1. Is satellite imaging a photograph of the Earth?
No. It is a pictorial representation of the electromagnetic energy recorded by a sensor, not by photography. A photograph is usually taken within a certain spectrum range (visible light). Satellites take images beyond this limited range.
2. What is the image resolution?
NOAA-AVHRR images have a spatial resolution of 1.1 km * 1.1 km in nadir.
The size of the pixel in DMSP-OLS images is equal to 560 m * 560 m (31.36 ha). From these data 5 pixels are averaged resulting in a 2.7 km resolution. This is known as the smooth mode.
3. What are spectral bands?
The capacity of the satellite to receive information in defined ranges of the electromagnetic spectrum.
In the case of AVHRR images, spectral bands are as follows.
1 = Visible, 0.58 – 0.68 µm
2 = Close infrared, 0.725 - 1.10 µm
3 = Thermal infrared, 3.55 – 3.93 µm
4 = Thermal infrared, 10.3 – 11.3 µm
5 = Thermal infrared, 11.5 – 12.5 µm
4. What is a satellite pass?
The satellite moves around the Earth following a route dependent on its design and altitude over the Earth. The pass describes the route over the earth´s surface in which the satellite travels.
5. What is repeat cycle (the revisit time) of the satellite over Mexico or other parts of the country?
NOAA satellites are in an orbit approximately 870 km over the Earth. The orbit is polar and sun-synchroneous, at a 98.5º inclination towards the Equator. Circulation time is 102 minutes resulting in 14.1 turns per day around the Earth. In each turn, the satellite pass moves 3º eastwards. Satellites cross Mexico twice daily which allows us to get a partial or total coverage of the country several times a day.
6. How can I find out what was the satellite pass on a certain date?
Visit the page
http://www.conabio.gob.mx/mapaservidor/incendios/tablas2001/marzo/nocturnas/n14_010301_1135.html
The page has daily information in the daily centroid section where you will see an icon named imagen diaria (daily image). Select this option to see how the satellite passed over Mexico on that date.
7. What does georeferenced information mean?
The data are referred to a latitude and longitude coordinate system.
8. What is a centroid?
This corresponds to coordinates (latitude, longitude) of a detected hot spot.
9. What is the nadir?
It is the point on Earth directly under the sensor at a 90º angle.
10. What is a hot spot? Is it a fire?
A hot spot is the minimum spatial element in the image (pixel) reporting an elevated temperature. The minimum value to consider a pixel as a heat point with night images is 25º C. In the case of day images, the minimum temperature is 38º C.
Therefore, a hot spot is any source of elevated temperature (above 38° C and significantly higher than background temperature), sufficiently strong to be detected by the sensor. This spot may be caused by fires, agricultural fires, insolated soils, gas flames in oil wells, active volcanoes, and so forth. CONABIO eliminates hot spots having a low likelihood of being actual fires by using additional data and trained staff.
11. Can you estimate the burnt surface area?
No. Pixel saturation is considered to occur at ~ 50ºC. However, you only need 0.1% of the pixel area covered by temperatures ranging between 250ºC and 500ºC to saturate the pixel. If the hot spot´s temperature is higher than 500ºC, even an area of only 0.01% of the pixel, will saturate it. Roughly calculated, hot spots ranging from 120 m2 to 1,200000 m2 will be detected. Thus, you cannot determine the number of fires or the size of the surface area affected.
12. Can you find out the number of fires in the country or in a pixel?
The sensor detects hot spots by pixel. Pixel size is 1.1 km * 1.1 km. In the pixel you cannot calculate or estimate the number of fires or the size of the surface area affected. One pixel may have a single very large fire or many small fires.
13. Can you detect fires in a city?
Theoretically you can. However, CONABIO eliminates all the points detected in cities by a mask (stable lights product from DMSP-OLS). This is done because:
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You cannot determine if a hot spots is a e.g. slot (for industrial slotes) and other sources (such as fires).
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Immediate detection of a fire and alerting the firemen in a settlement is very likely to occur without the need for help from remote sensing.
14. If no hot spots were detected in an area on a specific date is it because there were no fires?
No. The fact that no hot spots are reported in a certain area on a specific date may be due to the following reasons.
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No hot spots were detected with the algorithm used (below threshold)
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Cloud cover
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No image coverage
15. What is the purpose of the daily reports and why are they produced only once a year?
Hot spot detection is done every day to contribute to the prevention and control of forest fires.
The hot spot detection program is carried through during the fire season which is basically the dry season. During rainy season, high cloud cover occurs preventing observation of the Earth´s surface. And finally, there are very few, neglegible forest fires.
16. Can you get information on hot spots to work with them in a GIS?
Yes. In the statistics section for hot spots you can get the georeferenced information in a Shapefile format for each day. Also, you can get hot spots composits by month and year from the following sections.
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Access to monthly and daily statistics
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Daily statistics and access to data
17. Does CONABIO check for information on hot spots in the field?
No. Only in 1998 were some points selected to conduct a visit. However, this was not done in 1999 or 2000.
18. What is a Shapefile?
This is a format developed by ESRI, a company selling and developing GIS-systems such as Arc/Info and Arview. The Shapefile format is not topological. It stores geometric location and attribute information on geographic elements.
