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Most remote sensing texts begin by giving a survey of the main principles, to build a theoretical background, mainly in the physics of radiation. While it is important to have such a framework to pursue many aspects of remote sensing, we do not delve into this complex subject in much detail at this point. Instead, we offer on this and the next several pages an outline survey of the basics of relevant electromagnetic concepts. On this page, the nature of the photon is the prime topic. Photons of different energy values are distributed through what is called the Electromagnetic Spectrum. A full discussion of the electromagnetic spectrum (EMS) is deferred to page I-4. Hereafter in this Introduction and in the Sections that follow, we limit the discussion and scenes examined to remote sensing products obtained almost exclusively by measurements within the Electromagnetic Spectrum (force field and acoustic remote sensing are briefly covered elsewhere in the Tutorial). Our emphasis is on pictures (photos) and images (either TV-like displays on screens or “photos” made from data initially acquired as electronic signals, rather than recorded directly on film). We concentrate mainly on images produced by sensors operating in the visible and near-IR segments of the electromagnetic spectrum (see the spectrum map on page I-4), but also inspect a fair number of images obtained by radar and thermal sensors.

The next several pages strive to summarize much of the underlying theory – mainly in terms of Physics – appropriate to Remote Sensing. The reader can gain most of the essential knowledge just through those pages. The writer’s (NMS) original, but now unavailable, “Landsat Tutorial Workbook”, the information source from which this Remote Sensing Tutorial is an updated extension and expansion, contains a more detailed treatment of many aspects of the theory, including a different treatment of quantum theory and an examination of how spectroscopy helped to develop that theory. So, optionally you can choose to read a reproduction of extracts from the Landsat T W version to extend your basic understanding by clicking onto the hidden page I-2a. Or, if you choose not to, read this next inserted paragraph which synopsizes key ideas from both the present and the I-2a pages:

Synoptic Statement: The underlying basis for most remote sensing methods and systems is simply that of measuring the varying energy levels of a single entity, the fundamental unit in the electromagnetic (which may be abbreviated “EM”) force field known as the photon. As you will see later on this page, variations in photon energies (expressed in Joules or ergs) are tied to the parameter wavelength or its inverse, frequency. EM radiation that varies from high to low energy levels comprises the ElectroMagnetic spectrum (EMS). Radiation from specific parts of the EM spectrum contain photons of different wavelengths whose energy levels fall within a discrete range of values. When any target material is excited by internal processes or by interaction with incoming EM radiation, it will emit or reflect photons of varying wavelengths whose radiometric quantities differ at different wavelengths in a way diagnostic of the material. Photon energy received at detectors is commonly stated in power units such as Watts per square meter per wavelength unit. The plot of variation of power with wavelength gives rise to a specific pattern or curve that is the spectral signature for the substance or feature being sensed (discussed on page I-5).

Now, in more detail: The photon is the physical form of a quantum, the basic particle studied in quantum mechanics (which deals with the physics of the very small, that is, particles and their behavior at atomic and subatomic levels). The photon is also described as the messenger particle for EM force or as the smallest bundle of light. This subatomic massless particle comprises radiation emitted by matter when it is excited thermally, or by nuclear processes (fusion, fission), or by bombardment with other radiation. It also can become involved as reflected or absorbed radiation. Photons move at the speed of light: 299,792.46 km/sec (commonly rounded off to 300,000 km/sec or ~186,000 miles/sec). These particles also move as waves and hence, have a “dual” nature. These waves follow a pattern that can be described in terms of a sine (trigonometric) function, as shown in two dimensions in the figure below.

The distance between two adjacent peaks on a wave is its wavelength. The total number of peaks (top of the individual up-down curve) that pass by a reference lookpoint in a second is that wave’s frequency (in units of cycles per second, whose SI version [SI stands for System International] is known as a Hertz [1 Hertz = 1/s^-1]).

A photon travels as an EM wave having two components, oscillating as sine waves mutually at right angles, one consisting of the varying electric field, the other the varying magnetic field. Both have the same amplitudes (strengths) which reach their maxima-minima at the same time. Unlike other wave types which require a carrier (e.g., water waves), photon waves can transmit through a vacuum (such as in space). When photons pass from one medium to another, e.g., air to glass, their wave pathways are bent (follow new directions) and thus experience refraction.

