagery refers to photography or digital pictures taken from the air. There are many ways to obtain and use this imagery, which will be discussed below. The method you choose to obtain and use your imagery will depend upon the needs of your organization.

Differences in acquiring the imagery include:

• Equipment and aircraft
• Photographic vs. Digital Imagery
• Image (Color) Type
• Pixel Size
• Uncorrected vs. corrected (ortho) photography* (the most significant area of difference)

Differences in the use of the imagery include:

• Hard copy or paper prints for feature reference or photo-interpretation;
• Using a digital form as a computerized photographic backdrop or reference;
• Using a digital, corrected photograph as the foundation for mapping.
• Using corrected photographs in hard copy, mylar form as a basis for generating photographic copies.

Photography may be taken from low-altitude small craft, high-altitude airplanes, and satellites. Typically, the lower the altitude, the higher the resolution of the resulting photograph. Special equipment and techniques used before, during and after the flight are required to produce photography suitable for serving as the basis for mapping. Images are acquired by a variety of specialized cameras, costing up to $500,000 or more.

Note: The fact that imagery is taken from an aircraft or satellite does not imply that it is spatially correct* (see "Uncorrected vs. corrected (ortho) photography", below).

Analog photography is recorded directly onto film whereas digital images can be recorded via film, airborne scanners, digital cameras, or other equipment. Both analog and digital imagery can be stored in digital form - that is, on computer disks or tape; it can also be presented in paper or hard copy form. For the purpose of this publication, imagery in its digital form will be discussed from this point forward.

Photographs may be taken using black and white, color, infra-red, or other film. Current developments in technology have allowed for the direct capture of digital images corresponding to a wide range of radiation frequencies including ultraviolet, color, infra-red, thermal, and microwave frequencies.

• Black and white images are generally less expensive and typically require about 1/3 the digital storage space of color photographs. Properly taken, they can provide excellent resolution and can handle extensive scale enlargement. They will not be as useful for vegetation monitoring or for other analysis where color or heat is of importance; if photo-interpretation is required, more training is typically required in the case of black and white film.
• Color photography gives the closest rendition of a scene as viewed by the human eye and thus requires less training in its use. It is a more expensive product and requires about 3 times the digital storage space of black and white photography.
• Color Infra-red (or false color) digital photography is sensitive to green, red, and near-infrared radiation. A close inspection of the product will reveal that vegetated areas appear red. This product is particularly useful for delineating vegetation, since near-infrared radiation captures differences in vegetation type. Infra-red film typically produces sharper images than black and white or color film. In addition, sharp black and white photographs can be produced from infra-red film.
• Multi-spectral Imagery is a rapidly growing field with many experimental applications. Current applications include wetland delineation, vegetation mapping (including vegetation type and stress-levels), and mapping water pollution. Multi-spectral imagery refers to imaging with a large number of layers corresponding to particular solar radiation frequencies.
• Radar imagery is used to actively scan the earth's surface, yielding imagery capable of producing detailed digital elevation models and characterizations of surface texture. This includes SAR (synthetic aperture radar), an enhanced form of radar.

Note: Digital imagery can create very large files, especially those at higher resolutions. This can affect your computing speed. Various file compression systems are now available that reduce file size while not sacrificing detail. You should dicuss this option with your vendor; there is typically a small cost associated with file compression.

As noted in the Definitions section, the pixel is key to determining the visual resolution of the image. Produced in varying sizes, the pixel presents a uniform value of the ground covered in its range, which can range from several inches to several meters. As the photograph illustrates, smaller pixels typically result in higher resolution.

Images taken from the aircraft or satellite which are uncorrected will not permit accurate orientation to known or surveyed points on the ground. One will not be able to accurately measure between points on the photograph. The reasons for this are several: curvature of the earth, tip and tilt of the camera at the moment of exposure, differences in terrain elevation and the nature of photography itself.

Negotiating Tip: Quotes for "aerial imagery" do not include orthorectification unless that service is requested. Always specifiy that the imagery be orthorectified if that is your need and make certain that the quotation from your vendor states it clearly.

If you are planning to measure ground features or to create maps from your photography, you will require corrected photography. Corrected photography, or orthophotography (from the Greek ortho, meaning that which is right, straight or true), addresses and adjusts for the multitude of factors causing inaccuracies.

Generally, the development of orthophotos requires the acquisition of stereo photographic coverage (this means overlapping photographs of the same geography) and some combination of surveyed control on the ground and, increasingly, airborne GPS (Global Positioning System) collection at the time of photography. The photogrammetrist will need to perform aerotriangulation on the resulting block of photographs and may need to develop a digital terrain model. These numerous corrective features make orthophotography more expensive than uncorrected aerial photography.

