Isle of Wight - Image

Isle of Wight - Landsat Image N-30-50_2000


I recently had a visitor to this site request Landsat ETM+ imagery so they might digitize shorelines to create a map. Global Mapper gives me an option to save a screen shot, along with JGW and PRJ files which allows the image to be loaded into it's proper lat/lon grid. I took the image of Isle of Wight and zoomed to a scale of 1:150,000. I knew there would be image degradation, but I hoped it would be of small significance for my digitizing purposes.


JPEG, MrSid and TIFF

Isle of Wight - Image Detail (JPEG, MrSid and TIFF)


I am spoiled. I work with Landsat ETM+ images for much of my Enhanced Vector Shoreline (EVS) digitizing. All three images started at a scale of 1:150,000. The detailed images are zoomed to a scale of 1:12,500, where I typically do most of my digitizing. Anti-alias pixel smoothing was applied to all three images. I know that this algorithm is a "black box" smoothing technique. I happen to prefer it over the clunky pixel format. But that is another comparison.

The pristine central image is MrSid format. This is a proprietary image compression format, developed by LizardTech, that shrinks Landsat TIFF images into manageable file sizes. MrSid does to TIFF format what MP3 does to WAV format. In other words a tremendous reduction in file size. The top image is JPEG. I use this format for all of my EVS-Islands images. It shrinks the image typically to about 200kb or less. The bottom image is TIFF. This format, reputed to give the "best" image, I seldom work with.

It looks as if both JPEG and TIFF images degrade (blurry). If my work did not require extremely precise digitizing, these images, at this scale (1:12,500), would be fine. Between the JPEG and TIFF images, the TIFF image seems to offer the greatest clarity.

Should my visitor use either image (JPEG or TIFF) to do their digitizing? That is their choice. Both formats give one "okay" clarity. On the other hand, when you have used MrSid format, it is tough to recommend anything else. For the digitizing work that I do, MrSid format is the best available.


I am not a photogrammetrist. If you are, I am certain that there are many strong arguments for using TIFF imagery (original and uncompressed) versus MrSid imagery (2nd-generation and compressed). Any time you manipulate imagery you risk degradation and MrSid formatting manipulates imagery. However, I can see that the superior image for my work is MrSid format. I work with limited storage and MrSid formatted imagery fits easily onto my hard drives. My preferred mapping program, Global Mapper, effortlessly handles these MrSid formatted images. Once loaded, I can work at 1:12,500 scale throughout the world using MrSid format. In short, my EVS World Map will be made using MrSid formatted Landsat images.


10/20/2006 - More and more 1-meter resolution or better imagery is freely available. The Isle of Wight, used in the above example, is imaged at 1-meter or better resolution on both Google Earth and Windows Local Live. However, my last line in the previous paragraph still holds true, the Landsat ETM+ 2000 imagery, which is worldwide in scope, is still my choice for constructing an EVS precision World Shoreline Map. I am keenly aware of NGA's Prototype Global Shoreline (PGA) which used Landsat ETM+ 2000 imagery to construct a Global Shoreline. I have also demonstrated in a number of posts that EVS offers a more precise depiction of the world's shorelines

Enjoy!

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