Aerial Photography and Remote Sensing

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This unit introduces basic concepts of remote sensing of the environment. It is intended to provide you with the background information necessary to successfully use remotely sensed imagery in conjunction with GIS technology to answer questions about the world in which we live.
 

In recent years, technological advances have changed the way geographic analyses are done. Increasingly, computers are used to automate aspects of cartography and remote sensing, producing data that are easily integrated into a GIS.

Many GIS systems have the capability of incorporating aerial photography, satellite data, and radar imagery into their data layers. The process is simple, as images may be scanned or read off a data tape. However, to use this technology effectively, it is important to know the strengths and limitations of remotely sensed data, and to understand which types of imagery are suited to particular projects. This unit was developed with these concerns in mind. The information and exercises contained within it are intended to familiarize you with the interface between remote sensing and GIS.
 

Foundations of Remote Sensing

The Electromagnetic Spectrum

The USGS defines the electromagnetic spectrum in the following manner: "Electromagnetic radiation is energy propagated through space between electric and magnetic fields. The electromagnetic spectrum is the extent of that energy ranging from cosmic rays, gamma rays, X-rays to ultraviolet, visible, and infrared radiation including microwave energy."

Electromagnetic Waves

Electromagnetic waves may be classified by FREQUENCY or WAVELENGTH, and the velocity of ALL electromagnetic waves is equal to the speed of light, which we (along with Einstein) will refer to as c.
 

Wavelength and Frequency of common EM waves

Wave Phenomena Concepts

Electromagnetic waves are radiated through space. When the energy encounters an object, even a very tiny one like a molecule of air, one of three reactions occurs. The radiation will either be reflected off the object, absorbed by the object, of transmitted through the object. The total amount of radiation that strikes an object is referred to as the incident radiation, and is equal to:

reflected radiation + absorbed radiation + transmitted radiation

In remote sensing, we are largely concerned with REFLECTED RADIATION. This is the radiation that causes our eyes to see colors, causes infrared film to record vegetation, and allows radar images of the earth to be created.

Amplitude and Wavelength

Wave Descriptions
 

The electric field and the magnetic field are important concepts that can be used to mathematically describe the physical effects of electromagnetic waves.

The electric field vibrates in a direction transverse (i.e. perpendicular) to the direction of travel of the electromagnetic wave.

The magnetic field vibrates in a direction transverse to the direction of the em wave AND transverse to the electric field.

POLARIZATION: Polarization is defined by the orientation of the electrical field E. It is usually described in terms of HORIZONTAL POLARIZATION and VERTICAL POLARIZATION. Polarization is most important when discussing RADAR applications of remote sensing.

The Particle Nature of Light

Infrared Radiation

Sources of Electromagnetic Radiation

Aerial Photography

Introduction

Aerial photography has two uses that are of interest within the context of this course: (1) Cartographers and planners take detailed measurements from aerial photos in the preparation of maps. (2) Trained interpreters utilize arial 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.

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Basic Elements of Air Photo Interpretation

Novice photo interpreters often encounter difficulties when presented with their first aerial photograph. Aerial photographs are different from "regular" photos in at least three important ways:
 

• objects are portrayed from an overhead (and unfamiliar) position.
• very often, infrared wavelengths are recorded, and
• photos are taken at scales most people are unaccustomed to seeing
These "basic elements" can aid in identifying objects on aerial photographs.
 
• Tone (also called 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.
• 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.

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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.
Types of Aerial Photography
 
Black and White

Austin, Texas

Hidalgo County, Texas

Color

Color Infrared

In 1903 or 1904 the first reliable black and white infrared film was developed in Germany. The film emulsion was adjusted slightly from regular film to be sensitive to wavelengths of energy just slightly longer than red light and just beyond the range of the human eye. By the 1930s, black and white IR films were being used for landform studies, and from 1930 to 1932 the National Geographic Society sponsored a series of IR photographs taken from hot air balloons.

Throughout the 1930s and 1940s, the military was hard at work developing color infrared film, eager to exploit it for surveillance. By the early 1940s the military was successful in its attempts. It developed a film that was able to distinguish camouflaged equipment from surrounding vegetation. Within months, however, an IR reflecting paint was developed for use on military vehicles, effectively making IR film technology useless to the military. So, they dropped it.

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.

more to come
 

Basic Photogrammetry

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Applications of Aerial Photography
 
Introduction: The Scope of Air Photography

Land-Use Planning and Mapping

Geologic Mapping

Archaeology

Species Habitat Mapping

Integration of Aerial Photography into GIS

Digital Image Processing

Humans are adept at visually interpreting data. We can distinguish millions of colors, several shades of gray, and have a demonstrated ability to identify water, vegetation, and urban forms on several types of imagery. Why try to expand on this?
 