In spite of its added expense, many agencies are willing to spend the funds since there are so many benefits from using orthophotography. Good, accurate base maps are easily derived or created from the orthophotography because every physical feature on the photograph has been tied to a survey point that is horizontally accurate within certain tolerances. Streets and roads, centerlines, curbs, manholes, streetlights, traffic signs , water edge, tree inventories, fields, driveways, fire hydrants, and numerous other features can be accurately mapped from the orthophotos.

This can turn out to be a relatively inexpensive method of acquiring an excellent base or control layer for a GIS. In addition, once the original orthophotography is acquired and maps developed, future photography might be acceptable in its uncorrected mode, if the agency is merely looking at change.

• Satellite Imagery. Orthophotography is also available from various satellite imaging companies, although this is typically of lower spatial resolution and accuracy than that acquired through aerial photography. Several companies are anticipating the launch of satellites which are designed to deliver better spatial resolution and imagery suitable for urban mapping; however, it will be some months or years before we see these products and determine their quality.
  
  Satellite orthoimagery is very useful, however, for monitoring change and for mapping large areas of land where the surface is relatively uniform. Forestry and wide area land use are examples of appropriate use of satellite imagery.
• Accuracy Considerations. Typically, more horizontal accuracy is required in urbanized or developed areas than in rural or undeveloped lands. Again typically, planning agencies require less accuracy than engineering agencies. Funding may also play a role in the accuracy required. We strongly urge collaborative programs that are geared to the highest accuracy required by its members; it is futile to acquire imagery at a planning level need when engineers will be using the imagery and resulting maps. We recommend minimum standards for particular activities and agencies in Appendix B; however, agencies are encouraged to exceed these minimum standards if circumstances permit.
  
  Still, if an agency has both urbanized and rural areas, it may wisely choose to acquire high-accuracy imagery for the developed areas and a lesser accuracy for the undeveloped areas.

Flight planning refers to the work done by an imagery vendor prior to the acquisition and development of the photography. All of the steps are critical to the project's success and your ultimate satisfaction with the product.

• Setting the flight plan. Your vendor must know the boundaries of the region for which you wish to acquire imagery, the spatial accuracy you expect, and the map scale you anticipate creating. Once that is known, a flight plan will be developed that allows the aircraft to fully cover the area.
• Setting the ground control (for orthophotography only). Since orthophotography is a form of aerial survey, it follows that there must be control points on the ground to which location can be tied. The vendor will use known control points, but may wish to set others; they may ask you if you are aware of known control points. In some cases, you as the client may be able to assist the vendor and perhaps lower your costs somewhat by setting additional control points. The more control points, the more spatially accurate your imagery will be.
• Configuring the aircraft for the flight. Locational readings will be taken on the aircraft and the camera as well as on the GPS receivers on board, if the vendor offers airborne GPS as part of its orthophotography work. All of these steps are required to ensure the integrity of the locational information which will come from your photography.

Negotiating Tip: Check to see if your vendor (aerial or satellite) or others have existing orthophotography of the area for which you wish to acquire imagery. Some vendors, at regular intervals, prepare orthophotography of certain areas. In some cases, it may meet your requirements for current data and horizontal accuracy. If so, the cost will be considerably lower for you.

No two agencies or companies are alike. While we are providing guidelines to basic groups which share concerns, enough differences remain that will cause members of those groups to select differing imagery solutions. A sampling of these differing needs are listed below:

• One-Time vs. Periodic Photographic Coverage. Some imagery need only be obtained once, at a particular time. Other photography for an area must be repeated over a period of days, seasons or years. This variation depends on the agency and the intended function of the imagery.
• Image Type. See above descriptions.
• Standard vs. Ortho-rectified Photography. If an agency wishes to map from the photography, orthorectification is necessary. If change detection or resource monitoring is needed, uncorrected photography may be suitable.
• Accuracy. As noted earlier, some agencies may require a very generalized map or imagery product, while others may require engineering scale accuracy. Always map to the highest level of accuracy your use will require.
  
  A typical agency discussion of accuracy involves the terms "Planning Scale Accuracy" and "Engineering Scale Accuracy". This reflects the varying needs well: planners usually do not have a need for precise ground measurement, whereas engineers in the public works arena do have a need for precision. Planners may be well-served by mapping that is accurate within 30 feet, whereas an engineer will usually require sub-meter accuracy.

Negotiating Tip: If you are planning to have aerial imagery flown, search for adjacent agencies or companies with a similar interest or need for the product. (State and Federal agencies also frequently have imaging needs which could be obtained by a partnership endeavor.) Identify the highest level of accuracy and resolution required and pursue the project as a joint venture. Most vendors will work with you in this partnership effort. The price will likely be lower than if each entity pursues the imagery on its own. An initial expense is involved in flight planning, setting control points and just getting the craft into the air; once in the air, the greater flight area is typically of less concern and expense.

