4 Using the outputs
Once the MODIS satellite images level 1b and their corresponding geolocation files are on the local disk, they are processed in IDL. If volcano is not on the image, then no output is generated. Otherwise at least an XML file having the same name as the level 1b MODIS file is generated. If the volcano is not too close to the edge of the image, an RGB (8-bit) GEOTIFF is created. If a hotspot is detected, then XML becomes some additional tags that are not written to the file in the case of no hotspots. Additionally, 16-bit GOTIFF containing radiances of six selected channels is written when a hotspot is detected. All information from these data are available online. In the case of a hotspot, another IDL writes some additional files just to the local disk: 8-bit GEOTIFF with quality flags (0 – eveything OK, 20 – clouds, 21 – sun glint, 22 – nodata values; from the sum you can find out the situation), 16-bit GEOTIFF with NTI values, and ps file with NTI histogram. The last three files are written only to the specified path of the data on the local disk. All the files have the same names, just the suffix changes. All results are georeferenced in geographical coordinate system (decimal degrees) on WGS84.
- Add layer / WP2 / Infared image (or hotspots - vector data)
- Choose opaque for RGB style, or one of the six selected channels for the measured radiances
- In the opaque style, ash plume is usually pink, SO2 green, clouds orange, etc.
4.1 Data visualization with the Web GIS
Two hotspots features can be shown in Exupéry web GIS: image and vector data. Infrared image (choose “Infrared pseudoclour image” under WP2 in add layer menu) presents general situation of the volcano area (1.5 by 1.5 degree area, spatial resolution of 0.005 degree). Seven styles can be used. RGB style can be chosen if the style “opaque” is used; it shows the general meteorological situation (e.g. clouds colour depends mainly on the clouds height, usually in yellow or orange, ash plume in pink, ocean in blue, etc. but the colours are not always the same). The image is a combination of temperatures:
red: TIR 12.0 – TIR 11 ch. difference (stretch: -4 … +2 K)
green: TIR 11 – TIR 8.6 ch. difference (stretch: -4 … +5 K)
blue: TIR 11 (stretch: 243 … 303 K)
The other styles shows the measured radiances (units Watts/m2/micrometer/steradian) from 6 chosen channels (greyscale colourbar); the hotspots are obvious (the brightest) in the channel 3. Stretch to 8-bit grayscale image (0 – black, 255 – white) for each channel is presented below (radiances in Watts/m2/ μm /steradian):
1 NIR 0.8 μm 0–80
2 SWIR 1.6 μm 0–20
3 MIR 3.9 μm 0.5–3
4 TIR 8.6 μm 6–10
5 TIR 11 μm 6–10
6 TIR 12 μm 6–10
Each detected hotspot can be additionally described by a point layer (choose “Hotspots” under WP2 in add layer menu). Mohr styles are defined – you can either present hotspots temperature, area, or thermal flux.
4.2 Metadata structure
The main metdadata structure (witten to an XML file) is as follows:
- <infrared volcano_id="0101-06=" project_id="ExuperyTest">
+ <range_upperleft>
+ <range_upperleft>
+ <start_datetime>
+ <end_datetime>
+ <processing>
+ <files>
+ <hotspots_overview>
+ <hotspots>
</infrared>
Within the <range_upperleft> and <range_upperleft> tags the coordinates (longitude and latitude) of the corners with their uncertanties are written. Similar case is for <start_datetime> and <end_datetime> tags.
More information can be extracted from <processing> tag: who and when created results, which satellite was used, the solar and satellite geometry, used emissivities and atmospheric transmittance. The most siginificant tags here are <satellite_zenith_deg> that can be used to estimate the pixel size, <NTI_threshold> that seperates hotspost from background, and <cloud_coverage_percentage> because the cloud coverage might decrease the accuracy of the results.
If any images (there might be none, 8-bit, or 8- and 16-bit GEOTIFFs present) are created, then they are described in <files> tag. Size, path, resolution and the data in each channel are written in under this tag.
If no hotspots were detected, then only background temperature and alert level are written into <hotspots_overview> tag. This contains more information in the case of hotspots: number of pixels recognized as hotspots, pixel position in the original level 1b data, average NTI and radiance in each of the six selected channels, average temperature of the hotspot, also its area, area percentage, radiant flux, effusion rate and sun glint angle.
<hotspots> tag contains similar parameters as described above, but in this case for each cluster (group of adjacent hotspot pixels) and not for the whole volcano. In addition, the centre point of each cluster is given by its longitude and latitude. These parameters are of course not available if no hotspots were detected.