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Elements of Aerial Photography

Aerial photographs have been a main source of information about what is at the Earth's surface almost since the beginning of aviation more than 100 years ago. A good review of the history of aerial photography, by Prof. Paul Baumann of SUNY-Oneonta, is found at his website.

Until space imagery,aerial photos were the principal means by which maps are made of features and spatial relationships on the surface. Cartography, the technology of mapping, depends largely on aerial/satellite photos/images to produce maps in two dimensions or three (see next Section). Aerial photos are obtained using mapping cameras that are usually mounted in the nose or underbelly of an aircraft that then flies in discrete patterns or swathes across the area to be surveyed (see page 7-1 for more details as to flight plans). These two figures show a camera and a cutaway indicating its operation:

A typical camera used to obtain aerial photos.
Cutaway diagram (simplified) showing a mapping aerial camera.

For most flight surveys, the camera film is advanced automatically and wound onto reel spindles at a rate which is tied to the aircraft's speed.

A variant of this camera system is the multispectral camera (also discussed on page 11-1). This type uses separate lenses, each with its own narrow band color filter, that are opened simultaneously to expose a part of the film inside the camera. Here is one such camera developed for use in the Skylab space station program:

The six band Skylab multispectral camera.

Aerial photos are taken from a variety of platforms: airplanes; helicopters; unmanned drones; balloons; kites; tall buildings. For the most common platform - airplanes - most cameras are mounted in the underside of the aircraft. Propeller or JetProp aircraft are preferred, for two reasons: 1) they fly slower, allowing easier film advance; 2) they cost less to operate. This photo shows two such aircraft used by NOAA in its remote sensing programs:

The Dept. of Commerce's principal vehicles used in aerial photography: The larger aircraft is the Lockheed turboprop WP-30 Orion; the smaller plane is the GulfStream JetProp Commander.

In previous sections, we have employed aerial photography to look closer at areas of which we had satellite based images (such as Morro Bay in Section 1). In fact, satellite image interpretation is in essence an extension of the concepts underlying aerial photography, taken to higher altitudes that allow coverage of larger pieces of real estate. Space remote sensing uses devices that, while much more costly to build and operate, rely on the same physical principles to interpret and extract information content.

Most textbooks on remote sensing are outgrowths of earlier texts that once dwelt dominantly on acquiring and interpreting of aerial photos. New books still include one to several chapters on this basic, convenient approach to Earth monitoring. We shall allot only limited space to explore some essentials of this expansive topic in the present Section and the next. In Section 11, we consider photogrammetry as the tool for quantifying topographic mapping and other types of mensuration. For anyone seeking more details about aerial photography/photogrammetry, we recommend consulting the reading list in the RST Overview (first page), and/or going to Volume 1 (Module 1) of the Remote Sensing Core Curriculum. Below is a recommended entry from that reading list:

Avery, T.E. and Berlin, G.L., Fundamentals of Remote Sensing and Airphoto Interpretation, 6th Ed., 1992, MacMillan Publ. Co., 472 pp.

Examples of Aerial Photos

An aerial photo is just a black and white (b & w) or color "picture" of an area on the Earth's surface (plus clouds), either on print or in a transparency, obtained by a film or digital camera located above that surface. This camera shoots the picture from a free-flying platform (airplane, helicopter, kite or balloon) some preplanned distance above the surface. Two types depend on the angle of view relative to the surface. The first, oblique photography, snaps images from a low to high angle relative to vertical. The example below is the most common type (high oblique), showing Lyttleton Harbor, near Christchurch, on South Island of New Zealand, with more detail in the foreground and a panorama with reduced detail in the background.

Low oblique aerial photo of Lyttleton Harbor, Christchurch, South Island, New Zealand.

The second type of aerial photos is oriented vertically, that is, it results from pointing the camera straight down (to the nadir, at the photo center point) to show the surface directly from above. The size of the photo and the sizes of the features represented within the photos can vary depending on the following: the camera's optical parameters, the surface area of the exposed film (frame size), the subsequent printing sizes (e.g., enlargement), and the altitude of the camera platform.

Image Scale

The ratio of the size of any object, feature, or area within the photo to its actual size on the ground is called the scale (defined and discussed on the third page of this Section).

We now present a series of aerial photos, acquired at different times and scales, most covering areas that lie within this June, 1977, Landsat image (original scale = 1:1,000,000) of south-central Pennsylvania, a scene we have looked at in earlier Sections, and especially during the Exam at the end of Section 1.

False color Landsat-1 image of central Pennsylvania, including Harrisburg, originally printed at a 1:1,000,000 scale.

