Saturday, November 3, 2012

RS-from the Space to the "over head"

Hi, it has been a wonderful experience so far to explore some of the interesting topics in the field of remote sensing.As a matter of fact today we would jump from Satellite to the Aeroplane in order to grasp the techniques of RS...!

as we are well aware of the fact that the satellite imaging can be done with various objectives bearing in mind, i would like to summarise some of the characteristics or the abilities of some satellites(with common sense of course .... lol)

Ok, what is meant by the 2nd column header, it is simply an indication of the "spectral resolution" of the particular satellite-if it can grasp more and more bands and wavelengths the spectral resolution gets higher and higher..
Then in the next header it is named as "spatial resolution",well there is no need of big words to express what it means!!,we'll take an example mmm..
     take satellite sensor TM 4-5 (it belongs to the Landsat), it has a spatial resolution of 30m in first 5 bands,this means that the sensor can identify two objects as separate entities (or distinguish among each other ) only if the distance between them is greater than 30m !
of course by "repeat coverage" it means the temporal resolution, i'll give you a funny example,
            sri lankan A/L students can only sit for the A/L exam during the month of August,so the temporal resolution of A/L examination is (approximately) 365 days (1 year,if we count from the first day to the next first day), ***but any intelligent person may realise that not like the A/Ls the satellites do function on time so they would come to the top of a place on a sharp time....

if i dont say that by "orbit altitude" it means the height of the orbit of a particular satellite,this article would be incomplete.. but let us see what is meant by "swath width" >> !

as you can see the Swath width is the maximum distance that a sensor can grasp... if you took a rod with a length of (say) 1m and held it horizontally on a clear sand surface and dragged it,it would make a rectangle, 1m wide,similar story happens with the satellite sensor also.

Right, now lets step bit forward.

Satellite imaging is not the only possible method of taking images from above,there are so many other methods also, imagine somebody taking photographs from an aeroplane, let us not be so rude to imagine him falling to the earth
yes, there is a methodology like that and it is called "Aerial Photogrammetry"..!
Not like in satellite imaging, in aerial photogrammetry the wavelength used to acquire information is in the visible wavelength region (most of the time.).
the reason why i put the term "most of the time" is that,in aerial photogrammetry we take photographs using a  metric (usually an analog) camera so the images are captured in their visible range.
here a Mosaic of overlapping photographs are used for the interpretation.

i would tell it like this way................
 this is identical to the act of "we flying over that terrain with our both eyes open" ,as you may have heard how we,humans see objects in a 3-dimensional way,you might witness that the way in which the photographs are taken is pretty much close to it.
the overlapping image part is in an orthogonal direction to the flight direction (side lap) must be around 30% of the swath width and the overlapping image part to the flight direction (forward lap) must be 60% of the order to derive planimetric and topographic information......

let us talk about some more later. adios........!

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