Prior to contracting with an imagery vendor, you will need to decide the following items:

• Whether you want orthophotography or uncorrected imagery
• The geographic scope of the area to be flown
• The tile size you require: the ground coverage of each image, typically 2,000' x 3,000'
• The map scale you wish to create
• Pixel size
• The anticipated use of the imagery to be acquired
• The spatial accuracy required of the imagery
• Whether you prefer black and white or color photography
• Whether you will use the imagery in a Geographic Information System (GIS)
• If you have any partners to the project
• The products you want to receive from the vendor (See "What You Should Get from Your Vendor", below).

You may also have some subsidiary decisions to make. For example, you will need to decide in what season you wish to acquire the imagery. In some areas, tree cover is significant and blocks substantial portions of the terrain. If the trees are deciduous, a flight in winter would acquire imagery of terrain that would be obscured in the summer. Other seasonal issues such as fog and other cloud cover may have a significant bearing on the timing of your flight.

Related to this issue is your decision of how to handle features that are obscured. These include features hidden by trees, as noted above, but also a variety of other obstructions. Examples would include:

• Automobiles, which might obscure features on the curb or pavement;
• Rooflines
• Vertical obstructions such as tall buildings, signs, towers, etc.

Once the imagery is received and the obstructions identified, many agencies field check the features obstructed and obtain precise locations using GPS or other surveying techniques.

To say that imagery is provided sounds simple; in reality, imagery can be delivered in a variety of ways. In addition, other products can typically be provided by your vendor, including map work of your choosing and the imagery in a variety of formats. The following are typical products you can receive from your vendor:

• Diapositives. Photographic prints made on a clear film base used by a photogrammetrist in analytical aerotriangulation and traditional stereocompilation. Most vendors consider the diapositives to be the property of the client who commissioned the project and will willingly deliver them. A few vendors, however, hold on to the diapositives as a means of retaining the business of orthorectification. Specify your wishes in your pre-flight contract.
• Digital Elevation Model (DEM). Elevation data produced from overlapping stereo orthophotos. DEMs can usually be produced at little or no additional cost as they are a required component of producing the orthophoto. DEMs can be used to derive slope, aspect and shaded relief images.
• Flight Plan Map. Once you and your vendor have decided upon the area to be covered by orthophotographs, you should receive a confirming flight plan map from the company. This will look like an ordinary map, with the flight lines drawn in. You will see a certain amount of overlap in the areas covered by each line; this is required in creating horizontally accurate maps.
• Line work. Many vendors have the capability to develop the initial map base for you. They will do the necessary interpretation required to develop map layers derived from physical features shown on the imagery. Examples might include streets and roads, centerlines, curb lines, manhole covers, streetlights, rooflines, waterlines, hydrants, and signs. (You will want linework only if you are contracting for orthophotography.)
  
  You must specify how you wish to receive this data, particularly if you are going to use the imagery as part of a GIS. If it is delivered in a CAD format, you or another entity must work with the data to create topology, polygonize and tag the data, thus rendering it useful to the GIS. Incidentally, many vendors work in the CAD environment, while relatively few work in a full GIS environment. Nonetheless, the CAD format delivery is extremely useful and makes the final GIS work much easier.
• Mylars. The ortho or uncorrected photographic prints, corresponding to the tiles, can be delivered in mylar form. Some agencies find this useful if they wish to make copies for themselves or clients.
• Orthophotographs and uncorrected imagery are delivered digitally on a Compact Disc (CD). Orthophotographs are typically broken out into tiles of the size you specify in your project planning; uncorrected imagery is delivered by individual exposure The vendor should provide you with a grid that identifies each tile in the case of orthophotos, and a flight map with exposure numbers in the case of uncorrected images
• Prints (or Contact Prints). Typically in a 9" x 9" format, the individual tiles of the coverage are frequently useful to agencies wishing to use them on a daily basis for quick reference. These are uncorrected photographs, and you will see the same geography covered by several photographs. The apparent duplication is necessary for good aerial photography. The photographs are sequenced by flight line and photograph number; these are to be found in the upper margin of the photograph.
• Triangulation Report. Aerotriangulation is a photogrammetric procedure that uses relatively few survey control points to create other survey points across a block of photography. When the process is complete, each pair of overlapping photographs in a block can be set in a stereoplotting instrument and mapping can proceed. The procedure involves the precise measurement of stereo points and surveyed control targets on diapositives. Aerotriangulation is the mathematical adjustment of redundant measurements to reach a unique solution. It means that points on the ground are measured several times in several ways to reach the most precise description of their location.
  
