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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004
For this practical, you will be using Advanced Very High Resolution Radiometer
(AVHRR) images of Guatemala during an eruptive period of Fuego Volcano in
January 2004. This data will show the ability of AVHRR to detection volcanic ash cloud. In addition, you will see the significance of a locally based receiving station.
AVHRR data has a spatial resolution of 1.1 km at nadir and this increases as you move further along the swath, in both directions. Each AVHRR swath is approx 2000 km across and the repeatability of the NOAA system, means you have an AVHRR pass over Central America every 3 hours or so.
The AVHRR data is collected across 5 difference spectral ranges, Table 1. This means that Channels 1 and 2 are reflectance data. In Table 1, you will note that there are two
Channel 3 Bands. Band 3A is only for NOAA satellite 17 and is only during the day at 1.7 μm. Therefore, daytime NOAA 17 data has three reflectance channels and two temperature channels. NOAA 17 has Channel 3B at night at 3.6 μm and NOAA satellites 12, 15 and 16 have Channel 3 at 3.6 μm always.
Table 1 AVHRR Band Characteristics
Band Number Resolution (km) Channel Data
1
2
3A/3B
4
1.1
1.1
1.1/1.1
1.1
Reflectance
Reflectance
Reflectance/Brightness Temperature
Brightness Temperature
5 1.1 Brightness Temperature
The AVHRR data supplied in this practical has been captured at the AVHRR receiving station based at INETER in Nicaragua, see Figure 1. This station collects data for every pass that occurs across Guatemala, El Salvador, Nicaragua and Costa
Rica. The data is captured and processed automatically, analysing for 24 volcanoes, with the analysis outputs provided for users on a web-based interface.
(a) (b)
Figure 1: AVHRR receiving station at INETER, Nicaragua. (a) Antenna’s position for optimum view to track the satellites, (b) Data processing at INETER
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004
Fuego, Figures 2 and 3, forms part of the Guatemalan volcanic chain that extends throughout the Pacific coast of the country. It is located at Lat 14.473 N and Long
90.880 W and has an elevation of 3763 meters above sea level.
Figure 2: Central America and Guatemalan volcanoes
Figure 3: Volcan Fuego and surrounding region
There are 4 AVHRR data scenes supplied with this practical, Table 2. For each captured image, you will find a BIL, HDR and PTS file. The BIL file corresponds to the AVHRR channel information stored in Band Interleaved by Line format. The
HDR file provides the header information for the corresponding BIL file and the PTS file provides georeferenced point’s data created from the INETER system to allow the user to georeference the images from the image swath to the Lat/long grid. The image in bold in Table 2 will be the first one analysed. If time permits, you can analyse the others.
Table 2 AVHRR data for Fuego January 2004 ash cloud detection.
Date Time (GMT) Time (Local) Image name
8 th January 2004 20:17 14:17 16C18K11.BIL
9 th January 2004
9 th January 2004
9 th January 2004
00:13
04:31
20:06
18:13 (8/1/04)
22:31 (8/1/04)
14:06
15C1900D.BIL
17C1941F.BIL
16C19K06.BIL
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004
a) Loading the AVHRR Images
Firstly, you need to load the data into ENVI. To do this, choose the Select File ->
Open Image File option from the ENVI toolbar. Then enter Input Data File selection dialog appears. Navigate to the directory where your data are held, just as you would in any other application. Select the file for 9 th
January 2004 at 04:31 GMT
(17C1941F.BIL) from the list and click "OK". The Available Bands List dialog will appear on your screen. You will see that ENVI has loaded in the 5 AVHRR bands as well as the Latitude, Longitude, Satellite azimuth, Satellite elevation, Sun azimuth and
Sun elevation.
The data in Bands 1 and 2 are reflectance data and this can be used to create a multispectral image. For this, you need to load R (Band 1), G (Band 2) and B (Band
1). So click on the RGB color option, see Figure 4, and in turn click on each of the R,
G and B buttons, selecting the correct AVHRR band to assign to that screen colour.
When this is done click "Load RGB" to load the image. You will have loaded a False
Colour Composite (FCC) image. This FCC image for 09/01/04 at 04:31 GMT is black as it’s nighttime. Close this display and open up the Channel 3B Temperature data as a new grayscale display window. This is done by selecting the grayscale image option in the Available bands window.
