ENVI Tutorial

Itt Visual Information Solutions

Informations

• Publié par: Mocel
• Nombre de lectures: 31
• Langue: English

Extrait

E N V I T u t o r i a l :
H y p e r s p e c t r a l S i g n a t u r e s
a n d S p e c t r a l R e s o l u t i o n
H y p e r s p e c t r a l S i g n a t u r e s a n d S p e c t r a l R e s o l u t i o n 2
F i l e s U s e d i n t h i s T u t o r i a l 2
S p e c t r a l R e s o l u t i o n 3
S p e c t r a l M o d e l i n g a n d R e s o l u t i o n 3
C a s e H i s t o r y : C u p r i t e , N e v a d a , U S A 5
O p e n a n d V i e w U S G S L i b r a r y S p e c t r a 6
V i e w L a n d s a t T M I m a g e a n d S p e c t r a 7
V i e w G E O S C A N I m a g e a n d S p e c t r a 9
V i e w G E R 6 3 I m a g e a n d S p e c t r a 1 1
V i e w H y M a p I m a g e a n d S p e c t r a 1 3
V i e w A V I R I S I m a g e a n d S p e c t r a 1 6
D r a w C o n c l u s i o n s 1 8
R e f e r e n c e s 1 9
1E N V I T u t o r i a l : H yp e r sp e ct r a l S i g n a t u r e s a n d S p e ct r a l R e so l u t i o n
H y p e r s p e c t r a l S i g n a t u r e s a n d S p e c t r a l R e s o l u t i o n
This tutorial compares the spectral resolution of several different sensors and the effect of resolution on
the ability to discriminate and identify materials with distinct spectral signatures. The tutorial uses
Landsat Thematic Mapper (TM) data, GEOSCAN data, Geophysical and Environmental Resarch 63-
band (GER63) data, Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data, and HyMap data
from Cuprite, Nevada, USA, for intercomparison and comparison to materials from the USGS spectral
library.
F i l e s U s e d i n t h i s T u t o r i a l
All files are on the ENVI Resource DVD.
D i r e c t o r y : D a t a \ c u p _ c o m p
F i l e D e s c r i p t i o n
u s g s _ e m . s l i ( . h d r ) Subset of USGS spectral library
c u p t m _ r f . i m g ( . h d r ) TM reflectance subset
c u p t m _ e m . t x t Kaolinite and alunite average spectra from c u p t m _ r f . i m g
c u p g s _ s b . i m g ( . h d r ) GEOSCAN reflectance image subset
c u p g s _ e m . t x t Kaolinite and alunite average spectra from c u p g s _ s b . i m g
c u p g e r s b . i m g ( . h d r ) GER63 reflectance image subset
c u p g e r e m . t x t Kaolinite and alunite average spectra from c u p g e r s b . i m g
D i r e c t o r y : D a t a \ c u p 9 9 h y m
F i l e D e s c r i p t i o n
c u p 9 9 h y . e f f ( . h d r ) HyMap reflectance data
c u p 9 9 h y _ e m . t x t Kaolinite and alunite average spectra from c u p 9 9 h y . e f f
D i r e c t o r y : D a t a \ c 9 5 a v s u b
F i l e D e s c r i p t i o n
c u p 9 5 e f f . i n t ( . h d r ) AVIRIS EFFORT-polished, atmospherically corrected apparent reflectance
data, converted to integer format by multiplying the reflectance values by
1000 to conserve disk space. Values of 1000 represent reflectance values of
1.0.
c u p 9 5 e f f . t x t Kaolinite and alunite average spectra from c u p 9 5 e f f . i n t
Optional: USGS spectral library. Use if you want a more detailed comparison
u s g s _ m i n . s l i
( . h d r )
2E N V I T u t o r i a l : H yp e r sp e ct r a l S i g n a t u r e s a n d S p e ct r a l R e so l u t i o n
S p e c t r a l R e s o l u t i o n
Spectral resolution determines the way we see individual spectral features in materials measured from
imaging spectrometry. Many people confuse the terms spectral resolution and spectral sampling. These
are very different.
Spectral resolution refers to the width of an instrument response (band-pass) at half of the band depth, or
the full width half maximum (FWHM). Spectral sampling usually refers to the band spacing - the
quantization of the spectrum at discrete steps - and may be very different from the spectral resolution.
Quality spectrometers are usually designed so that the band spacing is about equal to the band FWHM,
which explains why band spacing is often used interchangeably with spectral resolution.
The exercises that follow compare the effect of the spectral resolution of different sensors on the
spectral signatures of minerals. The graph below shows the modeled effect of spectral resolution on the
appearance of spectral features for Kaolinite.
S p e c t r a l M o d e l i n g a n d R e s o l u t i o n
Spectral modeling shows that spectral resolution requirements for imaging spectrometers depend upon
the character of the material being measured. Kaolinite, for example (see the plot below), exhibits a
characteristic doublet near 2.2 µm at 20 nm resolution. Even at 40 nm resolution, the asymmetrical shape
of the band may be enough to identify the mineral, even though the spectral features have not been fully
resolved.
