This is another one of our discussion questions from GEOS 451, Data Capture II.
- During aerial photograph acquisition the aircraft is in motion, what issues can this raise with regards to introducing distortions into the imagery?
- What are some considerations that must be made regarding the camera used during aerial photograph acquisition?
- What are the basic characteristics used for aerial photograph interpretation?
One method of remote sensing is the acquisition of aerial photographs from aircraft. This can allow for quicker and more up to date imagery than satellites over an area, for a lower cost (Sean Kotz, 2018).
Taking pictures from aircraft can add distortions into an image (SAIT, n.d.). All photos will display some distortions such as relief and radial distortions, but the camera can also add some level of distortion as well (SAIT, n.d.). Some of the obvious types of distortions caused by cameras are focus and issues with the optics (Maribeth, Price, 2018).
Beyond these basic distortions there are a few critical aspects to take into account when photographing from an airplane. The two of most concern are vibrations and motion blur (SAIT, n.d.). Cameras will have modules and features incorporated to account for these things, but they are ever present with an aircraft (James B Campbell & Randolph H Wynne, 2012). We should also note that the lower and faster an aircraft is travelling will have the effect of adding in more motion blur, if you consider a satellite at 700km, it will have less motion blur than an aircraft at 2,200m (James B Campbell & Randolph H Wynne, 2012).
Distortions in the image are important things to consider, but if you haven’t made some correct decisions around your camera they may not matter, as your image may not be useful (SAIT, n.d.). Some of the aspects of the camera you need to consider are the shutter speed, aperture and the type of film/sensor and how sensitive it is (SAIT, n.d.). Having any of these incorrect for the time of day, lighting or mission could result in an image that is over exposed, under exposed or just contains the wrong information. Beyond these we will want to make sure the camera is suitable for the task at hand and has anti-vibration and motion compensators (SAIT, n.d.).
Once we have accounted for our distortions and made good decisions about camera settings and have acquired the aerial photograph, we need to interpret it. When we do this, we should gather all our materials together that will be useful, such as location of the photo, other images of the area, perhaps even oblique aerial images. Looking at a vertical aerial image such as we would use for interpretation can present objects to us in a way we are not used to seeing them, making it difficult to interpret them (SAIT, n.d.).
When we interpret a photo it is Important to start with the macro and work towards the micro, that is determine what areas are forests before deciding what is a hardwood and what is a softwood (SAIT, n.d.).
In order to interpret these elements we consider a few basic properties of objects: shape, size, pattern, texture, tone, shadow, site, association and resolution (SAIT, n.d.). Some of these are pretty self-explanatory, if it shaped like a boat, then it is probably a boat, the type of boat might be determined by the size of it compared to other interpreted objects, such as a city block (SAIT, n.d.). Others, like pattern and texture seem to be two names for the same thing, but they are referring to two distinct ideas. Texture is talking about the actual surface of an object, is it rough, smooth, rippled etc, while pattern is talking about how various objects might be related to each other spatially, such as airport runways (SAIT, n.d.). Tone is similar to the color of an object, but we must be careful to understand that not all images have color, so really what we are looking at is the relative brightness of the objects and not their hue (SAIT, n.d.). Thinking of this as color could also cause issues when dealing with false color images. Observing shadows can give us an idea of shape and profile of the object, however they can also obscure objects and make them hard to see, shadows can be a boon, but also an inconvenience (SAIT, n.d.). We can also consider where an object is in the world by thinking of it’s site, are we expecting to see a sandy desert in the arctic circle (SAIT, n.d.). We can also consider Tobler’s law that things closer are more similar than things distant (Maribeth, Price, 2018). We can sometimes determine objects based on what other objects are around them. Finally, our last property is resolution, if our image has a resolution of 25m, we can rest assured that any object we are identifying and interpreting is larger than this. (SAIT, n.d.)
Acquiring and interpreting an aerial photograph isn’t as simple as pointing as pointing a polaroid camera out of a moving plane. Getting a good image requires thought, planning and the correct equipment as well as an understanding of what might cause problems in the image (James B Campbell & Randolph H Wynne, 2012). Once you have the image the objects in it might seem unfamiliar from the angle they are being viewed, interpreting requires experience and the use of nine characteristics to help determine what each object in the image is (SAIT, n.d.).
Works Cited
James B Campbell & Randolph H Wynne. (2012). Introduction to Remote Sensing (5th ed.). Guilford Publications.
Maribeth, Price. (2018). Mastering ArcGIS, 8th Edition. McGraw-Hill Education.
SAIT. (n.d.). Module 2—Photographic Remote Sensing. GIS Data Capture II (GEOS-451-O4A).
Sean Kotz. (2018, March 13). What Is the Difference Between Satellite Imagery & Aerial Photography? Sciencing. https://sciencing.com/difference-satellite-imagery-aerial-photography-8621214.html
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