This scene contains heavily forested fold ridges. Some of the bluish-black areas are defoliation patches caused by the Gypsy Moth. Others areas near top right are surfaces covered with black dust from the Anthracite coal strip mining in fold valleys. Bluish areas in the wide valleys are fields still bare or with early stage growth. The Susquehanna River which empties into the top of Chesapeake Bay bisects the image. Near the left center, a blue pattern with spokes is Harrisburg, the state capital, with York below it and Lancaster to the right. Next, we show a standard medium-scale ( moderate area of coverage but with considerable detail [individual buildings still visible]), black and white aerial photo of part of Harrisburg. The scale value given is that of the original photo before it was reduced to your screen size; quoting this value helps to appreciate what can be seen (resolved) at that scale, no matter what the eventual picture size becomes through enlargement or reduction.

 Black and white vertical aerial photo showing the downtown part of the city of Harrisburg (right) and towns across the Susquehanna River to its west.

Harrisburg (Scale = 1:100,000)

The number in the upper left corner of this black and white photo of Harrisburg is the date; on its right is the Mission number; and in center is a number denoting the flight line and particular photo within that line. Individual fields, smaller rivers, bridges, and roads are easily picked out.

The next photo is large scale (small coverage area and high resolution for identifying features smaller than buildings, e.g., cars) and covers an area within Harrisburg, just east of the previous photo, bisected by Interstate 83. Note particularly the lake-filled quarry (left center).

Aerial photograph of urban Harrisburg (scale=1:4000).
Urban Harrisburg (1:4,000)

In the lower right corner of the Landsat image is an agricultural area along the Chesapeake and Delaware Canal. Its expression in a moderately large-scale, natural-color photo is shown here:

Color aerial photograph along the Chesapeake Bay and Delaware Canal (scale=1:24,000).
Natural Color Photo (1:24,000)

At a still smaller scale, we next show a false-color, IR image of the Susquehanna Water Gap passing through Blue Mountain just north of Interstate 81 (bottom of the picture) that, to the east, runs along the north side of Harrisburg.

Color IR aerial photograph of the Susquehanna Water Gap near Harrisburg (scale=1:8000).
Color IR Photo (1:8,000)

Much the same area is part of a small scale (large area coverage with reduced detail) aerial photo obtained from an RB-57, NASA aircraft, flown at an altitude near 15,200 m (about 50,000 ft) on February 5, 1974. On this date, the color-IR photo shows limited red tones from fields in which winter wheat is growing. The image is 25.2 km (15.7 mi) on a side (635 square km; 246 square mi).

Color infrared aerial photo, taken from the NASA RB-57 high altitude aircraft, again showing Harrisburg, and towns to the west.
High Altitude Aerial Photo (1:141000)

A word of caution at this point. Because of shadow orientation and other factors, features in a photograph (much rarer in space imagery) that represent relief (differences in elevation), such as hills, can appear to the eye as inverted, i.e., a high appears as a valley, a valley as a hill. The best example the writer (NMS) has found is a group of mesas and troughs on Mars. On the left is the correct expression (mesas look higher); on the right is the inverted case. If you ever see an aerial photo that does not look right (expected highs show up as lows), just reorient the photo (a few people may need to reorient their brain).

Normal view of mesas and troughs on Mars The same image when rotated 180 degrees causes the troughs to appear higher.

Among the most obvious features in a photograph are tones and tonal variations (as grays or colors) and patterns made by these. These, in turn, depend on the physical nature and distribution of the elements that make up a picture. These "basic elements" can aid in identifying objects on aerial photographs.

Tone (closely related to Hue or Color) -- Tone refers to the relative brightness or color of elements on a photograph. It is, perhaps, the most basic of the interpretive elements because without tonal differences none of the other elements could be discerned. Size -- The size of objects must be considered in the context of the scale of a photograph. The scale will help you determine if an object is a stock pond or Lake Minnetonka. Shape -- refers to the general outline of objects. Regular geometric shapes are usually indicators of human presence and use. Some objects can be identified almost solely on the basis of their shapes: for example - the Pentagon Building, (American) football fields, cloverleaf highway interchanges Texture -- The impression of "smoothness" or "roughness" of image features is caused by the frequency of change of tone in photographs. It is produced by a set of features too small to identify individually. Grass, cement, and water generally appear "smooth", while a forest canopy may appear "rough". Pattern (spatial arrangement) -- The patterns formed by objects in a photo can be diagnostic. Consider the difference between (1) the random pattern formed by an unmanaged area of trees and (2) the evenly spaced rows formed by an orchard. Shadow -- Shadows aid interpreters in determining the height of objects in aerial photographs. However, they also obscure objects lying within them. Site -- refers to topographic or geographic location. This characteristic of photographs is especially important in identifying vegetation types and landforms. For example, large circular depressions in the ground are readily identified as sinkholes in central Florida, where the bedrock consists of limestone. This identification would make little sense, however, if the site were underlain by granite. Association -- Some objects are always found in association with other objects. The context of an object can provide insight into what it is. For instance, a nuclear power plant is not (generally) going to be found in the midst of single-family housing.

These elements can be ranked in relative importance:

The elements of interpretation, ranked in complexity and value.

Since aerial photography is dependent on photographs, we need, at this juncture, some basic insight into how a photo is made.

Source: http://rst.gsfc.nasa.gov