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NWTCG NOAA AVHRR Information Site Frequently Asked Questions Questions Can I use this data with other programs such as OziExplorer? What are the best files to look at? Why are the images black? How do I use it? How often is it updated? What do all the colours on the images mean? What do fires look like? What did other dates in year 2000 look like? What other images are available? Why is the image so muddy/unclear? Why does it not fit on my screen? Why does it say file not found? How does all this work? What is UTC Time? What "day" is it? What else can it do? What are the image projection parameters Related links Can I use this data with other programs such as OziExplorer? Yes you can. Many GIS programs will use Geotif data. The projection information that you will need can be found at the bottom of this page. A warning the tif files are large, most are between 1 and 8 megabytes. I have created OziExplorer "map" files for the TIF files on this site. To use these files with OziExplorer download OziExplorerAVHRR.zip maps and save the contents to the "OziExplorer\maps" folder. OziExplorer needs the ".map" file to describe the TIF files. To view a particular image right click on the desired TIFF and select "Save target as" save the file to the same "OziExplorer\maps" folder that you placed the ".map" files. You can then do "Map / Open" in OziExplorer to load the file that you saved. Please remember that the AVHRR files are not precision geometrically corrected and there may be positional errors typically up to 10 Km. Rarely images will be totally incorrect. Return to questions What are the best files to look at? In the fall and winter the best images are the thermal images. The visual images are usually not useful because the sun is too low (or not present) in the sky. In the summer time both image sets have useful information. The thermal images show fires very well and the visual images show vegetation ore greenness. Return to questions Why are the images black? The reason that part of the image is black - Each "picture" is a composite of all the images for that day. The composite is cleared at the start of each day so parts of the image that have no data yet for that day are black. The "visual image set" requires sunlight and does not produce an image if the sun is too low in the sky. During the spring, fall and winter the sun angle is too low and it is possible that no valid "visual" data may be present in a pass. There is also a "Thermal" image set that uses the AVHRR thermal channels. These images do not require sunlight and may contain useful information. During the winter the thermal only AVHRR band combination of 4,4,5 is used. During the spring it's 1,2,3. During the fire season it's 3,2,1. Channels 1 and 2 are "visual bands". Return to questions How do I use it? The "Image set list" links point to jpeg and tiff files from recent NOAA AVHRR satellite passes. The reference names are in the form of: Day DDD Hour HHMM. Where DDD is the UTC Day-of-year and the HHMM is the UTC time-of-day that the satellite rose over the horizon (relative to Yellowknife). You must refresh this page to view the current images. The jpeg files can be viewed by your browser. The tiff files require additional software. Select an image date:time from the list of images in the top left frame. Select an image area and file type (I suggest you select from the jpegs.) Return to questions How often is it updated? The images are updated as they are received. The reception of any pass can not be guaranteed. The time provided in the "Image set list page" is the time the next satellite pass starts. Allow an additional 1 hour for pass reception and file processing. Subtract 6 hours to convert UTC time to MDT time. The update frequency will change without notice. Return to questions What do all the colours on the images mean? Have a look at our sample images with descriptions There are several band combinations that are used at different times of the year. The "Visual" set uses AVHRR bands 1,2,4. The visual set shows the cloud, snow, water, land and ice best. The sun must be at least 5 degrees above the horizon when the image is captured for valid data to be captured. This set is used all year. During the winter the "thermal" set uses 4,4,5 which do not require sunlight. Since these do not require sunlight valid data is available even when the sun is low or not present What you are seeing is close to the "surface temperature" of what ever is under the sensor. Warmer items are darker. You will notice that the land is colder than the water. The higher cloud is usually colder than lower cloud and snow. During the late spring the "thermal" set uses AVHRR bands 1,2,3. This combination is very similar to the "Visual" set. During the late spring the "thermal" set uses 3,2,1. In this set the recent burns usually show as dull red especially late in the day. Active fire shows as bright red often with smoke evident. Return to questions What do fires look like? Here are some samples from 1999 fire season. Select this link then pick a date from the "Fire image set list" in the top left frame. Look in the "thermal" images for red spots on some of the images. If you look at a number of dates you can see fires appear, get larger and go out. Sometimes fires are obscured by cloud and do not appear. Sometimes it is difficult to distinguish recent burns from current burns. Look for smoke plumes on current burns. Return to questions What did other dates in year 2000 look like? Sample images from 2000. Return to questions What other images are available? have a look at 5km decimated image jpgs from the archives Return to questions Why is the image so muddy/unclear? The images are large in many respects. They have many colours, lots of lines and pixels and are many kilobytes or megabytes in size. They look much better on the computer screen than on a printer. They also look better if your computer system can display more than 256 colours. To see what your computer is configured to display do the following: (these instructions are for Windows 95/Windows NT computers) Click on start/settings/control panel, then "display" and select the "settings" tab look at the "colour palette" value and the "desktop