Architecture M.Arch

Master in Architecture

(GIS)

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Module Detail

Subject Name M.Arch - Architecture

Paper Name GIS

Module Name/Title Aerial Photography – 2

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Role Name

National Coordinator

Subject Coordinator Dr. Monsingh D. Devadas Paper Coordinator Dr. Pratheep Moses Content Writer/Author (CW) Dr. S. Vijaysagar Content Reviewer (CR) Dr. Pratheep Moses Language Editor (LE)

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Aerial Photography Introduction

An aerial photograph, in broad terms, is any photograph taken from the air. Normally, air photos are taken vertically from an aircraft using a highly-accurate camera. There are several things you can look for to determine what makes one photograph different from another of the same area, including type of film, scale, and overlap. Other important concepts used in aerial photography are stereoscopic coverage, fiducial marks, focal length, roll and frame numbers, and flight lines and index maps.

Aerial photography has two uses that are of interest in various applications viz., Cartographers and planners take detailed measurements from aerial photos in the preparation of maps and Trained interpreters utilize aerial photos to determine land-use and environmental conditions, among other things.

Although both maps and aerial photos present a "bird's-eye" view of the earth, aerial photographs are NOT maps. Maps are orthogonal representations of the earth's surface, meaning that they are directionally and geometrically accurate (at least within the limitations imposed by projecting a 3- dimensional object onto 2 dimensions). Aerial photos, on the other hand, display a high degree of radial distortion. That is, the topography is distorted, and until corrections are made for the distortion, measurements made from a photograph are not accurate. Nevertheless, aerial photographs are a powerful tool for studying the earth's environment.

Because most GISs can correct for radial distortion, aerial photographs are an excellent data source for many types of projects, especially those that require spatial data from the same location at periodic intervals over a length of time. Typical applications include land-use surveys and habitat analysis.

This unit discusses benefits of aerial photography, applications, the different types of photography, and the integration of aerial photographs into GISs.

Advantages of Aerial Photography over Ground – Based Observation

 Aerial photography offers an improved vantage point.

 Aerial photography has the capability to stop action.

 It provides a permanent recording.

 It has broader spectral sensitivity than the human eye.

 It has better spatial resolution and geometric fidelity than many ground-based sensing methods.

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Black and White Color

Color Infrared

The Black and White and Colour photographs are common in all scientific studies. The scientific community, however, has made continuous use of the film technology.

Color infrared film is often called "false-color" film. Objects that are normally red appear green, green objects (except vegetation) appear blue, and "infrared" objects, which normally are not seen at all, appear red.

The primary use of color infrared photography is vegetation studies. This is because healthy green vegetation is a very strong reflector of infrared radiation and appears bright red on color infrared photographs.

Basic Concepts of Aerial Photography

Film: most air photo missions are flown using black and white film, however colour, infrared, and false-colour infrared film are sometimes used for special projects.

Focal length: the distance from the middle of the camera lens to the focal plane (i.e. the film). As focal length increases, image distortion decreases. The focal length is precisely measured when the camera is calibrated.

Scale: the ratio of the distance between two points on a photo to the actual distance between the same two points on the ground (i.e. 1 unit on the photo equals "x" units on the ground). If a 1 km stretch of highway covers 4 cm on an air photo, the scale is calculated as follows:

Another method used to determine the scale of a photo is to find the ratio between the camera's focal length and the plane's altitude above the ground being photographed.

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If a camera's focal length is 152 mm, and the plane's altitude Above Ground Level (AGL) is 7 600 m, using the same equation as above, the scale would be:

Scale may be expressed three ways:

 Unit Equivalent

 Representative Fraction

 Ratio

A photographic scale of 1 millimetre on the photograph represents 25 metres on the ground would be expressed as follows:

 Unit Equivalent - 1 mm = 25 m

 Representative Fraction - 1/25 000

 Ratio - 1:25 000

Two terms that are normally mentioned when discussing scale are:

 Large Scale - Larger-scale photos (e.g. 1:25 000) cover small areas in greater detail. A large scale photo simply means that ground features are at a larger, more detailed size. The area of ground coverage that is seen on the photo is less than at smaller scales.

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 Small Scale - Smaller-scale photos (e.g. 1:50 000) cover large areas in less detail. A small scale photo simply means that ground features are at a smaller, less detailed size. The area of ground coverage that is seen on the photo is greater than at larger scales.

Fiducial marks: small registration marks exposed on the edges of a photograph. The distances between fiducial marks are precisely measured when a camera is calibrated, and this information is used by cartographers when compiling a topographic map.