A photon is said to be quantized, in that any given one possesses a certain quantity of energy. Some other photon can have a different energy value. Photons as quanta thus show a wide range of discrete energies. The amount of energy characterizing a photon is determined using Planck’s general equation:

where h is Planck’s constant (6.6260… x 10^-34 Joules-sec)^* and v is the Greek letter, nu, representing frequency (the letter “f” is sometimes used instead of v). Photons traveling at higher frequencies are therefore more energetic. If a material under excitation experiences a change in energy level from a higher level E₂ to a lower level E₁, we restate the above formula as:

where v has some discrete value determined by (v₂ – v₁). In other words, a particular energy change is characterized by producing emitted radiation (photons) at a specific frequency v and a corresponding wavelength at a value dependent on the magnitude of the change.λ.

I-4 Is there something wrong with the equation just above? ANSWER

Wavelength is the inverse of frequency (higher frequencies associate with shorter wavelengths; lower with longer), as given by the relationship:

where c is the constant that expresses the speed of light, so that we can also write the Planck equation as

I-5 Come up with a very simple mnemonic phrase (one that helps your memory) for associating the energy level (amount of energy) with wavelength. ANSWER

I-6: Calculate the wavelength of a quantum of radiation whose photon energy is 2.10 x 10^-19 Joules; use 3 x 10⁸ m/sec as the speed of light c. ANSWER

I-7: A radio station broadcasts at 120 MHz (megahertz or a million cycles/sec); what is the corresponding frequency in meters (hint: convert MHz to units of Hertz). ANSWER

A beam of radiation (such as from the Sun) is usually polychromatic (has photons of different energies); if only photons of one wavelength are involved the beam is monochromatic. The distribution of all photon energies over the range of observed frequencies is embodied in the term spectrum (a concept developed on the next page). A photon with some specific energy level occupies a position somewhere within this range, i.e., lies at some specific point in the spectrum

How are the photon energy levels in EM radiation quantified and measured? The answer lies in the discovery of the photoelectric effect made by Albert Einstein in 1905. Consider this diagram:

Einstein found that when light strikes a metal plate C, photoelectrons (negative charges) are ejected from its surface. In the vacuum those electrons will flow to a positively charged wire (unlike charges attract) that acts as a cathode. Their accumulation there produces an electric current which can be measured by an ammeter or voltmeter. The photoelectrons have kinetic energy whose maximum is determined by making the wire potential ever more negative (less positive) until at some value the current ceases. The magnitude of the current depends on the radiation frequencies involved and on the intensity of each frequency. From a quantum standpoint (Einstein’s discovery helped to verify Planck’s quantum hypothesis), the maximum kinetic energy is given by:

This equation indicates that the energy associated with the freed electron depends on the frequency (multiplied by the Planck constant h) of the photon that strikes the plate plus a threshold amount of energy required to release the electron (φ, the work function). By measuring the current, and if that work energy is known and other adjustments are made, the frequency of the photon can be determined. His experiments also revealed that regardless of the radiation intensity, photoelectrons are emitted only after a threshold frequency is exceeded. For frequencies below the threshold, no electron emission takes place; for those higher than the threshold value (exceeding the work function) the numbers of photoelectrons released re proportional to the number of incident photons (this number is given by the intensities involved.

When energies involved in processes from the molecular to subatomic level are involved (as in the photoelectric effect), these energies are measured in electron volt units (1 eV = 1.602 x 10^-19; this number relates to the charge on a single electron, as a fraction of the SI unit for charge quantity, the Coulomb [there are about 6.24 x 10¹⁸ electrons in one Coulomb).

Astute readers may have recognized the photoelectric effect as being involved in the operation of vacuum tubes in early radio sets. In remote sensing, the sensors used contain detectors that produce currents (and voltages, remember V = IR) whose quantities for any given frequency depend on the photoelectric effect.

From what’s been covered so far on this page, let’s modify the definition of remote sensing (previous page) to make it “quantum phenomenological”. In this approach, electromagnetic remote sensing involves the detection of photons of varying energies coming from the target (after the photons are generated by selective reflectance or by internal emittance from the target material(s)) by passing them through frequency (wavelength)- dependent dispersing devices (filters; prisms) onto metals or metallic compounds/alloys which undergo photoelectric responses to produce currents that become signals that can be analyzed in terms of energy-dependent parameters (frequency, intensity, etc.) whose variations are controlled by the atomic level composition of the targets. The spectral (wavelength) dependency of each material is diagnostic of its unique nature. When these photon variations are plotted in X-Y space, the shapes of the varied distributions of photon levels produce patterns that further aid in identifying each material (with likely regrouping into a class or physical feature.

There is much more to the above than the brief exposition and summary given. Read the next page for more elaboration. Consult a physics text for more information. Or, for those with some physics background, read the Chapter on The Nature of Electromagnetic Radiation in the Manual of Remote Sensing, 2nd Ed., published by the American Society of Photogrammetry and Remote Sensing (ASPRS). This last source contains a summary table that lists and defines what can be called basic radiometric quantities but the print is too small to reproduce on this page. The following is an alternate table which should be legible on most computer screens.

