Image processing 2 Flashcards
What is atmospheric correction?
Removing the influence of the atmosphere on the imagery
‘Cleaning the data’ to be able to accurately measure land data
What is the atmosphere composed of?
Aerosols and molecules which vary spatially and in their concentration
What are 2 things both aerosols and molecules do?
Scatter light (best at shorter wavelengths)
Absorblight
Why is atmospheric correction necessary?
Quantitative analysis- relate reflectance to surface property
Multispectral data for visual analysis
Scattering increases inversely with wavelength
Multi-temporal analysis- atmosphere changes with time
What are the 3 methods of atmospheric correction?
Dark object subtraction method
Empirical line method
Radiative transfer model
How does the dark object subtraction method work?
It assumes that any pixels with values low and close to 0 should be 0
E.g. corrects them to dark shadows
How does the empirical line method work?
Two targets- one bright and one dark- where the satellites collect the data
Handheld sensor reading compared to aircraft- aircraft data corrected for any atmospheric differences from the different surface reflectance
How do the radiative transfer models work?
Uses lots of information to parameterise the model:
Geometric conditions
Atmospheric model
Aerosol model
Spectral condition
Ground reflectance
What is geometric correction?
Transforming the x-y dimensions of a remotely sensed image so it has the scale and project properties of a selected map projection
What does geometric correction allow you to do?
To merge two or more remote sensing image, including two images of the same area collected at different times
To merge the remote sensing image with other geographic data
What is the geometric correction procedure?
Find a point on a map and also find it in the image
At least 20 points dotted around the image
Build the relationship between the map co-ordinates and image co-ordinates
Connects image to the land map
Produces a new output grid with rows and columns aligned with eastings and northings of a map projection system
What are the 3 types of image resampling?
Nearest neighbour
Bilinear interpolation
Cubic convolution
How does nearest neighbour resampling work?
Uses the value of the CLOSEST input value for the output pixel
It doesn’t change the pixel value, it simply moves
How does bilinear interpolation resampling work?
Uses the average of the nearest 4 pixels
Does change the pixel
Results in a more accurate and smoother image
