What Are 3 Important Facts About Aerial Photography
Aerial Photography
Introduction
Aeriform photography is one the earliest forms of remote sensing and is still one of the nigh widely used and cost effective methods of remote sensing. Before the evolution of multispectral sensors and computers, people were using traditional photography to capture aerial images. Since its inception, aerial photography has gone from balloons and kites, to airplane, satellites and at present unmanned aircraft systems (UAS). While the quality, resolution and platforms have evolved, aerial photography is still a cornerstone of remote sensing and is becoming cheaper and more accessible than ever. Aerial photography is useful both for regional analysis and for evaluating specific sites. It tin also provide a historical perspective that allows us to view changes in landscapes overtime.
As we learned before in this class, the first aeriform photographs were taken from balloons, kites and fifty-fifty pigeons. Aerial photography quickly expanded with advances in aeronautics. The military potential of aeriform photography was obvious and aerial photography was widely used in WWI and WWII. The first non-war machine aerial photography programs were developed in the 1930'south as part of the Agricultural Adjustment Act. In the The states the U.South. Department of Agronomics (USDA) has been involved in the conquering, utilise and distribution of aerial photography for more than 65 years. Aerial photography has numerous applications and is used by catographers, engineers and scientists to clarify everything from urban expansion to the impacts of climate change.
History in the U.South.
In the United states the U.S. Section of Agriculture (USDA) has been involved in the acquisition, utilize and distribution of aeriform photography for more than 65 years. The earliest aerial photography in 1937 by USGS and Farm Security Agency. Later the National Aeriform Photography Program (NAPP) and the National Digital Orthophoto Programme (NDOP) were created to accept a more imagery of subcontract lands on a national scale and provide consistent coverage.
National High Altitude Program
The National High Altitude Program (NHAP) was an interagency federal try coordinated by the USGS which operated from 1980 to 1989. The goal of the programme was to provide deject free aerial images of the entire lower 48 states. The images were acquired at 40,000 ft. elevation. The NHAP drove includes black-and-white aerial photographs at a scale of ane:80,000 and color infrared aerial photographs at a calibration of 1:58,000 Larn More than About NHAP.
National Aerial Photography Program
The National Aeriform Photography Plan (NAPP) program began in 1987 as a replacement for NHAP, with the objective of acquiring complete uniform photo coverage of the conterminous 48 States over a 5 to seven twelvemonth period. The NAPP photos include blackness-and-white and color infrared images and all images have a scale of one:forty,000 (one inch equals virtually 0.6 miles). Learn More About NAAP.
National Agriculture Imagery Programme
Starting in 2003 the National Agriculture Imagery Program (NAIP) and acquiring aerial imagery during the agricultural growing seasons in the continental U.Due south. A primary goal of the NAIP programme is to make digital ortho photography available to governmental agencies and the public within a year of acquisition. NAIP imagery has a resolution of ane-meter ground sample distance (GSD). Learn More Nearly NAIP.
Types of Aerial Photography
Bending of Photograph
Aeriform photographs may be taken in vertical, low-oblique or high-oblique positions. Most of the air photos nosotros use in remote sensing are vertical photographs.
Verticle
Vertical photographs are taken looking straight down. Vertical (or nearly) angle to the footing surface, i.e. camera pointed direct down. Vertical photograph are often used in cartography and photogrammetry.
Oblique
Whatever aerial photographs taken at an angle are known as oblique photographs. In that location are two types of oblique photographs:
- High-oblique -Shows the surface, the horizon, and a portion of heaven.
- Low-oblique - Shows merely the surface, the horizon is non show.
Film or Digital
Early aerial photography was film based and remained the ascendant medium throughout the 20th century until the development of digital cameras. There are several different types of motion picture normally used in aerial photography.
Film Types
Black and White Film
- Black-and-white panchromatic (B/West) flick primarily consists of a black-and-white negative material with a sensitivity range comparable to that of the man eye. Information technology has skilful contrast and resolution with low graininess and a wide exposure range.
