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:
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:
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:
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.
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.
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.
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.
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.
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).
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:
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.
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).
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).
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.
These elements can be ranked in relative importance:
Since aerial photography is dependent on photographs, we need, at this juncture, some basic insight into how a photo is made.