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Tools, Tips, and Workflows

Multi-band Imagery and Bit Depth Terrasolid

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September 2014

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Multi-band (multi-spectral) imagery, as the name suggests, contains multiple bands which make up the image as a whole. In remote sensing imagery, a band would be considered discrete ranges of wavelengths sampled by the system. If you recall from science class, a wavelength is “the mean distance between maxima or minima of a periodic pattern.”1

For every raster cell location in multi-band imagery, there is one value for each band assigned to the cell. Traditionally, multi-band imagery consists of just three bands, red, green and blue (RGB), each of which are collected by a sensor that is specifically tuned to the specified wavelength. The RGB bands fall in the visible wavelength spectrum between 0.7 µm to 0.4 µm. Note, the SI symbol µm is a micrometer, which is one millionth of a meter, or 1×10−6 of a meter, and is commonly referred to as a micron. Figure 1 from the Computer Science department at Brown University in Rhode Island shows an example of the visible light spectrum.

Figure 1 - Visible Light Spectrum. Retrieved from http://cs.brown.edu/~tld/courses/cs148/02/sensors.html

Multi-spectral imagery generally includes the infrared and ultraviolet regions of the electromagnetic spectrum as well: ultraviolet 0.1 – 0.4 µm, near infrared (NIR) 0.7 – 1.3 µm, mid-infrared (MIR) 1.3-3.0 µm, and thermal (far) infrared 3.0 – 5.0 µm.

LANDSAT 8, is a United States Geological Survey (USGS) satellite, launched in 2013, that has eleven bands. Band one is deep blue and violet which are part of the ultraviolet spectrum, bands two through four are the visible light spectrum of blue, green and red. Band five covers the NIR, bands six and seven are shortwave infrared, while band eight is panchromatic. Band nine is something entirely new and it is designed to pick up cirrus clouds while bands ten and eleven are for the thermal infrared band.2 LANDSAT 8 products are all delivered as 16-bit images.

What is a bit and how does it pertain to imagery? What is the difference between an 8-bit image and a 16-bit image? What does it mean when someone says a 3 x 8 bit image? These questions can be answered by looking at how the information for a digital image is stored.

Computers store information using zeros and ones. A bit, or bit depth, as it is often referred to for imagery, refers to how many tonal variations can be displayed, or stored, per raster cell using a series of zeros and ones. Figure 2 is a chart explaining the number of variations per bit. One bit may only be one of

http://cs.brown.edu/~tld/courses/cs148/02/sensors.html


Multi-band Imagery and Bit Depth

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two values, a 0 or a 1. Two bits could then be 22, or one of 4 possible values; Eight bits would then be 28, or 256 possible values; While 16 bits would be 216, or 65636 possible values.

Figure 2

Having more possible values for a pixel of an image means that the image may contain more tonal values. Hence, a 16-bit per band image contains more depth than an 8-bit since 8-bit allows for 256 different tonal values, while 16-bit allows for 65,536 tonal values per band. If there are three bands in an image, and each band is 8 bits, then the image is a 3*8-bit, or a 24-bit image, or an 8-bit per band image. It is not an 8-bit image. Figure 3 and Figure 4 show LANDSAT 8 imagery from the USGS and demonstrates the difference between an 8-bit per band pan-sharpened image and a 16-bit per band pan-sharpened image. Bands 2, 3, and 4 were combined using ArcMap and the image exported, once as an 8-bit Tiff, and once as a 16-bit Tiff. Band 8 was then applied to both images for pan-sharpening.

Figure 3 - 8-bit per band Pan-sharpened Image




Figure 4 - 16-bit per band Pan-sharpened Image

In LP360 for Windows one may now load rasters via the Add LAS/Raster Layer button . Support for multiband imagery is handled in the Raster Layer Properties dialogue by allowing the user to select the desired bands for display from those available in the image. See Figure 5 and Figure 6.







Figure 5 (top) - RGB Bands of Image displayed in LP360 Figure 6 (bottom) - nIRGB Bands of Image Displayed in LP360

In early 2014, Terrasolid added support for multiband imagery to TerraPhoto. When referencing a raster,

using the TerraPhoto Raster Manager , the user may select the desired channels (bands) by modifying the attachment. The number of available channels (bands) in the image are listed in the selectable dropdowns.







Figure 7 (top) - RGB Bands of Image displayed in TerraPhoto Figure 8 (bottom) - nIRGB Bands of Image Displayed in LP360

TerraPhoto also supports the generation of orthophotos using multiband imagery. The channels to be used throughout the rectification process (color point preview mode and rectify mosaic) are controlled by the Mission Camera settings.




Figure 9 (left) - Mission Camera configured for RGB Figure 10 (right) - Mission Camera configured for nIRGB

The Rectify Mosaic process can output up to n-channels when the GeoTIFF format is selected. The number of available channels as defined in the Mission Camera will be available as options for n*8 and n*16 bit orthomosaic images. For example in Figure 9 and Figure 10 the Mission Camera is defined for four channels and so the options for rectifying the mosaic are as shown in Figure 11. For more information on the updated TerraPhoto Orthomosaic workflow attend the fall Terrasolid training event taking place Oct 21-24, 2014 in Huntsville, AL. Click here for more information, and to register before the class fills up.

http://www.geocuellc.com/support/training/index.html

http://www.geocuellc.com/support/training/index.html




Figure 11 - Rectify Mosaic Options for 4 Channel Mission Camera

With multi-band imagery now supported by both the production and exploitation software it is opening up the possibilities for different types of image data to be used in conjunction with LIDAR data for a wide range of applications.

1Drury, S.A., Images of the Earth: a Guide to Remote Sensing. Oxford: Oxford University Press, 1998. 2http://landsat.gsfc.nasa.gov/?page_id=5377

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Tools, Tips, and Workflows Multi-band Imagery … Tips, and Workflows Multi-band Imagery and Bit Depth Terrasolid Support Page 1 of 9 Terrasolid September 2014 Support September 2014