- Black-and-white infrared (BIR) motion-picture show, with some exceptions, is sensitive to the spectral region encompassing 0.4 micrometers to 0.nine micrometers. It is sometimes referred to every bit well-nigh-infrared film because it utilizes merely a narrow portion of the total infrared spectrum (0.vii micrometers to 0.9 micrometers).
- Natural color (also referred to as conventional or normal color) film contains three emulsion layers which are sensitive to blueish, greenish, and scarlet (the three primary colors of the visible spectrum). This flick replicates colors as seen by the human middle.
- CIR (color infrared) film, originally referred to every bit camouflage-detection film, differs from conventional color film because its emulsion layers are sensitive to green, cherry, and near-infrared radiation (0.5 micrometers to 0.9 micrometers). Used with a yellow filter to absorb the blue calorie-free, this moving-picture show provides sharp images and penetrates haze at high altitudes. Color infrared film also is referred to as false-colour pic.
Digital
Digital photography uses CCD (charge-coupled device) or CMOS (complementary metallic-oxide semiconductor) sensor to capture the image, as opposehd to an exposure on photographic film. Both sensors capture low-cal and convert it into electronic signals. The captured epitome is then digitized and stored as a calculator file ready for digital processing. Digital has replaced traditional moving picture photography in many applications, for example the NAIP imagery collected past the USDA is now acquired entirely with digital sensors
Information on Aerial Photographs
Often in that location is a meaning amount of information provided on the aerial photograph itself. This can include:
- Date
- Mission name and details
- Curl & frame number
- Fiducial marks
- Altitude
Click on the aerial photo to the right to enlarge the image
Determining Scale
The calibration of an aeriform photograph depends on the specific camera characteristics (focal length) and the flying height at which the prototype was captured. There are several methods for calculating the scale of an aerial photo. Which method you utilize depends on what information is already known.
Focal Length and Field of View
The scale of a photograph is determined past the focal length of the camera and the flying height to a higher place the ground. The focal length is the distance from the eye of the camera lens to the focal aeroplane. Focal length is precisely measured when cameras are calibrated and is typically expressed inn millimeters (mm). The focal length of a lens determines the magnification and the angle of the light ray. The longer the focal length, the greater the magnification of the image. Short focal length lenses cover larger areas. The area captured past a photographic camera is known equally the Field of View (FOV), which is typically expressed in degrees. Field of View is a function of the focal length of the lens and the size (sometimes chosen format) of digital sensors.
The scale of a photo is equal to the ratio betwixt the photographic camera's focal length and the plane's altitude above the basis level (AGL) being photographed. If the focal length and flying altitude to a higher place the surface is known, the scale can be calculated using the following formula:
Flying Height Above Basis Level (AGL) vs Above Mean Ocean Level (MSL)
In all of the scale calculations, information technology is important to know the flight height above the surface or above ground level (AGL). Onetime the altitude higher up sea level or MSL is given and yous may need to estimate the boilerplate flying height above footing. For example, the GSP on a unmanned aeriform vehicle (UAV) may record the altitude or height above sea level and not higher up basis level (AGL). To guess the AGL, you will demand to decide the average pinnacle of the terrain and subtract that from altitude to a higher place sea level. This volition requite you the boilerplate flying height above footing.
Example: A camera with a 152 mm focal length takes an aeriform photograph from a flying summit of 2280m above ground level. What is the scale of the photo? 
Feature of Known Size
The scale of an aerial photo can also be adamant if an object of a known footing size appears in the paradigm. I method is to find a feature of a known size (e.thou. football field or standard event field) in the photograph to summate the scale. The scale tin be adamant by measuring the distance or length of the feature on the photograph and comparing it to the real-life or ground altitude.
Example: Y'all measure the straight length of a runway to be 2.v mm and you lot know that the real ground distance is 100 meters, what is the calibration of the photo? 
