Lecture 8 - Introduction to image processing 2

Question 1

Atmospheric correction

Answer 1

Process of removing the influence of the atmosphere on the imagery

Question 2

Methods of atmospheric correction

Answer 2

1. Dark Object Subtraction method
2. Empirical line method
3. Radiative Transfer models

Question 3

What does Dark Object subtraction method involve?

Answer 3

Identify pixel values of low reflectance areas
e.g Dark shadows and clear water
Lowest pixel value in visible and NIR are approximation to atmosphere path radiance.
Assumes these surfaces should have zero reflectance
So near zero value due to scattering
Near zero e.g 4 is due to atmospheric scattering so subtract this from imag e get corrected image.
Limitation: doesn’t account for the spatial variation of the atmosphere.

Question 4

What does empirical line method involve?

Answer 4

Selection of one dark and one bright target.
Ground reflectance measured using field radiometer
- Sensor radiance computed from image is compared with ground reflectance

Question 5

What do radiative transfer models involve?

Answer 5

• Geometrical conditions e.g View/Solar angles
• Atmospheric model for gaseous components
• Aerosol model
• Spectral condition
• Ground reflectance

Question 6

What does geometric correction involve?

Answer 6

Transforming the x-y dimensions of a remotely sensed image so that it has the same scale and project properties of a selected map projection

Question 7

Ground Control Point

Answer 7

Matching points on an image with corresponding map co-ordinates

Question 8

Image Re-sampling

Answer 8

Re-sampling is used to assign a DN value to the output matrix pixel – determined on the basis of the pixel values which surround its transformed position in the original input image matrix

Question 9

Types of image re-sampling

Answer 9

1. Nearest neighbour
2. Bilinear interpolation
3. Cubic convolution

Question 10

Nearest Neighbour

Answer 10

Uses the value of the closes input pixel for the output pixel value.
Retain original value

Question 11

Bilinear Interpolation

Answer 11

Uses the weighted average of the nearest four pixels to the output pixel.
More spatially accurate

Question 12

Cubic Convolution

Answer 12

Uses the weighted average of the nearest sixteen pixels to the output pixel.
Can both sharpen and smooth out noise