Uniform Blocksize Experiments

in Chapter 3

The five test DEMs were subjected to a JPEG-like lossy compression algorithm using a variety of different block sizes (ranging from 4x4 cells to 64x64 cells), different minimum overlp between blocks, and different "quality" levels, where the higher the quality number, the greater the compression (and lower the quality, oddly enough).

All of the following "map tables" show the difference between the original DEM and a set of degraded DEMs created using different block sizes, minimum overlap specifications, and quality levels. In each table,  row One is quality 1, row Two is quality 2, row Three is quality 4, row Four is quality 8, and row 5 is Quality 16. The color scale for images in each table is the following:
 
 

Cactus, California

Minimum overlap of 0 pixels. Blocking artifacts dominate the higher quality settings and smaller block sizes.
 

>10

10 - 4

4 - 2

2 - -2

-2 - -4

4 - -10

< -10

 

Blocksize 4

Blocksize 8

Blocksize 16

Blocksize 32

Blocksize 64

Minimum overlap of 3 pixels. Error is reduced for the smaller block sizes. Larger sizes are unchanged. Blocking error is largely eliminated.
 

Krypton, Kentucky

Minimum overlap of 0 pixels. Blocking artifacts are present for smaller block sizes, but error is much higher overall than Cactus.
 

>10

10 - 4

4 - 2

2 - -2

-2 - -4

4 - -10

< -10

 

Blocksize 4

Blocksize 8

Blocksize 16

Blocksize 32

Blocksize 64

Minimum overlap of 3 pixels. Blocking artifacts are eliminated, but were not a major component of error.
 

Looney, Virginia

Minimum overlap of 0 pixels. Blocking artifacts are present for smaller block sizes. Low error even for large blocksizes, but in increases at higher quality settings.
 

>10

10 - 4

4 - 2

2 - -2

-2 - -4

4 - -10

< -10

 

Blocksize 4

Blocksize 8

Blocksize 16

Blocksize 32

Blocksize 64

Minimum overlap of 3 pixels. Blocking artifacts are eliminated, and little error remains for the smaller block size / lower quality setting images. At high blocksize/quality settings artifacts take on a characteristic "ripple" look; compare with McHenry's Peak, for example.
 

McHenry's Peak, Colorado

Minimum overlap of 0 pixels. Blocking artifacts are present in the small block size columns.
 

>10

10 - 4

4 - 2

2 - -2

-2 - -4

4 - -10

< -10

 

Blocksize 4

Blocksize 8

Blocksize 16

Blocksize 32

Blocksize 64

Minimum overlap of 3 pixels. Blocking artifacts remain in this high relief DEM.
 

Saline, Michigan

Minimum overlap of 0 pixels. Blocking artifacts are present for smaller block sizes. Error takes on the characteristic "ripple look" in the lower right-hand portion.
 

>10

10 - 4

4 - 2

2 - -2

-2 - -4

4 - -10

< -10

 

Blocksize 4

Blocksize 8

Blocksize 16

Blocksize 32

Blocksize 64

Minimum overlap of 3 pixels. Blocking artifacts are largely eliminated.