Ground Sampling Altitude in Digital Photographs
Ground Sampling Distance (GSD) refers to how big each pixel is on the ground. This linear measurement represents the ground width of a pixel and is typically expressed in meters. This is effectively the scale of a digital image. The size of the sensor, lens focal length along with flying height above the surface determines the GSD of an epitome.
In society to determine the GSD, yous demand to know the flying peak to a higher place the basis (non above sea level) and the digital camera sensor size and lens focal length. This information is typically available from the camera manufacturer.
Focal Length for Digital Cameras
The focal length for digital cameras is similar to that of analog picture show photographic camera. Focal length volition typically be expressed in millimeters (mm). At that place may exist a 35 mm equivalent focal length and real focal length listed for digital cameras. When determining GSD or other calculations you will desire to utilize the existent focal length.
Digital Camera Sensor Dimensions and Pixel Counts
Digital cameras use CCD (accuse-coupled device) or CMOS (complementary metallic-oxide semiconductor) to capture low-cal and convert it into electronic data. The photographic camera sensor is a rectangular grid containing millions of tiny square pixels. Each of these pixels detects and records the corporeality of calorie-free received. Sensors are divers by their concrete size (surface area for capturing light data), the dimension of the pixels and the number of pixels in the sensor. The physical size of the sensor may exist expressed in inches or centimeters. The dimension or width of each private pixel are usually expressed in micrometers, as each pixel on the sensor is extremely small. The sensors pixels sizes vary betwixt cameras, but are typically betwixt 1-ii micrometers. The number of pixels a sensor has is sometimes referred to has camera resolution. For case, a camera may have a sensor with dimensions of 3000 ten 2000 pixels. That means that the rectangular grid of the sensor is made up of 3000 pixels wide and 2000 pixels tall, for a full 6 one thousand thousand pixels or 6 Megapixels.
Computing Altitude and Area
Altitude and Length
If the scale of an aerial photograph is known distances lengths and areas of features can easily exist calculated. You simply mensurate the altitude on the photograph (photo altitude) and multiply the distance past the calibration cistron. Call back that scale is always equal to the ratio of the photo distance to the footing altitude.
Example: The calibration of an aerial photograph is 1:15,000. In the photo you mensurate the length of a bridge to exist 0.25 inches, what is the length of the bridge in feet in real life? 
Expanse
It is important to recollect that surface area is measured in square units. To determine rectangular area information technology is length multiplied past width, so if you measure both and convert these distances remember that if you are multiplying them together the resulting units are squared. For example, if an area is 100 meters by 500 meters, it is l,000 foursquare meters. Now if you lot wanted to change that number to square anxiety you lot wouldn't multiply by 3.28 (there are 3.28 feet per meter), you lot would multiply by 10.76 (iii.28 x 3.28).
Instance: An aerial photograph has a scale of 1:ten,000. On the photo, the length of a field is measured as 10 mm and the width 7mm. How big (in Hectares) is the field in existent-life? Note that 10,000 square meters = 1 Hectare. 
Calculating Object Heights
Equally with calculating calibration, there are multiple methods to determine the top of alpine objects (e.thou. trees or buildings) in aerial photos. In unmarried aerial photos the two chief methods are the relief/radial displacement method and the shadow methods.
Relief/Radial Displacement Method
The magnitude of the displacement in the image between the top and the lesser of an object is known as its relief displacement and is related to the height of the object and the distance of the object from the principal point. This method tin only exist used if the object that is being measured is be far enough from the principal betoken to measure the displacement and the top and bottom of the object are visible in the photograph.
Example: The length of a a displaced edifice is measured at 2.01 mm and the radial distance to the principal point is 56.43 mm. If the the flying height almost the surface is 1220 1000, what is the acme of the building? 
Shadow Method
If you lot can measure the length of a shadow and know the angle of the dominicus, the height of the object can be calculated using simple trigonometry.
If you know when and where the aerial photo was taken y'all can determine the angle of the sun using the NOAA Solar Figurer. When using this estimator you want to utilize the solar elevation angle (El) for your calculations. This is the angle of the lord's day at the time and location specified.
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