The previous couple of articles have investigated free sources of high quality satellite imagery providing suitable overlay material for the creation of digital terrain models. The two free data sources described offer coverage for most of the world’s surface. However, there are still some large gaps as discussed in the previous articles, for example all of Russia and much of South America. The next data source, NASA’s EarthKam imagery taken from the Space Station can help fill in those gaps when no other source is available. This data can be obtained from NASA's ISS EarthKam DataSystem Website
This data set has severaL drawbacks as compared with the data sets discussed in previous articles. The coverage, although evenly distributed across the earth’s surface, is very sparse. Also, in many of the images, the surface is completely covered with clouds. (Judging from information on the website, many of the images were produced "on-demand" from requests by school children in the United States. It looks like many of these demands were filled by simply pointing the camera out the window and shooting the target even if cloud coverage was 100%. Perhaps NASA would do better to develop a systematic procedure for acquiring high quality data rather than wasting film on a public relations gimmick.)
The data is not in fact satellite data in the true sense. There is only one sensor, i.e. a visible spectrum camera apparently hand held by the crew member. As a result, there is no uniformity or predictability as to how the camera is trained on the earth’s surface. In short, compared to precision satellite data acquired from ASTER or Landsat it is the equivalent of a vacation picture taken by leaning out of the car window with a Kodak Disposable.
The best thing about EarthKam is the novel user interface. You start at the ISS EarthKam DataSystem Website. Several search options are provided. Although you can ‘Search by Country’, this is not the most useful option. Instead, choose ‘Geographic Search’. Zoom in on your area of interest by successively clicking on the map. (Note: to zoom in you must click on the map, not on one of the colored diamonds. If you hit a diamond by accident, an image will come up rather than a zoomed map.)
Once you have zoomed in sufficiently you can see if coverage exists for your area of interest. If you move your cursor over the image without clicking on it you will see a preview thumbnail. Now you can click on the ‘Metadata’ link in order to view the metadata for the image, including the coordinates of the image center, image height and width, and the maximum available resolution. The latter varies but seems to be around 30 to 50m/pixel.
If the image is acceptable, click on ‘High Resolution’ to get the most zoomed in view. You can change the viewing area by clicking on various areas of the thumbnail. Your actual viewing area will appear as a red rectangle on the thumbnail.
The next interesting feature is the ‘Maps of Image Area’ selection. Clicking on this option presents the user with a NIMA Jet Navigation Chart for the area covered by the EarthKam image. By selecting the ‘Image on JNC’ link, your satellite image will be overlaid onto the JNC map. This helps in the georeferencing of the image.
The image is typically presented with North pointing some direction other than up. This is done in order to save a little image space, although the savings should be minimal if JPEG or similar compression is used. After overlaying the image you may wish to view it with North pointing more conventionally. This can be done by selecting ‘Rotated Image’. In addition to your aligned image, a small icon appears showing the position of the sun when the image was taken. Apparently you cannot download the rotated image (which I would consider unrotated) but you can screen capture it at medium resolution. For maximum resolution, zoom in to the area of interest using ‘High Resolution’ and then click ‘Download’. You will be offered several selections. For practical purposes, you will probably only be interested in the Zoomed Image using the JPEG radio button. (Downloading will yield a superior image as compared copying the image from the image viewer. Since the rotation angle is provided in the metadata, the orientation of the image with respect to true north can be re established after the zoomed image is downloaded.)
The first sample shown to the right is a region of the Himalayan foothills on the China Nepal border centered at 30.2N 82.5E. Image height is 62.2 km and image width is 85.3 km. Maximum resolution is 27 meters/pixel. The image is striking in its total lack of cloud cover, which occurs more frequently north of the range and in the dry season before the monsoon (which starts at the end of June). This image, taken in February, 2000 enjoys particularly cloud free conditions. The second image shows the rotated image superimposed on the JNC chart. The third image is a section of the overall image at maximum resolution.
It appears as if images are still being added to the data set as the latest ones are dated March, 2002. Hopefully NASA will fill in the coverage and produce more cloud free images according to some type of acquisition plan so that this can become a more meaningful data set for amateurs and researchers.