Overlap: is the amount by which one photograph includes the area covered by another photograph, and is expressed as a percentage. The photo survey is designed to acquire 60 per cent forward overlap (between photos along the same flight line) and 30 per cent lateral overlap (between photos on adjacent flight lines).

Stereoscopic Coverage: the three-dimensional view which results when two overlapping photos (called a stereo pair), are viewed using a stereoscope. Each photograph of the stereo pair provides a slightly different view of the same area, which the brain combines and interprets as a 3-D view.

Roll and Photo Numbers: each aerial photo is assigned a unique index number according to the photo's roll and frame. For example, photo A23822-35 is the 35th annotated photo on roll A23822. This identifying number allows you to find the photo in NAPL's archive, along with metadata information such as the date it was taken, the plane's altitude (above sea level), the focal length of the camera, and the weather conditions.

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Flight Lines and Index Maps: at the end of a photo mission, the aerial survey contractor plots the location of the first, last, and every fifth photo centre, along with its roll and frame number, on a National Topographic System (NTS) map. Photo centres are represented by small circles, and straight lines are drawn connecting the circles to show photos on the same flight line.

This graphical representation is called an air photo index map, and it allows you to relate the photos to their geographical location. Small-scale photographs are indexed on 1:250 000 scale NTS map sheets, and larger-scale photographs are indexed on 1:50 000 scale NTS maps.

Preparation of Topographic map from Aerial Photograph

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Mapping from aerial photos is the best mapping procedure yet developed for most large projects.

 Used successfully for maps varying in scale from 1:1,000,000 1:120 with contour intervals as small as 0.5 m

 Topographic mapping is the most common form.

 Used to reconstruct a scaled 3-dimensional optical model of the land’s surface using a stereoplotter.

Uses: Aerial photos

 Geological investigations

 Soil surveys

 Land surveys

 Tax mapping

 Reconnaissance and military intelligence

 Urban and regional development

 Transportation system investigations

 Quantity estimates

 Shore erosion, etc.

 Architecture – Urban Planning

Types of Photogrammetry

► Aerial – series of photographs of an area of terrain in sequence using a precision camera.

► Terrestrial – photos taken from a fixed and usually known position on or near the ground with the camera axis horizontal or nearly so.

► Close range – camera close to object being observed. Most often used when direct measurement is impractical.

Types of Aerial Cameras

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► Aerial mapping camera (single lens),

► Multilens camera, the multiband aerial camera,

► Reconnaissance camera,

► Strip camera,

► Panoramic camera,

► Digital camera – in vogue

A camera for tracking missiles launched at night

Images taken with Telescopic cameras

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Aerial mapping camera

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As mentioned above aerial photographs can be classified as:

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Terrestrial Aerial

Vertical Oblique True Tilted High Low

We can define vertical aerial photographs as a photo taken from an aerial platform (either moving or stationary) wherein the camera axis at the moment of exposure is truly vertical. In actuality, vertical airphotos with less than 3* tilt are considered vertical (for most photo interpretation purposes); while those with more than 3* tilt are considered oblique. There are two basic types of oblique aerial photography. These two types are:

1. High angle oblique; and 2. Low angle oblique.

In a high angle oblique, the apparent horizon is shown; while in a low angle oblique the apparent horizon is not shown. Often because of atmosphere haze or other types of obscuration the true horizon of a photo cannot really be seen. However we often can see a horizon in an oblique air photo. This is the apparent horizon.

Types of Aerial Photographs

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These images are usually taken at an angle, typically 45 degrees but as they are often taken manually, they can be whatever angle gives the best view of the feature or landscape. The oblique image is primarily used in archaeology to take a wider context of a feature and the area around it, and also to give depth. Nearly always taken at a much lower elevation than the vertical image and in few numbers, its application is fairly limited and often taken for a specific purpose. There is a problem in perspective because the farther away a feature is, the smaller it will appear: nearer objects of comparable size appear larger than those that are farther away so it is often best to take a selection or to use a frame of reference on the ground for perspective purposes. These images are taken from small fixed-wing aircraft and helicopters and are perfectly suited for monitoring erosion of features and monuments throughout the year and over the course of many decades.

Oblique Photographs: When Best to Take Them?