area". Higher numbers are better. You can try changing them, but don't call me if it causes you other problems. The TIFs have better detail than the JPGs but are much larger. The JPGs range in size from 300-800 Kilobytes the TIFs can be several Megabytes. You will also need to have a program that can view them. PCI has a "Freeware" viewer that you could use called handler (about 10 Megabytes in size). The tiffs can also be imported into other graphics programs as well as Spans GIS and Arc/View The HTML pages were tested with Internet Explorer 5.0/5.5 and probably don't work very well on Internet Explorer 3.x or early versions of Internet Explorer 4.x. They seem pretty good on Netscape Navigator 4.6 Return to questions Why does it not fit on my screen? The images are bigger than your screen. See the note about muddy/unclear pictures. If you want to see the image in a large window then "right click" on the image reference and select "open in new window". Return to questions Why does it say file not found? You must refresh to load the current "image set list" (the green table) (click reload/refresh or press "F5")Only the most recent 20 images are kept on disk. The "Image set list" is updated after every pass to remove the old image and add the new image. If you have a stale image set list then the images it refers to may not exist any longer. The image set list will update automatically every 60 minutes. Return to questions How does all this work? The National Oceanic & Atmospheric Administration (NOAA) has a series of polar orbiting satellites. The data on this site is derived from these satellites. All the work done on these images is done automatically and has not been verified in any way. Some things you may note is that these images do not always line up correctly and sometimes very little is recognisable. This is due to automatic nature of the processing. I can only put a limited amount of "smarts" into the process. Currently my procedure is in place to receive 6-10 images per day of NOAA 17 and 18. (There are also images from NOAA 12 in the archives.) The images are: Received by the Satlantic AVHRR receiving station located on the Scotia Centre, Yellowknife, NT Imported to PCI Geomatics image analysis system. Calibrated using methods from NOAASIS and the Canada Centre for Geomatics. The thermal channels are converted to radiance and the visible channels are converted to percent reflectance. Clipped to a 55 degree satellite viewing angle and 85 degree solar zenith angle. Geo-corrected to various scales and geographic areas, composited with the previous images of that day. Various versions of bands 1,2,3,4 and 5 are exported as JPG and geo-TIF for viewing. At midnight (MST time) I clear all the channels so you only see data from the current day in any image. During the winter this may mean that some visual channel sets are all black. The time of the next pass is the time the satellite rises above the horizon. The satellite pass takes 10-15 minutes. At the end of that time the file is copied to the PCI system. Processing by the PCI procedures usually starts with in 5 minutes of the completion of the satellite pass. The PCI system processing time varies from 10 to 30 minutes and is determined by other processing loads on the system. The unprocessed data is archived to tape. The oldest processed files that you see on the web are deleted when a new image is processed. I keep the 20 most recent images on disk. I can retrieve from tape and reprocess files as needed for specific projects. All of this happens automatically. I check the system occasionally and receive alerts if there is a system failure. Although it is quite reliable it does stop occasionally. If this happens on a weekend it won't be fixed until I'm back to work on Monday. All the images collected on the weekend will be processed before new images are processed so this will also delay the Monday images. Depending on the cause of the disruption some images may be lost. Return to questions What is UTC Time? All dates and times are Universal Time Coordinated or UTC. UTC time is 6 hours ahead of Mountain Daylight Savings Time (MDT)(summer). Subtract 6 hours to convert from the times given to get MDT. Subtract 7 hours to get Mountain Standard Time (MST)(winter). Return to questions What "day" is it? This table shows you the "day of year" and date for leap years and non-leap years Return to questions Image Projections The TIF images are PCI GeoTIF, so some software will be able to read the projection data right out of the image. 4-km NWT Lambert Conformal Conic Origin 105.00W 50.00N Standard Parallels 60.00N and 75.00N 2500000.00 x-offset, 0.00 y-offset NAD 83 Pixel size 4000m by 4000m 1-km NWT Mainland Lambert Conformal Conic Origin 115.00W 0.00N Standard Parallels 60.00N and 66.00N 0.00 x-offset, 0.00 y-offset NAD 83 Pixel size 1000m by 1000m 1-km Inuvik Lambert Azimuthal Equal-Area True origin 135.00W 72.00N 0.00 x-offset, 0.00 y-offset NAD 83 Pixel size 1000m by 1000m 1-km Yukon Albers Conical Equal-Area True origin 132.00W 59.00N Standard Parallels 62.00N and 66.00N NAD 83 Pixel size 1000m by 1000m 1-km Great Slave Lake Lambert Conformal Conic Origin 113.00W 55.00N Standard Parallels 62.00N and 64.00N 1000000.00 x-offset, 0.00 y-offset NAD83 Pixel size 1000m by 1000m 5-km Canada Albers Conic Equal-Area True origin 114.0W 45.0N Standard Parallels 62.00N and 66.00N 0.00 x-offset, 0.00 y-offset NAD 83 Pixel size 5000m by 5000m Return to questions What else can it do? There are many possibilities for AVHRR data. If you have a specific need that is not addressed by these images please contact the Centre. We offer services that may be able to address your needs. Return to questions Related links NOAA AVHRR (HRPT) images available from the Canadian Meteorological Centre (CMC) NOAA, the US National Oceanic and Atmospheric Administration NWT Centre for Geomatics AVHRR Home http://www.gnwtgeomatics.nt.ca/RemoteSensing/avhrr/photo.asp For more information or to make any comments contact gnwtmaps_admin at gnwtmaps_admin@gov.nt.ca NWT Centre for Geomatics 5102-50th Ave Yellowknife NT X1A 3S8 CANADA | GNWT Home | ENR Home | ITI Home | | Geomatics Home | Expertise | Tools | Data | | AVHRR Satellite Images | AVHRR Archived Images | Mandate | Contacts | | Why Login |