This page is about Landsat-7 satellite imagery available for Canada, although some of the information may apply to Landsat images available for other parts of the world. To learn more about Landsat, and remote sensing, you can visit the Canada Center for Remote Sensing website, including their Learning Resources page , or NASA's Landsat-7 Science Data User's Handbook or Remote Sensing Tutorial. Other sources of information are the USGS Landsat Project page, the History of Remote Sensing: Landsat's Thematic Mapper page from the NASA Remote Sensing Tutorial, or this Primer on Landsat-7.
Satellite imagery for Canada is available in a number of formats, from a number of sources. There is information about the Toporama orthoimages and the NASA MrSid satellite images on my Map Comparisons page.
The Natural Resources Canada GeoGratis website has a variety of data available for free, including older Landsat-5 images for 17 Canadian cities.
Also available from the GeoGratis website is Landsat-7 Level 1-G Imagery over Canada. This data is available in Hierarchical Data Format(HDF) files. There are examples of how to use this data on John Childs' Digital Terrain Modeling and Mapping Journal website, on the Free Landsat-7 Satellite Imagery and Landsat HSL Composite from Panchromatic Band pages.
The balance of this page is about the GeoGratis Landsat-7 Orthorectified Imagery over Canada, which are cloud-free(less than 10%) orthoimages covering the Canadian landmass using Enhanced Thematic Mapper(ETM+) data from the Landsat-7 satellite. You can see some examples of this data on my Map Comparisons page. There is a GeoConnections webpage where image metadata is available.
The Landsat-7 Orthorectified Imagery over Canada are large files. The band 743 composite image files are typically in the range of 40 to 80 megabytes, the individual images for the 30 meter bands are typically over 50 megabytes, and the 15 meter band 8 image files are typically over 200 megabytes. With the exception of the band 743 composite image file, which is compressed, the images are uncompressed GeoTiff files.
If you compare the X/Y pixel dimensions of the GeoTiff image for a scene in both the Lambert Conformal Conic projection and the UTM projection, you may find them to be different sizes, however, the area of the satellite image is identical in both. If you look at this thumbnail image, which is using the Lambert Conformal Conic projection, and imagine rotating the satellite image so it was more 'straight up and down', then the overall size of the image could be smaller, which is the case for this same scene when using the UTM projection.
The Landsat WorldWide Reference System (WRS) catalogues the world's landmass into 57,784 scenes, each 115 miles (183 kilometers) wide by 106 miles (170 kilometers) long. Each scene has a Path (orbit) number and Row (scene center) number.
In order to obtain the GeoGratis Landsat-7 imagery, you either have to use the GeoConnections Discovery Portal to perform a search, or else know the Path and Row numbers, which you will need for FTP or HTTP access. If you have the latitude and longitude, you can use the USGS WRS - Lat-Long Converter to get the Path and Row.
You can access the images via the GeoGratis FTP server at ftp-geogratis.ccrs.nrcan.gc.ca using anonymous FTP. The initial directory to navigate to is /landsat_7/ortho/geotiff. The next directory level determines the image projection - lcc for Lambert Conformal Conic, utm for Universal Transverse Mercator. Within the lcc and utm directories are directories with 6-digit names, made up of the 3-digit Path number(with leading zeros) and 3-digit Row number(with leading zeros). It is possible that some are actual directories, and some are links to directories, so you may have to check your FTP program's listing closely, because some FTP programs present such a directory listing with the two types separated.
Because the data capture, and scene generation, is ongoing, it is possible that the scene you are looking for doesn't yet exist on the server. If your desired scene does exist, go into it's directory. The naming conventions for the files are described in the /landsat_7/ortho/doc/readme_lisezmoi.txt file.
For example, the GeoGratis Landsat-7 composite satellite image shown on my Map Comparison page is for Path 47 and Row 26, and I used the UTM projection image, so it's directory is /landsat_7/ortho/geotiff/utm/047026. The image was extracted from the 047026_0100_000730_l7_743_utm10.tif.zip file. In the same directory is the file used for the GeoGratis Landsat-7 Band 8 satellite image shown on the same page, which also used the UTM projection image, 047026_0100_000730_l7_08_utm10.tif.
There are 'thumbnail' JPEG images available on the server for the band 743 composite images, in the /landsat_7/ortho/images directory. Once you know the name of the scene's composite image file, you can access the JPEG files directly by replacing the last part of the filename (after the "743") with "_thumb.jpg" for the smaller thumbnail image, or "_browse.jpg" for the larger thumbnail image. The thumbnails for the above example are /landsat_7/ortho/images/047026_0100_000730_l7_743_thumb.jpg and /landsat_7/ortho/images/047026_0100_000730_l7_743_browse.jpg. These thumbnail images were created from the Lambert Conformal Conic composite images, and may have been distorted in order to all be the same size.
You can access the same data as via FTP by pointing your web browser to http://geogratis.cgdi.gc.ca/download/ and starting at the /landsat_7/ortho/geotiff directory.