The time of year is vital and many see winter as the perfect season to take aerial photographs. There are many reasons for this, not least of all that it is easier to see features in fields that do not have crops and will not be ploughed for several more months. Surviving features beneath the surface will often show up darker due to the shallower levels of soil. Snowy and frosty conditions perfectly emphasise ridges and features and they can be photographed with a clarity not seen at any other time of the year. The low level to which the sun rises casts much longer shadows, making visibility of above ground features much easier to spot. The perfect example here is relict medieval ridge and furrow features. That's not to say that the warmer months and longer hours of light are not conducive to aerial photography. If there are stone remains beneath a surface, crops will grow shallower as they cannot put down as much root and features will show up as crop marks. Late evening conditions also cast longer shadows and the differing light levels between morning, afternoon and evening can add depth when comparing multiple images of the same feature(s) over

the course of a day.

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Vertical

Taking a photograph straight down over a landscape is the more familiar form of aerial photograph.

It is a plan view so there is no perspective to distort the image. This also means that it is difficult to read the lay of the land such as changes in height - though there is a work around to create 3D image through stereoscopic views, using a device to examine two at once. This usually gives a good impression of the variation in the elevation of land. They are taken at regular heights for consistency so it is easier to compare contexts of a landscape taken on the same day, or many years apart to examine development. Rarely used in archaeological applications except perhaps sometimes to find interesting earthworks and other sites that are easily missed on the ground, they cover a much wider area and focus on topography rather than specific details.

Vertical Photographs: When Best to Take Them?

As a rule, vertical aerial photographs are easier to interpret than oblique photographs because of the standardised ways in which they are taken - with set scales and at a single non- arbitrary angle (10). The same advantages generally apply to vertical as they do for oblique, but you will lack the perspective, the depth and the 3D effect even with the weather conditions mentioned above. At higher levels, you may miss crop and soil marks. If it is an overview you require, then vertical photography is the best way to go.

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Geometry of Aerial Photography

The basic advantages of vertical air photos are:

1. The scale is essentially constant;

2. Measurements of directions are easier than on oblique photograph. Directions can also be measured more accurately;

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3. Within limits a vertical aerial photograph can be used as a map (if grids and marginal data are added); and,

4. Vertical aerial photographs are often easier to interpret than oblique and are better for stereo (there is no masking).

The advantages of an oblique aerial photograph include:

1. Given a constant altitude and camera you can cover a much larger area on a single photo;

2. The view of some objects is more familiar to the interpreter; and, 3. Some objects not visible on vertical photos may be seen on oblique.

(Paine talks about clearance and cloud cover; but that's a tricky one (too cloudy for vertical but maybe enough clearance for an oblique).

Three terms need defining here, they are Principal Point, Nadir and Isocenter. They are defined as follows:

1. Principal Point - The principal point is the point where the perpendicular projected through the center of the lens intersects the photo image.

2. Nadir - The Nadir is the point vertically beneath the camera center at the time of exposure.

3. Isocenter - The point on the photo that falls on a line half- way between the principal point and the Nadir point.

On a true vertical aerial photograph all three of these would be at the same point. There is no such thing as a true vertical aerial photo. All air photos have some degree of tip or tilt.

A quick review.

Vertical Airphotos (0-3* tilt)

3 Photo Centers: Principal Point, Nadir, Isocenter

These points are important because certain types of displacement and distortion radiate from them.

It is the Isocenter of the aerial photo from which tilt displacement radiates. It is Nadir from which topographic displacement radiates.

II. Perspective and Projection

First lets consider the viewing perspective of a map. On a map objects and features are both planimetrically and geometrically accurate. That is objects are located on the map in exactly the

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same position relative to each other as they are on the surface of the Earth, except with a change in scale. This is due to the fact that maps use an orthographic projection (i.e. using parallel lines of site) and constant scale to represent features.

Aerial photographs on the other hand are created using a central or perspective projection.

Therefore, the relative position and geometry of the objects depicted depends upon the location from which the photo was taken.

Now because of this we get certain forms of distortion and displacement in Air Photos.

III. Distortion and Displacement

There are basically four types of distortions and three types of displacement.

Types of distortion include:

1. Film and Print Shrinkage;

2. Atmospheric refraction of light rays;

3. Image motion; and, 4. Lens distortion.

Types of displacement include:

1. Curvature of the Earth;

2. Tilt; and,

3. Topographic or relief (including object height).

The effects of film shrinkage, atmospheric refraction and the curvature of the Earth are usually negligible in most cases - the exception is precise mapping projects. These types of distortions and displacement will not be discussed here. Image motion will be dealt with further in our lecture on camera systems. That leaves only lens distortion, tilt and topographic displacement to be discussed here. Of these lens distortion is usually the smallest of these. So displacement is typically the largest problem/effect impacting our analyses.