To find the Latitude and Longitude of each pixel, you will need to display the
Latitude and Longitude arrays as new displays. Open both in as grayscale images.
This is done by loading in the Latitude band in a new grayscale display. You will need to do this again for the Longitude band, again opening a new display. To search for
Fuego, you need to link the three displays together, Channel 3B, Latitude and
Longitude.
Select the Tools ->Link- >Link Displays option from one of the display windows.
Click "OK" in the Link Displays dialog, making sure that you have chosen ‘Yes’ for all the displays and try scrolling or zooming in one display. You will observe that your changes are mirrored in the second and third displays. You have been able to link together the corresponding pixels in each display. You can use the Cursor
Value/Location option to move around the image, by right clicking on the zoom window. The cursor window will show you the pixel location of the cursor and also the Channel 3B brightness temperature, Latitude and Longitude of each pixel.
You will notice that to find the location of Fuego volcano you will need to search the
Latitude and Longitude arrays until you find the nearest pixel in both directions to the true Latitude and Longitude of Fuego, Lat 14.473N and Long 90.880W. This would require determining a window of +/- 0.001
o
until a pixel was detected and assigning this as the summit pixel. However, geo-referencing the data to a true WGS-84
Latitude/Longitude grid will make the process of finding Fuego much easier. b) Registration of the AVHRR Images
Choose the Map > Registration > Warp from GCPs: Image to Map option from the
ENVI toolbar. Here, you will be referencing from the Ground Control Points (GCP)
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004 generated during the INETER data processing. You will then be prompted to choose the name of your GCP filename. Choose the PTS file with the same name as the BIL file [the 17C1941F.PTS file for 17C1941F.BIL file]. Once you have done this you will be prompted, as in Figure 4, to decide upon the Registration Projection, Datum,
Units and X and Y size for pixels. You need to choose the Geo Lat/Lon Projection, the WGS-84 Datum, the Units of degrees and the Xsize and Ysize = 0.01 (As 1.1 km
~ 0.01
o
). Next select the BIL file that you want to register.
Figure 4: Image to Map Registration
Here you will want to change the Spectral Subset to just Channels 1, 2, 3, 4 and 5.
The spatial subset also needs to be changed. If you click on this option and then the
Image button, you will see a window appear, which is the full image. You need to move the Red box around the image and resize it until the red box covers all of
Guatemala see Figure 5. Then click on ‘OK’. The next window will ask you to decide upon the Warp Method, Resampling Method, Background Value, Zero Edge and
Output Format. Here, you need to choose the Triangulation Warp method, Nearest
Neighbor Resampling, 0.00 as the background, Zero edge as ‘YES’ and save the output to memory, see Figure 6.
Figure 5: Georeferencing window Figure 6: Registration Parameter
Then click on ‘OK’ and ENVI will carry out the registration of the image and load them into the available bands window. You can now view the image using the new band for Channel 3. e.g. Band 3 = Warp (Channel 3B Temperature). This time you will notice that the image array is much smaller and the image loads much quicker into the grayscale display window. If you choose the Cursor Value/Location option to move around the image, by right clicking on the zoom window, you will see that the
Latitude and Longitude can be used to find Fuego on the image, 14.473 N (14 o
28’ N)
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004 and 90.880 W (90 o
53’ W). Moving the cursor east-west and the Latitude will not change and by moving the cursor north-south, the Longitude will not change.
There are several methods available to detect for volcanic ash using satellite remote sensing data of the sort provided by the AVHRR. For the AVHRR sensor, the presence of volcanic ash means the brightness temperature difference in AVHRR
Channels 4 (wavelength of ~ 10.5μm) and 5 (wavelength of ~ 11.5μm) (i.e. T
4
– T
5
) becomes negative. This simple method offers the potential to automatically identify volcanic ash clouds, and to discriminate them from meteorological cloud cover.
Figure 7 shows cross sections through regions of meteorological cloud and ash cloud recorded in an AVHRR image. Figure 7a shows the positive temperature difference across metrological cloud (between dashed lines) and Figure 7b the negative difference across ash cloud (between dashed lines).