• (1) There are limits to a person's ability to distinguish small differences in color. We are especially limited in our resolution of shades of gray. If data are collected using 256 shades of gray, but an analyst can only distinguish 8-10 (optimistically) of them, a great deal of information is potentially lost. The human interpreter is outpaced by the precision of the data. Computers, however, have no trouble distinguishing 256 shades of gray. Each one is individually recognizable. And, the analyst has control over the conputer's presentation of the data. She can group it any way she pleases, extract a portion of it, or display it in false color. Data sets can also be combined, compared, and contrasted with more ease and precision (not to mention speed) than if the task were left to humans alone.
• (2) Human interpretations are highly subjective, hence, not perfectly repeatable. Conversely, results generated by computer--even when erroneous--are usually repeatable.
• (3) When very large amounts of data are involved (a series of photos of an orange grove taken at 5 day intervals over an entire growing season) the computer may be better suited to managing the large body of detailed (and tedious) data.

Sources of Digital Data

Image Enhancement

Data Classification

Satellite Imaging

Landsat
 

LANDSAT refers to a series of satellites put into orbit around the earth to collect environmental data about the earth's surface. The LANDSAT program was initiated by the U.S. Department of Interior and NASA under the name ERTS, an acronym which stands for Earth Resources Technology Satellites. ERTS-1 was launched on July 23, 1972, and was the first unmanned satellite designed solely to acquire earth resources data on a systematic, repetitive, multispectral basis. Just before the launch of the second ERTS satellite, NASA announced it was changing the program designation to LANDSAT, and that the data acquired through the LANDSAT program would be complemented by the planned SEASAT oceanographic observation satellite program. ERTS-1 was retroactively named LANDSAT-1, and all subsequent satellites in the program have carried the LANDSAT designation. Over time, the sensors carried by the LANDSAT satellites have varied as technologies improved and certain types of data proved more useful than others. The table which follows outlines the sensors onboard each satellite, their launch dates, and the dates they were decommissioned.

Table 1
 

The various Landsats have had Multispectral Scanners (MSS), Return Beam Vidicon (RBV) scanners, and Thematic Mapper (TM) scanners. Each type has its own spectral range and spatial resolution.

Interpreting Landsat Data

The images discussed in this section are the property of the University of California, Santa Barbara. Click here to get to the Center for Ecological Health Research Home Page, then click on the image indicated below, then back up to this page with the image still visible to read the discussion that pertains to the image. Detailed explanations of the images will be added soon.
 

• Click on the first image, labeled "California". This is a false color image that has been processed by computer.
• Now, close the image of California and return to the CEHR Home Page. Click on the third image, labeled "San Francisco Bay Delta, Northern California". more to come

NOAA AVHRR

NOAA Geostationary and Polar Orbiting Satellites

NOAA GOES mission overview and history. The GOES graphic was prepared by the NASA Goddard Space Flight Center, which provides additional information about the GOES project.

The first visible GOES-8 image. Look carefully and you can make out Baja California on the lower left and Lake Michigan on the upper right.

Applications of Satellite Imagery

Integration of Satellite Imagery into GIS

Bauer, M.E., T.E. Burk, A.R. Ek, P.R. Coppin, S.D. Lime, T.A. Walsh, D.K. Walters, W. Befort, and D.F. Heinzen. Satellite Inventory of Minnesota Forest Resources. Photogrammetric Engineering and Remote Sensing, in press.

MSS, Thermal, and Hyperspectral Scanning

Thermal infrared radiation refers to electromagnetic waves with a wavelength of between 3.5 and 20 micrometers. Most remote sensing applications make use of the 8 to 13 micrometer range. The main difference between THERMAL infrared and the infrared discussed above is that thermal infrared is emitted energy, whereas the near infrared (photographic infrared) is reflected energy.

Multispectral Scanning

Interpreting Thermal Scanning Imagery

Limitations of Thermal Infrared Imaging

There are some limitations of thermal imagery you should be aware of if you plan to use it in your GIS:

• It is very expensive.
• Most thermal imaging systems have very strict operational parameters. For example, the detector must be kept extremely cold during use.
• Thermal infrared imaging systems are notoriously difficult to calibrate.
• The data collected has extensive processing requirements. A PC isn't going to cut it.
• Thermal images can be quite difficult to interpret when compared with other types of imagery.
• Thermal imagery is NOT geometrically correct.
• Thermal images of water measure only the very top layer of the water. They tell you nothing of the water's characteristics below the top few micrometers.

Imaging Spectrometry

Radar (Microwave) Scanning

SLAR

LIDAR

ERS Program

Radar Images

The following radar images come from sites all over the world. The files at NASA's Jet Propulsion Laboratory have explanations accompanying the images.

Spaceborne Synthetic Aperture Radar, Oetxal, Austria. This file was created by NASA's Jet Propulsion Laboratory in Pasadena, CA.

Remote Sensing and GIS

• Remote sensing provides a regional view.
• Remote sensing provides repetitive looks at the same area.
• Remote sensors "see" over a broader portion of the spectrum than the human eye.
• Sensors can focus in on a very specific bandwidth in an image.
• They can also look at a number of bandwidths simultaneously.
• Remote sensors often record signals electronically and provide geo-referenced, digital, data.
• Some remote sensors operate in all seasons, at night, and in bad weather.

Last updated 2000.2.6. LNC.