Both distortion and displacement cause changes in the apparent location of objects in photos. The distinction between the types of effects caused lies in the nature of the changes in the photos.

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Distortion - Shift in the location of an object that changes the perspective characteristics of the photo.

Displacement - shift in the location of an object in a photo that does not change the perspective characteristics of the photo (The fiducial distance between an object's image and it's true plan position which is caused by change in elevation.)

These types of phenomena are most evident in terrain with high local relief or significant vertical features.

As stated above we will consider here three main types of problems/effects caused by specific types of distortion and displacement. These are the problems/effects associated with:

1. Lens distortion;

2. Tilt Displacement; and, 3. Topographic Displacement.

Lens distortion - Small effects due to the flaws in the optical components (i.e. lens) of camera systems leading to distortions (which are typically more serious at the edges of photos). Car windows/windshields, carnival mirrors are probably the best know examples of this type of effect.

These effects are radial from the principal point (making objects appear either closer to, or farther from the principal point than they actually are); and may be corrected using calibration curves.

Tilt Displacement - A tilted photograph presents a slightly oblique view rather than a true vertical record. All photos have some tilt. The perfect gyro stabilization unit, like the perfect lens, has yet to be built. Tilt is caused by the rotation of the platform away from the vertical. This type of displacement typically occurs along the axis of the wings or the flight line. Tilt displacement radiates from the isocenter of the photo and causes objects to be displaced radially towards the isocenter on the upper side of the tilted photo and radially outward on the lower side. If the amount and direction of tilt are known then the photo may be rectified.

Topographic Displacement - This is typically the most serious type of displacement. This displacement radiates outward from Nadir. Topographic displacement is caused by the perspective geometry of the camera and the terrain at varying elevations.

A close look at the equations involved in the calculations of relief displacement show that some important general relationships are involved. These relationships can be stated as follows:

1. There is no topographic displacement at Nadir. If r is zero, then so is d.

2. Assuming datum elevation to be at Nadir, points above the datum are displaced radially away from Nadir while points below datum are displaced radially towards Nadir.

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3. Topographic displacement varies directly with the radial distance from the Nadir to the object.

A particular elevation two inches from the Nadir will have half the displacement as that same

elevation four inches from the Nadir.

4. Topographic displacement varies directly with the height of an object. A 100 ft. tree would be displaced twice as far as a 50 ft. tree the same distance from Nadir.

5. Topographic displacement varies inversely with the flying height of the base of the object. As a result there is little apparent topographic displacement on space photography.

The reason for small relief displacement from space is that to achieve a given scale a shorter focal length lens requires flying at a lower altitude. The effect of using short focal length lenses is to increase topographic displacement, distortion and the apparent depth of the third dimension (vertical exaggeration) in stereoscopic images).

To get a scale of 1: 20,000 you fly at 10,000 ft. with a six inch focal length lens; but at 20,000 ft.

with a 12 inch focal length lens.

Basically, then the most important cause of object displacement on aerial photography is local relief. Remember here that there are times when increased displacement can be a good thing (e.g.

for height measurements). So in flat areas you may want to use a short focal length lens to achieve a given scale.

From Space then you can still use extremely long lenses with little displacement.

IV. Orthophotography

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Briefly, here there is a growing use of orthophotography today. If you remember back to our discussion of maps at the beginning of the lecture you will remember that orthographic projection depict thing in their true plan position. Basically what happens in the production of orthophotographs is that the original photographs are employ to create a stereo-model which is scanned, by very expensive (today) equipment (orthophotoscope), in very small segments;

displacements are corrected and the resulting strips are merged to create a photograph that is essentially a map, actually a planimetrically accurate photo-map. On an orthophoto distances, areas, and directions can all be accurately measured more easily. The U.S. Geological Survey’s National Mapping Division is doing all revisions of the 1: 24,000 topographic map updates using digital orthophotography. These ortho photo maps show more detail than the older cartographic product due to the actual image background.

Original & Ortho Rectified Air Photo So, the advantages here are:

1. Greater detail than a map 2. Equal planimetric accuracy But, the disadvantages are:

1. Lower resolution than the original photos 2. Some loss of information with loss of resolution 3. Loss of stereoscopic capability.