The 'thumbnail' JPEG images mentioned above can be accessed via your web browser by using URLs such as http://geogratis.cgdi.gc.ca/download/landsat_7/ortho/images/047026_0100_000730_l7_743_thumb.jpg for the smaller thumbnail, or http://geogratis.cgdi.gc.ca/download/landsat_7/ortho/images/047026_0100_000730_l7_743_browse.jpg for the larger thumbnail image. You may not be able to use your browser to list the /landsat_7/ortho/images directory contents.
You can use the GeoConnections Discovery Portal as a means to find out the Path and Row numbers for an image, but it also provides easy access to the thumnail JPEG images, and, if you are willing to complete the free registration process, will allow you to order the image with format translation, scene sub-setting, and a choice of projection and compression.
The GeoConnections Discovery Portal makes use of Java, as well as Javascript, so some functionality may be missing, or problematic, if either or both are not available in your browser, or have been partially disabled. In addition, there are some bugs in the GeoConnections Discovery Portal website.
To fully make use of the satellite image data, you need some knowledge about a variety of topics, but to just use the images as "maps", some basic steps should suffice.
The first thing is to understand that what you see in an image depends on what data was captured, and how it was processed. For example, you can see an example of one of the band 743 color composite images on my Map Comparison page. If you were to download the individual files for bands 7, 4, and 3, and combine them together as red, green, and blue, you wouldn't get the same result. This is because the color composite images available from GeoGratis have had some image enhancement applied. As part of the image generation, a process called histogram trimming was applied, which dropped the low and high 5 percent of the sample for each band. The remaining data then had a linear stretch applied to resample it back to a range of 0 to 255.
If you look at the Landsat-7
Band 8 image
on my Map Comparisons page, you may consider it to be too dark to be usefull.
As with the band 743 color composite image, it can make a substantial difference
if some image enhancemnt is applied. The image above shows a portion of
the band 8 data as a black and white image. The left half is the image with no
enhancement, and the right half shows the same portion of the image, with some
basic image enhancement applied.
You should be familiar with the basics of how a Landsat-7 color composite image is produced, as outlined in this article on Three Band Color Composite Imagery. Note that on page 3 of the article it covers band 743 composite images, the combination used for the GeoGratis Landsat-7 color composite images, and band 742 composite images, the combination used for the NASA MrSid color composite images.
There are a variety of methods that can be used to produce a 3-band composite image, including the use of image-editing programs such as Photoshop. One possible limitation of programs that are oriented towards image-editing is that they may not preserve the geo-referencing information in the GeoTiff images.
Chips, the Copenhagen Image Processing System, is a general-purpose software package for remote sensing image processing, and the standard version of Chips for Windows is now free. I've listed below some basic steps for using WinChips to produce a Landsat-7 3-band color composite image. WinChips is a powerfull program, and these are just the minimum steps needed to do a basic task. The WinChips User Guide is 460 pages, if you need more information. The steps below use the menus, but in some cases there are other ways to do the same step, such as toolbar buttons. These steps explain how to combine the images from three bands of the same resolution(e.g. 30 meters), so using band 8(15 meters) or either of the band 6 images(60 meters) is not covered.
Open WinChips, and from the File menu, select New Project. From the File menu, select Add Object(s), and select the three images for the three bands you want to use in your composite image, and add them as Chips Images. From the File menu, select New Object, then Lookup Table, then save the new Lookup Table file. From the View menu, select New Image View. In the Image View Configuration dialog, on the Image tab, select Display Mode as RGB. For each of the (Red, Green, Blue) choices, select the image for the band you want to use for the respective color. For Lookup Table select the Lookup Table you added earlier, then click OK.
You should now have a Default Image View window open, showing your 3-band color composite image. From the Image View menu, select Lookup Table, then Enhance using Statistics for Entire Image. Once the statistics are calculated for each of the colors, a Modify Lookup Table dialog will be presented. To see the effect of some basic image enhancement, click on Hist. Equal.. You can use Reset to undo the enhancement. Click on Cancel to close the dialog window.
To save the 3-band color composite image, from the Image View menu select Export View To Standard Format. Once you have saved the image file, you have the option of saving geocoding for the image.
OziExplorer doesn't support the automatic import of map images that use the Lambert Conformal Conic projection, so it makes more sense to use the UTM images. OziExplorer can read the geo-referencing information from the GeoTIFF image files, but you need to make sure you have the "Geotiff Support File", available from the OziExplorer Optional Extras page.
Because the GeoTIFF images are uncompressed, and the band 743 composite images are 24-bit color, the images as received from GeoGratis may not be in the best format, taking up extra disk space, and/or having to be fully loaded into memory. It may be worthwhile to consider converting them to another image format. One approach is to first import the GeoTIFF image into OziExplorer in order to create the map calibration file, convert the GeoTIFF image (keeping it the same size), and then use OziExplorer to change the map calibration file to point to the converted image.
If you use steps similar to those outlined above to create 3-band color composite images, the easiest way to use the image will be if you don't change the size or orientation of the composite image. Then, instead of using geo-reference information such as the Arc/Info world file produced by WinChips, you can import one of the GeoTiff images for the three bands, then use OziExplorer to change the resulting map calibration file to point to the 3-band composite image.