  One of the first and probably most confusing products you will receive from the vendor is the triangulation report. This usually takes the form of a paper report with lists of mysterious numbers, letters and abbreviations. What the vendor is trying to convey to you are the adjustments made to the survey measurements. This is meant to reassure you of the quality of the measurements and to indicate that the orthorectification process is proceeding as it should.The triangulation report should state the RMS (root mean square) error for the surveyed control points and the RMS error for the image measurements made on the diapositives. The first of these values is measure of the control survey quality , the second is a measure of the photogrammetric measurement's quality. While RMS for the control points will be influenced by the scale of the photography, the quality of the photogrammetric measurements is independent of scale. A good adjustment will have an RMS of 0.010 mm (10 microns) or less on the image measurements.

Quality checking and quality assurance are a substantial component of your imagery product. You must be prepared to check the photography against your contractual agreement. Like any other product, imagery can contain errors, many of which can be corrected. Linework in particular may contain errors, since a great deal of interpretation is involved in its development.

Examples of interpretation errors may include simple misidentification of a feature, such as a stop light or manhole cover, or faulty presentation of the feature.

Appendix A: Definitions of Terms Frequently Used in Aerial Imaging

Accuracy, Horizontal: This is usually expressed as "plus or minus 10 feet" or "+/- 15'. A more formal definition has been written:

"...a measure of the confidence that a particular location on an orthophoto can be located to within a specified distance. For instance, the accuracy standards for a 1:24,000 scale digital orthoquad state that 90% of well-defined point features must fall within 40 feet of the actual ground coordinates^c. Areas of high terrain relief, and which fall on the edge of a photo will experience greater horizontal inaccuracies."

Metadata. Usually defined as data about data. For full utility of your imagery and the imagery you may acquire from other sources, you must have metadata. This means you must prepare metadata for your own imagery (or require it from your imagery provider) and you must demand it from the producers of imagery you acquire.

The US Geological Survey (USGS) has led the efforts of the Federal Geographic Data Committee (FGDC) to create standards for metadata development and tools for its acquisition. The national standards for metadata can be found on the Internet at the USGS site, http://www.usgs.gov/ Many states have metadata coordinating councils that participate in the national program and adhere to the national standard. The USGS website also has a complete listing of these agencies, along with contact information.

Why bother with metadata? Think of it as a prescription. You probably don't want to use a medication unless you know what its intended use is, who produced it, how old it is, and any particular problems that are known about it. Similarly, with any data, but particularly with imagery, you need to know where it came from, what its intended use is, its scale, its format, whether or not it has been orthorectified and so on.

The description of data, or metadata, for imagery should include, at a minimum:

The Central Coast Joint Data Committee is a consortium of public, private, academic and non-profit entities agreeing to share spatial data in the Monterey Bay region of California. It has taken on the task of building a metadata clearinghouse on the Internet, so that the community and its agencies can search for existing information about the region. It is also helping to fill in the gaps of spatial data, providing spatial data on streets and roads, parcels and census tracts.

The Imagery Special Interest Group's interest is in identifying regional imagery needs. Its efforts will be reflected both in the metadata clearinghouse and in the data building efforts. It is therefore setting forth desirable standards for the acquisition of aerial imagery (both digital and non-digital) for California's Central Coast; in addition, it is providing guidelines to agencies that are pursuing the acquisition of imagery. Please visit our website at http://www.centralcoastdata.org/

These standards have arisen from a series of discussions among area researchers, government representatives, and GIS professionals regarding the types of imagery that are most useful to them. Special thanks to:

• Peter Ashley, LSIT of Hammon, Jensen, Wallen and Associates in Oakland, California, for providing technical review, and to
• Jonathan Van Coops of the California Coastal Commission for providing graphics

A variety of resources exist for learning more about imagery and its uses. The following is a small sampling of those resources:

American Society for Photogrammetry and Remote Sensing (ASPRS)

http://www.asprs.org/

NASA

http://www.nasa.gov

US Geological Survey

http://www.usgs.gov

Digital Imaging, Ron Graham, Whittles Publishing, 1997

Digital Photogrammetry: An Addendum to the Manual of Photogrammetry, Clifford W. Greve, Editor, ASPRS, 1996

Fundamentals of Remote Sensing and Airphoto Interpretation, 5th edition, Thomas Eugene Avery and Graydon Lennis Berlin, 1992, Prentice Hall, Upper Saddle River, New Jersey

Remote Sensing & GIS: An Integration of Technologies for Resource Management, Ann Maclean, ASPRS, 1994