Triangulation - Aerotriangulation

Triangulation is the principle used by both photogrammetry and theodolites to produce 3- dimensional point measurements. By mathematically intersecting converging lines in space, the precise location of the point can be determined. However, unlike theodolites, photogrammetry can measure multiple points at a time with virtually no limit on the number of simultaneously triangulated points.

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In the case of theodolites, two angles are measured to generate a line from each theodolite. In the case of photogrammetry, it is the two-dimensional (x, y) location of the target on the image that is measured to produce this line. By taking pictures from at least two different locations and measuring the same target in each picture a "line of sight" is developed from each camera location to the target. If the camera location and aiming direction are known (we describe how this is done in Resection), the lines can be mathematically intersected to produce the XYZ coordinates of each targeted point.

ELEMENTS OF PHOTOGRAPH / IMAGE INTERPRETATION

Although most individuals have had substantial experience in interpreting “conventional”

photographs / images in their daily lives, the interpretation of aerial photographs / satellite imagery often departs from every day image interpretation in three different respects: (1) the portrayal of features from an overhead often unfamiliar perspective; (2) the frequent use of wavelengths outside the visible portion of the electromagnetic spectrum: and (3) the depiction of the earth’s surface at unfamiliar scales and resolutions. While these factors may be insignificant to the experienced image interpreter they can represent a substantial challenge to the novice image analyst. A systematic study of aerial photograph / satellite imagery usually involves several basic characteristics of features shown on a photograph / an image. The exact characteristics useful for any specific task and the manner in which they are considered depend on the field of application. However, most applications consider the following basic characteristics or variations of them: shape, size, pattern, tone (or hue), texture, shadows, site, and association.

Shape: refers to the general form, configuration or outline of individual objects. In the case of stereoscopic photograph / imagery the object’s height also defines its shape. The shape of some objects is so distinctive that their images may be identified solely from this criterion. The pentagon Building near Washington D.C is a classic example. All shapes are obviously not this diagnostic but every shape is of some significance to the photograph / image interpreter.

Size: of objects on photograph / imagery must be considered in the context of the photograph / image scale. A small storage shed for example might be misinterpreted as a barn if size were not considered. Relative sizes among objects on photograph / images of the same scale must also be considered.

Pattern: relates to the spatial arrangement of objects. The repetition of certain general forms or relationships is characteristic of many objects both natural and constructed, and gives objects a pattern that aids the photograph / image interpreter in recognizing them. An out-door drive-in theater for example has a particular layout and pattern of parking spaces that aid in its identification.

Drive-in theaters have been misidentified as housing subdivisions by novice photograph / image interpreters, who did not carefully consider size, shape ad pattern. Likewise the ordered spatial arrangements of trees in an orchard is in distinct contrast to that of forest tree stands.

Tone (or hue): refers to the relative brightness or color of objects on photograph / images. It is important to know how relative photograph / image tones could be used to distinguish between

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deciduous and coniferous trees on black and white, infrared photograph / images. A striking pattern of light toned and dark-toned soils where the tonal pattern vary according to the drainage conditions of the soil (the lighter toned areas are topographically higher and drier, the darker toned areas are lower and wetter) making it easy to differentiate the soil type. Without tonal differences the shapes, pattern, and textures of objects could not be discerned.

Texture is the frequency of tonal change on the photograph / image. Texture is produced by an aggregation of unit features that may be too small to be discerned individually on the photograph such as tree leaves and leaf shadows. It is a product of their individual shape, size, pattern, shadow and tone. It determines the overall visual smoothness or coarseness of image features. As the scale of the photograph / image is reduced, the texture of any given object or area become progressively finer and ultimately disappears. An interpreter can often distinguish between features with similar reflectance based on their texture differences. An example would be the smooth texture of green grass as contrasted with the rough texture of green tree crowns on medium scale photograph / images.

Shadow: is important to interpreters in two opposing respects: (1) the shape or outline of a shadow affords an impression of the profile view of objects (which aids in interpretation), (2) objects within shadows reflect little light and are difficult to discern on photograph / images (which hinders interpretation). For example the shadows cast by various tree species or cultural features (bridges, towers, etc.,) can definitely aid in their identification on photograph / image. Also the shadows resulting from even subtle variations in terrain elevations especially in the case of low sun angle photograph can aid in assessing natural topographic variations that may be diagnostic of various geologic landforms.

Site: refers to topographic or geographic location and is a particularly important aid in the

identification of vegetation types. For example, certain tree species would be expected to occur on well-drained upland sites, whereas other tree species occur only on certain geographic areas.