(a) (b)
Figure 7: AVHRR T
4
– T
5 temperature difference at: (a) Meteorological cloud
(Positive difference), (b) Ash cloud (Negative difference)
Furthermore, since the method relies on thermal remote sensing methods, it will work as well by night as by day, which is obviously important since ash clouds are just as likely to occur at night. a) Single channel (AVHRR Band 4 or 5)
Now that you have seen that using the brightness temperature Bands 4 – 5 is a useful tool for detecting any ash cloud. Firstly, you will need to load in Band 4 and 5 to show that it is difficult to use a single AVHRR Band. Load in Channel 4 into a grayscale window and Channel 5 into a separate grayscale display window. Then choose the Cursor Value/Location option to move around the image, by right clicking on the zoom window, you will see that the Latitude and Longitude can be used to find
Fuego on the image, 14.473 N (14 o
28’ N) and 90.880 W (90 o
53’ W). Once you have found the volcano, you will see that whilst clouds are generally cold and the land/sea warm, it is difficult to isolate the ash cloud precisely. Figure 8 shows a screenshot of
AVHRR Channels 4 and 5 in ENVI.
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004
Figure 8: ENVI screenshot of
Channels 4 and 5 b) Production of Band 4 – 5 brightness temperature difference
To detect for any evidence of ash clouds around Fuego you need to carry out a band math of AVHRR Bands 4 and 5. Firstly, choose the Basic Tools > Band Math option from the main ENVI toolbar (see below Figure 9a). Then insert the equation B4 – B5 into the expression window, Figure 9b. Then you will be prompted to choose the B4 and B5, see Figure 9c. Save the output to either a file, so you can save it for future use, or to memory. Now you can load the B4 – B5 data into a new grayscale display window. To best illustrate evidence of ash clouds you will need to change the image color bar and scale.
(b) (c)
(a)
Figure 9: Band Math for Brightness Temperature from Channels 4 and 5 c) Changing color scale
The default color bar is black and white. Firstly, load the Band 4 – 5 data in as a grayscale image. You can then change this colour so that it best illustrates the ash cloud. You choose the Tools > Color Mapping > ENVI Color Tables option from the
Image window once you have displayed the B4 – B5 data, see Figure 10.
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004
Figure 10: Changing ENVI
Colour scale
You will now see the window in Figure 11a. If you scroll down until you find the
EOS A options, Figure 11b. Next you need to reverse the color scale so that the
‘purple’ is on the right and the ‘red’ is on the left. This will give you the color bar as seen in the image on the Figure 11c.
(b) (c)
(a)
Figure 11: Changing from Black and White to EOS A colour scale d) Interactive Stretching
Now that you have the EOS A colour scale, you can change the image scale so that only pixels where Band 4 – 5 < 0 are shown in the color bar and the rest is left as black. To do this you will need to ‘RIGHT CLICK’ in the image window and select the Interactive Stretching option .
You will then get the window as shown in Figure
12. You will then need to select the Histogram_source > Zoom option, as shown in
Figure 12. This will scale the color by the max and min values in the zoom window.
Figure 12: Interactive
Stretch of data
You will need to set these values to have a maximum of 0. You need to click in the right hand one of the boxes next to the Apply button. Type in ‘0’ and then click
‘RETURN’. You will see the value change to the closest value in the zoom window to zero and the dotted line in the Input histogram change. To apply this to the display window, you will need to click the ‘Apply’ button. You will now see the image change so that the only areas where B4 – B5 < 0 will show up.
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004 e) Ash eruption width, size and length
This option will allow you to determine the length, width and horizontal area extent of the volcanic ash eruption at Fuego. You need to choose the Tools > Measurement
Tool option from the Image window, see Figure 13.
Figure 13: Measurement tool option
Then choose the ‘zoom’ option from the next window. You then need to ‘Right Click’ in the zoom window at one end of the ash cloud. You will see if you move around the zoom window, then you will extend a ‘red’ line. Move this red line so that it extends to the other end of the ash plume and ‘right click’ again. In the Measurement tool window, you will see the number of pixels that are between each end. Record this value and multiply by 1.1. This value will equal the N-S extent in km of the ash cloud.
Next ‘Right click’ at one side of the maximum width of the ash cloud. Move the red line to the other end and ‘Right Click’ again. Make a record of this measurement and multiply by 1.1 to give the maximum width in km.
To determine the size of the ash cloud, you will need to carry out a Band Threshold to Region of Interest (ROI).
Choose the Tools > Region of Interest > Band Threshold to ROI option from the
Image window, Figure 14.