The spectral resolution required for a specific sensor is a direct function of the material you are trying to
identify, and the contrast between that material and the background materials. The following figure from
Swayze (1997) shows modeled spectra for kaolinite from several different sensors.
3E N V I T u t o r i a l : H yp e r sp e ct r a l S i g n a t u r e s a n d S p e ct r a l R e so l u t i o n
4E N V I T u t o r i a l : H yp e r sp e ct r a l S i g n a t u r e s a n d S p e ct r a l R e so l u t i o n
C a s e H i s t o r y : C u p r i t e , N e v a d a , U S A
Cuprite has been used extensively as a test site for remote sensing instrument validation (Abrams et al.,
1978; Kahle and Goetz, 1983; Kruse et al., 1990; Hook et al., 1991). Refer to the following alteration
map of the region.
This tutorial illustrates the effects of spatial and spectral resolution on information extraction from
multispectral and hyperspectral data. You will use Landsat TM, GEOSCAN MkII, GER63, HyMap and
AVIRIS images of Cuprite, Nevada, USA, and you will see the effect of different spatial and spectral
resolutions on mineralogic mapping through remote sensing.
All of these datasets have been calibrated to reflectance. Only three of the numerous materials present
at the Cuprite site are used for comparison. Average kaolinite, alunite, and buddingtonite image spectra
were selected from known occurrences at Cuprite. Laboratory spectra from the USGS spectral library
(Clark et al., 1990) of the three selected minerals are provided for comparison to the image spectra.
5E N V I T u t o r i a l : H yp e r sp e ct r a l S i g n a t u r e s a n d S p e ct r a l R e so l u t i o n
O p e n a n d V i e w U S G S L i b r a r y S p e c t r a
1. From the ENVI main menu bar, select S p e c t r a l > S p e c t r a l L i b r a r i e s > S p e c t r a l L i b r a r y
V i e w e r. A Spectral Library Input File dialog appears.
2. Click O p e n and select S p e c t r a l L i b r a r y. A file selection dialog appears.
3. Navigate to D a t a \ c u p _ c o m p and select u s g s _ e m . s l i. These spectra represent USGS
laboratory measurements for kaolinite, alunite, buddingtonite, and opal, in Cuprite, measured with
a Beckman spectrometer. Click O p e n.
4. Select u s g s _ e m . s l i in the Spectral Library Input File dialog, and click O K. The Spectral
Library Viewer dialog appears.
5. In the Spectral Library Viewer dialog, select each mineral. The spectra appear in a Spectral
Library Plots window.
6. Examine the detail in the spectral plots, particularly the absorption feature positions, depths, and
shapes near 2.2 - 2.4 µm. For better comparison, use the middle mouse button to draw a box in the
plot window from 2.0 to 2.5 µm. F  ollowing is an annotated plot of laboratory spectra for kaolinite,
alunite, and buddingtonite, showing the absorption features of interest:
6E N V I T u t o r i a l : H yp e r sp e ct r a l S i g n a t u r e s a n d S p e ct r a l R e so l u t i o n
V i e w L a n d s a t T M I m a g e a n d S p e c t r a
The following plot shows region of interest (ROI) mean spectra for kaolinite, alunite, and buddingtonite.
The small squares indicate the TM band 7 (2.21 µm) center point. The lines indicate the slope from TM
band 5 (1.65 µm). The spectra appear very similar, and you cannot effectively discriminate between the
three endmembers.
V i e w T M M e a n K a o l i n i t e a n d A l u n i t e I m a g e S p e c t r a
1. From the ENVI main menu bar, select W i n d o w > S t a r t N e w P l o t W i n d o w. A blank ENVI Plot
Window appears.
2. From the ENVI Plot Window menu bar, select F i l e > I n p u t D a t a > A S C I I. A file selection
dialog appears. 
3. Select c u p t m _ e m . t x t and click O p e n. An Input ASCII File dialog appears. Click O K to plot
the mean kaolinite and alunite spectra.
C o m p a r e M e a n S p e c t r a a n d L i b r a r y S p e c t r a
Refer to these steps throughout the rest of the tutorial whenever you compare library spectra and ROI
mean spectra from different sensors.
1. Right-click in the Spectral Library Plots window and select P l o t K e y.
2. Click and drag the Kaolinite and Alunite spectrum names from the Spectral Library Plots window
to the ENVI Plot Window.
3. Right-click in the ENVI Plot Window and select P l o t K e y.
7E N V I T u t o r i a l : H yp e r sp e ct r a l S i g n a t u r e s a n d S p e ct r a l R e so l u t i o n
4. For easier comparison, select E d i t > D a t a P a r a m e t e r s from the ENVI Plot Window menu bar,
and change the M e a n : K a o l i n i t e and M e a n : A l u n i t e colors to match the colors of the
corresponding library spectra.
O p e n L a n d s a t T M I m a g e
1. From the ENVI main