Association: refers to the occurrence of certain features in relation to others. For example, a Ferris wheel might be difficult to identify if standing in a field near a barn but would be easy to identify if in an area recognized as an amusement park.

PHOTOGRAPH / IMAGE INTERPRETATION STRATEGIES

As previously mentioned the photograph / image interpretation process can involve various level of complexity from a simple direct recognition of objects in the scene to the inference of site conditions. An example of direct recognition would be the identification of a highway interchange.

Assuming the interpreter has some experience with the vertical perspective of aerial photographs, recognition of a highway interchange should be a straightforward process. On the other hand, it may often be necessary to infer rather than directly observe the characteristics of features based on their appearance on photograph / image. In the case of a buried gas pipeline for example, the actual pipeline cannot be seen but there are often changes at the ground surface caused by the buried pipeline that are visible on photos / images. Soils are typically better drained over the pipeline because of the sand and gravel used for backfill and the presence of a buried pipeline can often be

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inferred by the appearance of a light toned linear streak across the photograph / image. Also the interpreter can take into account the probability of certain ground cover types occurring on certain dates at certain places. Knowledge of the crop development stages (crop calendar) for an area would determine if a particular crop were likely to be visible on a particular date. For example, corn peas and winter wheat would each have a significant vegetative ground cover on different dates.

Likewise, in a particular growing region one crop type may be present over a geographic area many times larger than that of another crop type therefore the probability of occurrence of one crop type would be much greater than another.

In a sense the photograph / image interpretation process is like the work of a detective trying to put all the pieces of evidence together to solve a mystery. For the interpreter the mystery might be presented in terms of trying to understand why certain areas in an agricultural field look different from the rest of that field of that field. At the most general level the interpreter must recognize the area under study as an agricultural field. Beyond this consideration might be made as to whether the crop (e.g., alfalfa) Based on the crop calendar and regional growing conditions a decision might be made that the crop is indeed corn rather than another row crop such as soybeans. Furthermore, it might be noted that the anomaly areas appearing in the field are associated with areas of slightly higher topographic relief relative to the rest of the field. With knowledge of the recent local weather conditions, the interpreter might infer that the anomaly areas appearing are associated with drier soil conditions and the corn in these areas is likely drought stressed. Hence, the interpreter uses the process of convergence of evidence to successively increase the accuracy and details of the interpretation.

AIRPHOTOGRAPH / IMAGE INTERPRETATION KEYS

The photograph / image interpretation process can often be facilitated through the use of photograph / image interpretation keys. Keys can be valuable training aids for novice interpreters and provide useful reference or refresher materials for more experienced interpreters. A photograph / image interpretation key helps the interpreter evaluate the information presented on aerial

photograph/satellite imagery in an organized and consistent manner. It provides guidance about the correct identification of features or conditions on the photograph / images. Ideally, a key consists of two basic parts: (1) a collection of annotated or captioned stereograms illustrative of the features or conditions to be identified and (2) a graphic or word description that sets forth in some systematic fashion the image recognition characteristics of those features or conditions. Two general types of photograph / image interpretation keys exist differentiated by the method of presentation of diagnostic features. A selective key contains numerous photograph / images examples with supporting text. The interpreter selects the example that most nearly resembles the feature or condition found on the photograph / images under study.

An elimination key is arranged so that the interpretation proceeds step-by-step from the general to the specific and leads to the elimination of all features or conditions except the one being identified. Elimination keys often take the form of dichotomous keys where the interpreter makes a series of choices between two alternatives and progressively eliminating all but one possible answer. The use of elimination keys can lead to more positive answer than selective keys but may result in erroneous answer if the interpreter is forced to make an uncertain choice between two unfamiliar image characteristics. As a generalization keys are more easily constructed and more reliably utilized for cultural feature identification (houses, bridges, roads, towers) than for

vegetation or landform identification. However a number of keys have been successfully employed

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for agricultural crop identification and tree species identification. Such keys are normally developed and used on a region-by-region and season-by-season basis in that the appearance of vegetation can vary widely with location.

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In Urban Studies

Urban development and the history of urbanism is a growing niche of landscape studies which has a wide range of uses through history and archaeology, the history of cartography, the history of commerce, sociology and even for modern urban planning. Town developers need to study the impact of expansion and development of urban centers on the landscape and the impact on the environment. New facilities (for example a new sports stadium) will require a rethink of the infrastructure and the impact that the new facility will have on people living in the area - will we need to build more houses? Upgrade the roads? Will this affect protected areas? Aerial photography taken at low levels is vital to examining the existing infrastructure.