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BIA4100: Location Analytics
Exercise 3: MultiSpec
Exercise Version: SP2021b
Exercise Author: Erskine
Exercise Editor: Johnson
Credits: Original Lab Content Author Authored by Kurt Menke, GISP; Content
Authored by Richard Smith, Ph.D., GISP Texas A&M University - Corpus
Christi; The development of the original document was funded by the
Department of Labor (DOL) Trade Adjustment Assistance Community College
and Career Training (TAACCCT) Grant No. TC-22525-11-60-A-48; The National
Information Security, Geospatial Technologies Consortium (NISGTC) is an
entity of Collin College of Texas, Bellevue College of Washington, Bunker Hill
Community College of Massachusetts, Del Mar College of Texas, Moraine
Valley Community College of Illinois, Rio Salado College of Arizona, and Salt
Lake Community College of Utah.
Objective: Explore and Understand How to Display and Analyze Remotely Sensed Imagery
Introduction
In this exercise, you will learn how to display and inspect multi-band imagery
in QGIS Desktop. They will use QGIS data processing tools to conduct an
unsupervised classification of multi-spectral imagery. They will then use
MultiSpec to perform a more advanced analysis. MultiSpec is a freeware
multispectral image data analysis system created at the Purdue Research
Foundation. MultiSpec provides the ability to analyze and classify imagery
data, among other tasks. This lab has been adapted from four tutorial
exercises provided by the MultiSpec team and introduces the software
package.
This exercise includes the following tasks:
•
Task 1 – Display and Inspection of Image Data
•
Task 2 – Supervised Classification
Required and Recommended Tools
QGIS, MultiSpec
Objective: Learn the Basics of using QGIS Desktop and MultiSpec for Image
Analysis
Image analysis is one of the largest uses of remote sensing imagery,
especially with imagery that has recorded wavelengths beyond the visible
spectrum. There are proprietary software packages designed specifically for
remote sensing work such as ENVI and ERDAS Imagine. QGIS Desktop can
now be used in combination with two additional FOSS4G software's, SAGA
and GRASS, to also conduct image analysis. SAGA and GRASS are both
standalone software packages that can be installed separately. However, the
main analysis tools from both are now bundled with QGIS Desktop. This
means that no additional installations are required in order to use GRASS and
SAGA analysis tools via QGIS Desktop. However, some of this functionality is
for more advanced users. For this reason, you will also learn how to use
MultiSpec which is a very simple and intuitive, but powerful, freeware image
analysis software package. This lab was adapted from the first four tutorials
provided by the MultiSpec team.
Task 1: Using data dictionaries and attribute selections
There are many ways to view multi-band image data. Here you will explore
some display options for a multi-band image in QGIS Desktop.
1.
Open QGIS Desktop.
2.
Click the Add Raster Layer button and navigate to the ex3data folder. Set
the filter to All files (*)(*.*).
3.
Select the file named ag020522_DPAC.img and click Open.
This raster layer does not have a defined coordinate reference system (CRS).
Therefore, QGIS opens the Coordinate Reference System Selector window. If
this window does not open automatically, double-click the layer and click the
Select CRS button under the General tab. This interface lets you define the
CRS before the layer is added to the Layers panel. This raster is in UTM, zone
16, WGS84.
4.
Type ‘zone 16’ into the Filter window. In the Coordinate reference
systems of the world box you’ll see a list of all the CRSs with zone 16 in
the name. Scroll through until you find WGS/84 UTM zone 16N (EPSG:
32616). Select it so that it appears in the Selected CRS box (see figure
below) and click OK .
Coordinate Reference System Selector
5.
The image will be added to QGIS (shown in figure below). This is an aerial
photograph of a portion of the Davis Purdue Agriculture Center in
Randolph County, Indiana.
Multi-band Image in QGIS Desktop
6.
Save your QGIS Desktop project to your lab folder as ex3.qgs
7.
Double click on the layer name in the Layers panel to open the Layer
Properties.
8.
Click on the Symbology tab.
When an image has multiple color bands, QGIS defaults to a Multiband color
rendering of that image. Colors on your computer monitor are created by
combining three color channels: red, green and blue (RGB). By selecting
three bands from a multiband image, and illuminating them with either red,
green or blue light we create a color image. The multiband color renderer
defaults to displaying Band 1 through the red channel, Band 2 through the
green channel and Band 3 through the blue channel. However, we can
change which bands are displayed through which channels.
9.
Click the drop-down arrow for the Red band and change it to Band 3.
Change the Blue band to Band 1 (see figure below).
Changing the band combination in QGIS
10.
Click Apply and move the Layer Properties window so you can see the
raster.
Note
: The difference between using Apply and using OK. Clicking OK saves
the changes and closes the dialog window. Apply saves the changes and
leaves the window open. If you want to change a setting, see the result and
change another setting use Apply.
11.
The image should now look like the figure below. This band combination
creates what is known as a false color composite. Vegetation reflects a
lot of near-infrared energy. You are now looking at the near-infrared
through the red channel so vegetation shows up as red tones. The
brighter the red, the more vigorous and healthier the vegetation.
False Color Composite
The Symbology tab also allows you to adjust Contrast enhancement. This
setting gives you options to modify the appearance of the image when used
in combination with the Load min/max values settings. Each band has values
from 0-255. By default, the renderer is set to use Cumulative count cut
values from 2% to 98%. This setting eliminates the bottom and top 2% of the
values. Many images have some outlying very low and high data values.
These outlying data values can be eliminated by using the Cumulative count
cut option. The Contrast enhancement is set by default to No enhancement.
12.
The values currently being used for each band will appear in the Min/max
boxes in the Band rendering area.
13.
Change the Contrast Enhancement to Stretch to MinMax and click Apply.
MinMax Stretch
The Accuracy setting lets you either estimate the range of values from a
sample or get the actual values. Obtaining actual values can take longer
since QGIS has to look at all the values in the image, instead of a sample.
14.
Now choose a Load min/max values setting of Mean +/- standard
deviation to specify within 1 standard deviation and click Load. Click
Apply to see the image change.
15.
Change the Accuracy setting to Actual, and click the Load button to see
the values change slightly.
The raster gets a more saturated appearance (shown in figure below). These
are the values within one standard deviation of the mean value. This is useful
when you have one or two cells with abnormally high values in a raster grid
that are having a negative impact on the rendering of the raster.
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