From the Manual of Remote Sensing Chapter 2, the writer has extracted the following useful information that explains some of the radiometric terminology and the concepts they represent as used by specialists in the remote sensing field: Radiant energy (Q), transferred as photons moving in a radiation stream, is said to emanate in minutely short bursts (comprising a wave train) from a source in an excited state. This stream of photons moves along lines of flow (also called rays) as a flux (φ) which is defined as the time rate at which the energy Q passes a spatial reference (in calculus terms:φ = dQ/dt). The energy involved is capable of doing work. The SI units of work are Joules per second (alternately expressed in ergs, which equal 10^-7 Joules). The flux concept is related to power, defined as the time rate of doing work or expending energy (1 J/sec = 1 Watt, the unit of power). The nature of the work can be one, or a combination, of these: changes in motion of particles acted upon by force fields; heating; physical or chemical change of state. Depending on circumstances, the energy spreading from a point source may be limited to a specific direction (a beam) or can disperse in all directions.

Radiant flux density is just the energy per unit volume (cubic meters or cubic centimeters). The flux density is proportional to the squares of the amplitudes of the component waves. Flux density as applied to radiation coming from an external source to the surface of a body is referred to as irradiance (E); if the flux comes out of that body, it’s nomenclature is exitance (M) (see below for a further description).

. The intensity of radiation is defined as the power P that flows through unit area A (I = P/A); power itself is given by P = ΔE/Δt (the rate at which radiant energy is generated or is received). The energy of an EM wave (sine wave) depends on the square of its amplitude (height of wave in the x direction; see wave illustration above); thus, doubling the amplitude increases the power by 4. Another formulation of radiant intensity is given by the radiant flux per unit of solid angle ω (in steradians – a cone angle in which the unit is a radian or 57 degrees, 17 minutes, 44 seconds); this diagram may help to visualize this:

Thus, for a surface at a distance R from a point source, the radiant intensity I is the flux Φ flowing through a cone of solid angle ω on to the circular area A at that distance, and is given by I = Φ/(A/R²). Note that the radiation is moving in some direction or pathway relative to a reference line as defined by the angle θ.

From this is derived a fundamental EM radiation entity known as radiance (commonly noted as “L”). In the ASPRS Manual of Remote Sensing, “radiance is defined as the radiant flux per unit solid angle leaving an extended source (of area A) in a given direction per unit projected surface area in that direction.” This diagram, from that Manual, visualizes the terms and angles involved:

As stated mathematically, L = Watt � m-2 � sr-1; where the Watt term is the radiant flux (power, or energy flowing through the reference surface area of the source [square meters] per unit time), and “sr” is a solid angle Ω given as 1 steradian. From this can be derived L = Φ/Ω times 1/Acos θ, where θ is the angle formed by a line normal to the surface A and the direction of radiant flow. Or, restated with intensity specified, L = I/Acosθ. Radiance is loosely related to the concept of brightness as associated with luminous bodies. What really is measured by remote sensing detectors are radiances at different wavelengths leaving extended areas (which can “shrink” to point sources under certain conditions). When a specific wavelength, or continuum of wavelengths (range) is being considered, then the radiance term becomes L_λ.

In practical use, the radiance measured by a sensor operating above the ground is given by:

where L_tot is the total spectral radiance (all wavelength) received by the sensor; ρ is the reflectance from the ground object being sensed; E is the irradiance (incoming radiant energy acting on the object); T is an atmospheric transmission function; and L_p is radiance from the atmospheric path itself.

Radiant fluxes that come out of sources (internal origin) are referred to as radiant exitance (M) or sometimes as “emittance” (now obsolete). Radiant fluxes that reach or “shine upon” any surface (external origin) are called irradiance. Thus, the Sun, a source, irradiates the Earth’s atmosphere and surface.

The above radiometric quantities Q, φ, I, E, L, and M, apply to the entire EM spectrum. Most wave trains are polychromatic, meaning that they consist of numerous sinusoidal components waves of different frequencies. The bundle of varying frequencies (either continuous within the spectral range involved or a mix of discrete but discontinuous monochromatic frequencies [wavelengths]) constitutes a complex or composite wave. Any complex wave can be broken into its components by Fourier Analysis which extracts a series of simple harmonic sinusoidal waves each with a characteristic frequency, amplitude, and phase. The radiometric parameters listed in the first sentence can be specified for any given wavelength or wavelength interval (range); this spectral radiometric quantity (which has a value different from those of any total flux of which they are a part [unless the flux is monochromatic]) is recognized by the addition to the term of a subscript λ, as in L_λ and Q_λ. This subscript denotes the specificity of the radiation as at a particular wavelength. When the wavelengths being considered are confined to the visual range of human eyes (0.4 to 0.7 µm), the term “luminous” precedes the quantities and their symbols are presented with the subscript “v”, as Φ_v for a luminous flux.