Figure 14: Choosing Band
Threshold to ROI
Then select the Band 4 – 5 as the input band. You will need to change the spatial subset so that it covers the area of the ash cloud. Once you have done this, you will then select the Maximum and Minimum values for the Threshold that you are going to
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004 use. As we are using the Band 4 – 5 data, then the minimum needs to be 0. Once you have done this make a record of the number of pixels. The horizontal area in km
2
is this value * 1.21.
The production of fully georeferenced images as JPEG, GIF or TIFF’s is useful as it allows you to have a record of the captured image. For this, you can add coastline and political boundaries to the georeferenced data. Select the Vector > Create World
Boundaries option from the ENVI toolbar. When prompted choose the [1] and [2] options, which are the Political Boundaries (High Res) and Coastlines (High Res) and save the output to memory. Then click on ‘OK’. You will be warned that the file is
20 MB. Click ‘YES’ here. Now you need to click on the ‘Coastline (High Res) [Full range]’ and then the ‘Load Selected’ button. When prompted choose the display window and the coastline will be added, Figure 15. You need to do the same for the
Political Boundaries Vector as well.
Figure 15: Vector options, available vectors and vectors added to AVHRR image.
On the Image window for the Band 4 – 5 display (Coloured to EOS A, scaled to < 0 and with annotation) choose the File > Save Image as > Image File option. You will then have the option of choosing the resolution, Spatial Subset, Output file type and
Output file name, see Figure 16.
For the best option you need to choose
Resolution : 8 bit (Grayscale)
Spatial Subset : By image for Guatemala
Output File Type : JPEG (Factor = 1.0)
Output Filename : Suitable for Channels
Figure 16: Output Display to Image File
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004
th
You have been able to show that AVHRR can detect an ash cloud from the January
2004 eruption of Fuego volcano, Guatemala. This next image is from earlier on 8 th
January. Firstly open up the 16C18K11.BIL in ENVI. Load in Channel 1 into a grayscale window and you will see that this time there is captured reflectance data, as it is a daytime image. You will need georeference this data as with the previous image. Choose the Map > Registration > Warp from GCPs: Image to Map option from the ENVI toolbar. Choose the PTS file with the same name as the BIL file.
You need to choose the Geo Lat/Lon Projection, the WGS-84 Datum, the Units of degrees and the Xsize and Ysize = 0.01 (As 1.1 km ~ 0.01
o
). Next select the BIL file that you want to register. The next window will ask you to decide upon the Warp
Method, Resampling Method, Background Value, Zero Edge and Output Format.
Here, you need to choose the Triangulation Warp method, Nearest Neighbor
Resampling, 0.00 as the background, Zero edge as ‘YES’ and save to memory.
Once you have georeferenced the data load in the Bands for 1 and 2 into a RGB display, with R (Band 1), G (Band 2) and B (Band 1). So click on the RGB color option and in turn click on each of the R, G and B buttons, selecting the correct
AVHRR band to assign to that screen colour. When this is done click "Load RGB" to load the image. You will have loaded a False Colour Composite (FCC) image. You will see that the image shows the region of Central America using the visible channels.
Load in Band 4 and 5 to show that it is difficult to use a single AVHRR Band. Load in
Channel 4 into a grayscale window and Channel 5 into a separate grayscale display window. Then choose the Cursor Value/Location option to move around the image, by right clicking on the zoom window, you will see that the Latitude and Longitude can be used to find Fuego on the image, 14.473 N (14 o
28’ N) and 90.880 W (90 o
53’
W). Once again you will see that it is not easily possible to determine if there is evidence of ash cloud.
Then carry out the B4 – B5 band math. Choose the Basic Tools > Band Math option from the main ENVI toolbar and insert the equation B4 – B5 into the expression window. Now you can load the B4 – B5 data into a new grayscale display window.
To best illustrate evidence of ash clouds you will need to change the image color bar and scale. Also go to Fuego (14.473 N, -90.880 W) to see if there are any ash pixels.
