What is an image?What is an image?

What is an image and which What is an image and which image bands are “best” for image bands are “best” for

visual interpretation?visual interpretation?

http://cbc.rs-gis.amnh.org/

An image is made up of individual elements called An image is made up of individual elements called pixels that are arranged in a grid of rows and pixels that are arranged in a grid of rows and columns.columns.

Each pixel represents an area on the ground defined by its resolution. The resolution determines the detail that can be seen in an image. For example, a 1 meter pixel will provide more detail than a 30 meter pixel

The sensor acquires several images (bands) at The sensor acquires several images (bands) at once, each recording a specific color or range of once, each recording a specific color or range of colors. When viewed, each individual band looks colors. When viewed, each individual band looks like a black and white photographlike a black and white photograph

                                                            

Landsat band 2 - (wavelength range = 0.52-0.60 µm = blue light)

Electromagnetic Spectrum

Source: http://observe.ivv.nasa.gov/nasa/education/reference/reflect/ir.html

For each band the amount of energy (intensity) for a specific range of wavelengths (colors) is measured

Spectral signaturesSpectral signatures

Spectral signatures with TM bandsSpectral signatures with TM bands

RGB Band CompositeRGB Band Composite

At the pixel level digital numbers from three bands At the pixel level digital numbers from three bands are converted to grayscale colors, passed through are converted to grayscale colors, passed through a color filter, and then composited to make a color a color filter, and then composited to make a color pixelpixel

Certain bands or band combinations are better Certain bands or band combinations are better than others for identifying specific land cover than others for identifying specific land cover features.features.

Landsat TM Red=band 3 Green=band 2 Blue = band 1

Landsat TM Red=band 4 Green=band 5 Blue = band 3

Landsat ETM+ band 1 (0.45-0.52 µm, blue-Landsat ETM+ band 1 (0.45-0.52 µm, blue-green)green)

Penetrates water better Penetrates water better than the other bands so than the other bands so it is often the band of it is often the band of choice for aquatic choice for aquatic ecosystemsecosystems

Used to monitor Used to monitor sediment in water, sediment in water, mapping coral reefs, mapping coral reefs, and water depthand water depth

The “noisiest”  of the The “noisiest”  of the Landsat bands since Landsat bands since short wavelength blue short wavelength blue light is scattered more light is scattered more than the other bandsthan the other bands

Rarely used for "pretty Rarely used for "pretty picture" type images picture" type images

Landsat ETM+ band 2 (0.52-0.60 µm, green)Landsat ETM+ band 2 (0.52-0.60 µm, green)

Similar qualities to Similar qualities to band 1 but not as band 1 but not as noisynoisy

Matches the Matches the wavelength for the wavelength for the color greencolor green

Landsat ETM+ Landsat ETM+ bandband 3 (0.63-0.69 µm, 3 (0.63-0.69 µm, red)red)

Since vegetation Since vegetation absorbs nearly all red absorbs nearly all red light (it is sometimes light (it is sometimes called the chlorophyll called the chlorophyll absorption band) this absorption band) this band can be useful for band can be useful for distinguishing distinguishing between vegetation between vegetation and soil and in and soil and in monitoring vegetation monitoring vegetation healthhealth

Landsat ETM+ band 4 (0.76-0.90 µm, near Landsat ETM+ band 4 (0.76-0.90 µm, near infrared)infrared)

Since water absorbs Since water absorbs nearly all light at this nearly all light at this wavelength water bodies wavelength water bodies appear very dark. This appear very dark. This contrasts with bright contrasts with bright reflectance for soil and reflectance for soil and vegetation so it is a good vegetation so it is a good band for defining the band for defining the water/land interfacewater/land interface

Sensitive to vegetation Sensitive to vegetation covercover

Less affected by Less affected by atmospheric atmospheric contaminationcontamination

Landsat ETM+ band 5 (1.55-1.75 µm, mid-Landsat ETM+ band 5 (1.55-1.75 µm, mid-infrared)infrared)

Very sensitive to Very sensitive to moisture and is moisture and is therefore used to therefore used to monitor vegetation monitor vegetation water stress and soil water stress and soil moisturemoisture

Useful to Useful to differentiate differentiate between clouds and between clouds and snowsnow

Landsat ETM+ band 6 (10.40-12.50 µm, thermal Landsat ETM+ band 6 (10.40-12.50 µm, thermal infrared)infrared)

Measures surface Measures surface temperaturetemperature

Geological Geological applications applications

Differentiate clouds Differentiate clouds from bright soils since from bright soils since clouds tend to be very clouds tend to be very coldcold

The resolution is twice The resolution is twice as course as the other as course as the other bands (60 m instead bands (60 m instead of 30 m) of 30 m)

Landsat ETM+ band 7 (2.08-2.35 µm mid-Landsat ETM+ band 7 (2.08-2.35 µm mid-infrared)infrared)

Can detect high Can detect high surface temperaturessurface temperatures

Also used for Also used for vegetation moisture vegetation moisture although generally although generally band 5 is preferred for band 5 is preferred for that applicationthat application

Commonly used in Commonly used in geologygeology

Good at detecting Good at detecting burn scarsburn scars

Landsat ETM+ bands Landsat ETM+ bands 3,2,1 Penetrates shallow 3,2,1 Penetrates shallow water and shows water and shows submerged shelf, water submerged shelf, water turbidityturbidity

Landsat ETM+ bands Landsat ETM+ bands 4,3,2 Peak chlorophyll, 4,3,2 Peak chlorophyll, land/water boundary, land/water boundary, urban areasurban areas

Landsat ETM+ bands 4,5,3 Landsat ETM+ bands 4,5,3 Land/water boundary, Land/water boundary, Vegetation type and Vegetation type and condition, soil moisturecondition, soil moisture

Landsat ETM+ bands 7,4,2 Landsat ETM+ bands 7,4,2 Moisture content in Moisture content in vegetation and soils, vegetation and soils, geological mapping, geological mapping, vegetation mappingvegetation mapping

“Raw” values After enhancement

Image colors need to be enhanced to highlight specific features or simply to look nice.

Image enhancementImage enhancement Common enhancement tools include:

Brightness/contrast, Linear histogram stretching, Histogram equalization, Customized histogram stretching,

and Many more options

Experience with different tools provides a sense for what works best for different situations.

Software tools with a graphical display of the input (before) and output (after) histogram are helpful to understand how the enhancement process works.

Enhancements can focus on a specific feature although this often produces an image that is less visually pleasing.

Image fusionImage fusion

Image sharpening combines lower Image sharpening combines lower resolution multispectral images with a resolution multispectral images with a higher spatial resolution panchromatic higher spatial resolution panchromatic (single, broad band) image to create a finer (single, broad band) image to create a finer resolution multispectral image.resolution multispectral image.

There are several algorithms for doing this There are several algorithms for doing this and it is best to experiment with different and it is best to experiment with different ones to see what works best.ones to see what works best.

The sharpened image is visually more The sharpened image is visually more pleasing but the pixel values have been pleasing but the pixel values have been altered from the original images so altered from the original images so automated classification methods might automated classification methods might produce unexpected results. produce unexpected results.

Effects of image fusion

Multispectral Landsat ETM+ 30 meter image before sharpening

Image after sharpening using Landsat ETM+ 15 meter panchromatic image