EM radiation can be incoherent or coherent. Waves whose amplitudes are irregular or randomly related are incoherent; polychromatic light fits this state. If two waves of different wavelengths can be combined so as to develop a regular, systematic relationship between their amplitudes, they are said to be coherent; monochromatic light generated in lasers meet this condition.

The above, rather abstract, sets of ideas and terminology is important to the theorist. We include this synopsis mainly to familiarize you with these radiometric quantities in the event you encounter them in other reading.

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

If you have heard the term “remote sensing” before you may have asked, “what does it mean?” It’s a rather simple, familiar activity that we all do as a matter of daily life, but that gets complicated when we increase the scale at which we observe. As you view the screen of your computer monitor, you are actively engaged in remote sensing.

A physical quantity (light) emanates from that screen, whose imaging electronics provides a source of radiation. The radiated light passes over a distance, and thus is “remote” to some extent, until it encounters and is captured by a sensor (your eyes). Each eye sends a signal to a processor (your brain) which records the data and interprets this into information. Several of the human senses gather their awareness of the external world almost entirely by perceiving a variety of signals, either emitted or reflected, actively or passively, from objects that transmit this information in waves or pulses. Thus, one hears disturbances in the atmosphere carried as sound waves, experiences sensations such as heat (either through direct contact or as radiant energy), reacts to chemical signals from food through taste and smell, is cognizant of certain material properties such as roughness through touch (not remote), and recognizes shapes, colors, and relative positions of exterior objects and classes of materials by means of seeing visible light issuing from them. In the previous sentence, all sensations that are not received through direct contact are remotely sensed.

I-1 In the illustration above, the man is using his personal visual remote sensing device to view the scene before him. Do you know how the human eye acts to form images? If not, check the answer. ANSWER

However, in practice we do not usually think of our bodily senses as engaged in remote sensing in the way most people employ that term technically. A formal and comprehensive definition of applied remote sensing * is:

Remote Sensing in the most generally accepted meaning refers to instrument-based techniques employed in the acquisition and measurement of spatially organized (most commonly, geographically distributed) data/information on some property(ies) (spectral; spatial; physical) of an array of target points (pixels) within the sensed scene that correspond to features, objects, and materials, doing this by applying one or more recording devices not in physical, intimate contact with the item(s) under surveillance (thus at a finite distance from the observed target, in which the spatial arrangement is preserved); techniques involve amassing knowledge pertinent to the sensed scene (target) by utilizing electromagnetic radiation, force fields, or acoustic energy sensed by recording cameras, radiometers and scanners, lasers, radio frequency receivers, radar systems, sonar, thermal devices, sound detectors, seismographs, magnetometers, gravimeters, scintillometers, and other instruments.

I-2 To help remember the principal ideas within this definition, make a list of key words in it. ANSWER

This is a rather lengthy and all-inclusive definition. Perhaps two more simplified definitions are in order: The first, more general, includes in the term this idea: Remote Sensing involves gathering data and information about the physical “world” by detecting and measuring signals composed of radiation, particles, and fields emanating from objects located beyond the immediate vicinity of the sensor device(s). The second is more restricted but is pertinent to most of the subject matter of this Tutorial: In its common or normal usage (by tacit implication), Remote Sensing is a technology for sampling electromagnetic radiation to acquire and interpret non-contiguous geospatial data from which to extract information about features, objects, and classes on the Earth’s land surface, oceans, and atmosphere (and, where applicable, on the exteriors of other bodies in the solar system, or, in the broadest framework, celestial bodies such as stars and galaxies).

I-3 What is the meaning of “geospatial”? Are there any differences in meaning of the terms “features”, “objects”, and “classes”? ANSWER

Or, try this variation: Applied Remote Sensing involves the detecting and measuring of electromagnetic energy (usually photons) emanating from distant objects made of various materials, so that the user can identify and categorize these objects – usually, as rendered into images – by class or type, substance, and spatial distribution. Generally, this more conventional description of remote sensing has a specific criterion by which its products point to this specific use of the term: images much like photos are a main output of the sensed surfaces of the objects of interest. However, the data often can also be shown as “maps” and “graphs“, or to a lesser extent, as digital numbers that can be input to computer-based analysis, and in this regard are like the common data displays resulting from geophysical remote sensing. As applied to meteorological remote sensing, both images (e.g., clouds) and maps (e.g., temperature variations) can result; atmospheric studies (especially of the gases in the air, and their properties) can be claimed by both traditionalists and geophysicists.