If you carry out the interactive stretch you will find that there is no evidence of an ash cloud in this image. Then move onto the next image to detect for an ash cloud.
th
Firstly open up the 15C1900D.BIL in ENVI. You will need georeference this data as with the previous image. Choose the Map > Registration > Warp from GCPs: Image to Map option from the ENVI toolbar. Choose the PTS file with the same name as the
BIL file. You need to choose the Geo Lat/Lon Projection, the WGS-84 Datum, the
Units of degrees and the Xsize and Ysize = 0.01 (As 1.1 km ~ 0.01
o
). Next select the
BIL file that you want to register. The next window will ask you to decide upon the
Warp Method, Resampling Method, Background Value, Zero Edge and Output
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004
Format. Here, you need to choose the Triangulation Warp method, Nearest Neighbor
Resampling, 0.00 as the background, Zero edge as ‘YES’ and save the output to memory
Load in Band 4 and 5 to show that it is difficult to use a single AVHRR Band. Load in
Channel 4 into a grayscale window and Channel 5 into a separate grayscale display window. Then choose the Cursor Value/Location option to move around the image, by right clicking on the zoom window, you will see that the Latitude and Longitude can be used to find Fuego on the image, 14.473 N (14 o
28’ N) and 90.880 W (90 o
53’
W). Once again you will see that it is not easily possible to determine if there is evidence of ash cloud.
Then carry out the B4 – B5 band math. Choose the Basic Tools > Band Math option from the main ENVI toolbar and insert the equation B4 – B5 into the expression window. Now you can load the B4 – B5 data into a new grayscale display window.
To best illustrate evidence of ash clouds you will need to change the image color bar and scale to between 0 and –2.7 as before. Also go to Fuego (14.473 N, 90.880 W) to see if there are any ash pixels. You will find that there is evidence of an ash cloud, if smaller than the first image analysed. You will again want to determine the length, width and size using the measurement tool and the Band Threshold to ROI. Choose the Tools > Measurement Tool option from the Image window and the Tools >
Region of Interest > Band Threshold to ROI option from the Image window.
Finally, you can create a georeferenced image as a jpeg, gif as with the first image.
You can add the vector files for the coastline and political boundaries and save the file with the File > Save Image as > Image File option.
Figure 17 shows the ability of AVHRR to detect the volcanic ash cloud from the
January 2004 Fuego eruption using the simple T
4
– T
5
< 0 threshold. At 20:17 GMT
(2:17pm local time) on 8 th
January 2004 (Figure 17a) there is no evidence of an ash cloud at Fuego. However, four hours later (6:13pm local time) a 38 km long ash cloud is apparent (Figure 17b), spreading mainly towards the north. Figure 17c indicates that after a further four hours, the ash cloud now extends for approx 100 km, and is ~ 40 km wide at maximum. After yet another four hours, the main ash cloud has dissipated towards the northwest and there have been no subsequent ash emissions, so the cloud appears detached from the volcano (Figure 17d). By the following afternoon, the ash cloud has fully dissipated (Figure 17e).
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004
(a) (b)
(c) (d)
(e)
Figure 17: Evidence of volcanic ash cloud detected by AVHRR station at INETER.
(a) 8 th Jan 20:17 GMT, (b) 9 th Jan 00:13 GMT, (c) 9 h Jan 04:31 GMT, (d) Jan 9 th
10:16 GMT and (e) Jan 9 th
20:06 GMT. The location of Fuego is indicated by the red cross, and the T
4
– T
5 colour scale is from 0 to –2.7 Kelvin.
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AVHRR Ash Cloud Detection Remote Sensing Workshop November 2004
You have been able to detect a volcanic ash cloud at Fuego volcano using a basic method of Band 4 – 5 < 0. You have seen that at 20:17 GMT on January 8 th
2004
(14:17 LT) there was no evidence of an ash cloud. By 18:13 GMT or 12:13 LT on the
8 th
, an ash cloud was detected. This had grown to over 100 km long and at its maximum 40 km wide by 8 th
January 2004 at 22:31 LT (09/01/04 at 04:31 GMT).
You can see from Figure 19 that this ash cloud disperses by the following image and there is no evidence at all by the afternoon of January 9 th .
This has shown the use of AVHRR to detect and monitor volcanic ash clouds. This data was collected from the Local Receiving station at INETER in Nicaragua. This data shows the ability of AVHRR to detect the ash cloud even at nighttime. During this eruption, INSIVUMEH and CONRED in Guatemala had local observers monitoring the volcano and stated that this data would of great benefit as detect ash is almost impossible for their local observers. There also stated the use that this may have in operational use for aircraft movement and ash deposits on the local surrounding area.
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