Week 6

Remote sensing: aerial photography, oblique, ortho, measurements.


Remote Sensing: collection of data about the object without direct contact, using reflected or emitted electromagnetic energy.


Examples of Remote Sensing Data at the Earth Resources Observation and Science (EROS) Center: https://eros.usgs.gov/remote-sensing


Passive and Active Remote Sensing: https://www.slideshare.net/VivekSrivastava22/passive-and-active-sensors


Aerial Photography: collection of images taken by camera attached to the aircraft (airplane, drone, balloon, etc.)


Examples of Aerial Photography on Terra Server: https://www.terraserver.com/






History of photography: https://en.wikipedia.org/wiki/Photography#History


One of the first aerial photography: 18 century, Felix Nadar: https://en.wikipedia.org/wiki/Nadar_(photographer)


Cuban Crisis (1962) Aerial Photos: http://nsarchive.gwu.edu/nsa/cuba_mis_cri/photos.htm


Historical Aerial Photo Viewer by NETRonline: https://www.historicaerials.com/


Use in modern archaeology: https://www.nytimes.com/2015/11/09/arts/international/ted-grant-goes-to-archaeologist-who-combats-looting-with-satellite-technology.html?partner=rss&emc=rss&_r=2







National Wide Data: http://earthexplorer.usgs.gov/


New York State: http://gis.ny.gov/gateway/mg/





  1. Vertical

Advantage: measurements and directions (azimuth) are more accurate and easy; scale is almost uniform on the photo; tall objects do not obscure short objects; easy to use for stereopair processing.




Lehman College, 2017


  1. Oblique (high and low)

Advantage: cover larger area; better show texture (i.e. high/low objects) on the ground because of shadows and perspective; this also helps to assess relative height of the objects; does not require aircraft to fly directly over the object.



City of Halifax, ca 1965





Nadir: point below aircraft, direction orthogonal to the earth surface


Principal Point: geometric center determined by fiducial marks on the image, location of least distortion.


Isocenter: point between nadir and principal point.


Relief displacement: visual distortion when tall objects tend to be displaced from the center of the image


Fiducial marks: reference points on the aerial photo for use as a measure or fixed coordinate pairs.













ORTHOPHOTO: corrected image with uniform scale. The process of correcting is called orthorectification; it removes distortion effects caused by terrain relief and camera tilting.






Application of image interpretation in relation to population density estimates:




Introduction to Air Photo Interpretation (CANADA):




Aerial Photo Interpretation (University of Iowa):






Height of objects can be calculated using Pythagoras Law:



tanα is a constant for the image, i.e. by knowing at least one height of any object on the image we can identify heights (H) of other objects as: H = tanα • L


To make linear measurements (e.g. L) on the image we need to know image scale that shows relationship between units of measurement on the image and on the terrain. In cartographic tradition scale is usually represented as a fraction, e.g. 1:2,000 or 1:300,000. This also can be written as 2K or 300K scales. Therefore in scale calculations we always need to keep nominator as 1.


Image scale is calculated as: Focal Length (f)  /  Aircraft altitude (H)


Equation specifics:


  1. The units of nominator and denominator should be the same. Because focal lens is usually expressed in mm we need to convert aircraft height into mm as well. This is easy since 1 m = 100 cm and 1 cm = 10 mm. Therefore 1 m = 1000 mm.

  2. The aircraft altitude is the height above ground, not the absolute altitude above mean sea level

Example: if you use lenses with focal length 135 mm and the height of the flight is 700 m  (conversion to mm makes it 700,000) then the image scale will be = 135 / 700000 = 1/ 5185 or 1:5185.


For aerial photo with well-defined Principal Point the object height can be calculated using this equation (of course units should be the same):


H = m / r * h,


m – relief displacement

r – distance from the Principal Point to the top of the object

h – Aircraft height



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