All of these statements are valid and, taken together, should give you a reasonable insight into the meaning and use of the term “Remote Sensing” but its precise meaning depends on the context in which it is spoken of.

Thus, as the above comments suggest, some technical purists arbitrarily stretch the scope or sphere of remote sensing to include other measurements of physical properties from sources “at a distance” that are more properly included in the general term “Geophysics”. (Geophysics has a scientific connotation: it is pertinent to the study of the physical properties of Earth and other planets. It likewise has an applied connotation: it is the technology often used to search for oil and gas and for mineral deposits.) This latter is especially conducted through such geophysical methods as seismic, magnetic, gravitational, acoustical, and nuclear decay radiation surveys. Magnetic and gravitational measurements respond to variations in force fields, so these can be carried out from satellites. Remote sensing, as defined in this context, would be a subset within the branch of science known as Geophysics. However, practitioners of remote sensing, in its narrower meaning, tend to exclude these other areas of Geophysics from their understanding of the meaning implicit in the term.

Still, space systems – mostly on satellites – have made enormous contributions to regional and global geophysical surveys. This is because it is very difficult and costly to conduct ground and aerial surveys over large areas and then to coordinate the individual surveys by joining them together. To obtain coherent gravity and magnetic data sets on a world scale, operating from the global perspective afforded by orbiting satellites is the only reasonable alternate way to provide total coverage.

One could argue that Geophysics deserves a Section of its own but in the remainder of this Tutorial we choose to confine our attention almost entirely to those systems that produce data by measuring in the electromagnetic radiation (EMR) spectrum (principally in the Visible, Infrared, and Radio regions). We will reserve our treatment of Geophysics to three pages near the end of this Introduction. There you are given examples of the use of satellite instruments to obtain information on particles and fields as measured inside and around the Earth; in Sections 19 and 20 (Planets and Cosmology) there will also be some illustrations of several types of geophysical measurements.

One mode of remote sensing not treated in the Tutorial is acoustic monitoring of sound waves in atmospheric and marine environments. For example, volcanic eruptions or nuclear (testing) explosions can be detected by sensitive sound detectors. Sonar is used to track submarines and surface ships in the oceans. Sound through water are also involved in listening to marine animals such as whales and porpoises.

It may seem surprising to realize that going to the doctor can involve remote sensing. Most obvious, on a miniature scale, is listening to a heartbeat using the stethoscope. But in the field of modern medical technology, powerful, often large, instruments such as CATscans and Magnetic Resonance Imaging, are now almost routinely used for non-invasive subskin investigation of human tissue and organs. This is indeed another major application of remote sensing that will be surveyed on pages I-26c through I-26e.

The traditional way to start consideration of what remote sensing is and means is to set forth its underlying principles in a chapter devoted to the Physics on which remote sensing is founded. This will be done in the next 5 pages. The ideas developed may seem arcane. These pages contain the “technical jargon” that remote sensing specialists like to banty about. With this caveat in mind, work through the pages, try to understand the esoteric, and commit to memory what seems useful.

_{* The term “remote sensing” is itself a relatively new addition to the technical lexicon. It was coined by Ms Evelyn Pruitt in the mid-1950’s when she, a geographer/oceanographer, was with the U.S. Office of Naval Research (ONR) outside Washington, D.C.. It seems to have been devised by Ms Pruitt to take into account the new views from space obtained by the early meteorological satellites which were obviously more “remote” from their targets than the airplanes that up until then provided mainly aerial photos as the medium for recording images of the Earth’s surface. No specific publication or professional meeting where the first use of the term occurred is cited in literature consulted by the writer (NMS). Those “in the know” claim that it was verbally used openly by the time of several ONR-sponsored symposia in the 1950s at the University of Michigan. The writer believes he first heard this term at a Short Course on Photogeology coordinated by Dr. Robert Reeves at the Annual Meeting of the Geological Society of America in 1958.}SOURCE:http://rst.gsfc.nasa.gov

What is an aerial photograph?

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. The following material will help you understand the fundamentals of aerial photography by explaining these basic technical concepts.

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.

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.
• 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.

The National Air Photo Library has a variety of photographic scales available, such as 1/3 000 (large scale) of selected areas, and 1/50 000 (small scale).

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.

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.