EVS-Islands

Some time ago, I did a comparison of images taken from a Landsat ETM+ mosaic - JPG, TIFF and MrSID. I thought I knew what I was doing, but I didn't. Finally, after many months, I intend to right my wrong.

This comparison looks at images taken from a Landsat ETM+ mosaic (N-32-65_2000). It looks a whole lot better if you select the image and follow it to my Flickr EVS-Islands area. Look at these images full size (1280x1024). After studying them for a few minutes, you will be able to discern differences in colors and clarity of details. The master image is the Landsat ETM+ (N-32-65_2000) in MrSID format. The images derived from the master image are in JPG, TIFF 8-bit and TIFF 24-bit. TIFF images tend to be larger (MBs to KBs), sometimes much larger (GBs to MBs) than JPG images. All second sets of images use an effect found in Global Mapper called Anti-Alias. It smooths the pixels, using an industry standard algorithm. I prefer to map with this effect on, as it gives a smoother shoreline to follow when I digitize. On with the visual comparison.

MrSID and JPG

Tranoy Area NO - Landsat N-32-65_2000 (1-10,000) MrSID (L) JPG (R) without Anti-Alias

Tranoy Area NO - Landsat N-32-65_2000 (1-10,000) MrSID (L) JPG (R) with Anti-Alias

(Again - look at the full-sized images in Flickr EVS-Islands - select the image, in Flickr, select "All Sizes" and you will be viewing the image at it's original size of 1280x1024.)

JPG has noticeable loss of detail. When I look at the image without Anti-Alias, it appears fewer colors are used in the image creating the illusion of larger pixels. I use JPG when I post images, because the files are many times smaller in size (KBs vs MBs) and the clarity is sufficient for viewing.

MrSID and TIFF 8-bit

Tranoy Area NO - Landsat N-32-65_2000 (1-10,000) MrSID (L) TIFF 8-bit (R) without Anti-Alias

Tranoy Area NO - Landsat N-32-65_2000 (1-10,000) MrSID (L) TIFF 8-bit (R) with Anti-Alias

TIFF 8-bit is an improvement over JPG, but larger in size. The TIFF format is typically much larger in size. MrSID came about because of gigabyte sized TIFF images. MrSID format could shrink the TIFF images into something more manageable and viewable. That said, TIFF 8-bit gives a wider range of colors, doing a better job with subtle shading. The image is clear and almost as good as the original MrSID image. It is a good duplication of the MrSID base image.

MrSID and TIFF 24-bit

Tranoy Area NO - Landsat N-32-65_2000 (1-10,000) MrSID (L) TIFF 24-bit (R) without Anti-Alias

Tranoy Area NO - Landsat N-32-65_2000 (1-10,000) MrSID (L) TIFF 24-bit (R) with Anti-Alias

TIFF 24-bit is as good as it gets compared to the other two options (JPG and TIFF 8-bit). When viewing the original size image, it is difficult to distinguish between the MrSID image and the TIFF 24-bit image. The colors most nearly duplicate those found in the MrSID image. Clarity and sharpness of detail are a near mirror of the MrSID image. It is a solid reproduction of the MrSID master image.

If the size of the image doesn't matter, but the clarity of details is important, the TIFF options are far superior than JPG. However, if size of the image is of concern and the sharpness and clarity of details is not, go for the JPG.

Okay, I know that all of you image specialist are guffawing at my layman's descriptions. However, most of my viewers are laymen, and a few laywomen. They appreciate the non-technical descriptions.

Enjoy!

Labels: Flickr, Global Mapper, JPEG, Landsat, MrSid, TIFF

Posted by Mr Minton at 7/12/2007 10:16:00 AM | Permalink | Comments | links to this post

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.

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!

Labels: JPEG, Landsat, map, MrSid, TIFF

Posted by Mr Minton at 10/20/2006 06:20:00 AM | Permalink | Comments | links to this post