Raster Tips # 1 - Choosing a raster format

To make a proper raster format choice, you should read the entire blog entry, however, having said that, my favourite is TIFF/LZW because it is every where, it has a good compression (10:1), uses little memory and is fast.

		Bits	Lossy	Files	Memory	Drawing
BMP	1:1	any	n	large	tiny	fast raw
CADRG	55:1	8	n	small	tiny	fast tiles
GIF/LZW	10:1	8	n	small	tiny	slow
JPEG	20:1	24	y	tiny	large	fast memory
JPEG2000	20:1	any	y	tiny	medium	fast tiles
MrSID	20:1	any	y	tiny	medium	fast tiles
PNG	10:1	any	n	small	tiny	slow
TIFF/none	1:1	any	n	large	tiny	fast raw
TIFF/CCITT	100:1	2	n	small	tiny	slow
TIFF/LZW	10:1	any	n	small	tiny	fast strip
TIFF/DEFLATE	10:1	any	n	small	tiny	fast strip
ECW *	20:1	24	y	tiny	medium	fast tiles

* ECW requires a 3rd party extension

In the table Compression, Memory and Drawing speed are a rough guide based on my personal experience. You should always check by running your own performance tests on your data. You may get different results which could and should affect your choice of raster format.

JPEG, JPEG2000, MrSID and ECW are lossy. They achieve high 20:1 compression by losing/replacing bits of image often undetected by the human eye. Choose lossy compression for captured imagery (e.g. scan images, digital camera photos and aerial photography) otherwise choose a non lossy compression for computer generated images such as a cartographic map. If you inadvertently choose a lossy compression for a cartographic map the edges of lines will distort noticeably producing a poorer looking map. JPEGs should be used for small photos like 1 mega pixel images. This is because when JPEGs are loaded into ArcPad they are decompressed in their entirety and can consume large amounts of memory. MrSID, JPEG2000 and ECW are hierarchical tile formats. They are fast when viewing the overall image but may slow down when you zoom in since higher detailed tiles are being retrieved and decompressed which consumes more memory and more CPU time. MrSID and JPEG2000 are supported out of the box by ArcPad whereas ECW requires a 3rd party extension downloadable from ERMapper's website: http://www.ermapper.com/downloads/plugins.aspx.

GIF and PNG are widely used on the internet and both achieve a decent compression of 10:1. These formats are ideal for small images, but are definitely not ideal for large images since both of these formats are sequential meaning they must be read from start to finish. If we zoom in to the centre of these images, ArcPad cannot jump to the right place in the file, it must always read from the beginning of the file and often has to decode large volumes of off screen data before it finds the portion required.

CADRG file format is used by the National Geospatial-Intelligence Agency and is claimed to have compression ratios up to 55:1. I suspect this figure is an overestimation due to mixing 8 bit and 24 bit file formats and the real figure lying between 10:1 and 20:1. The file sizes are small. It also does not use much memory or require much speed to draw in ArcPad. The only limitation is that the format is extremely restrictive in its geographic area and resolution.

BMP, TIFF/none are extremely fast to draw because ArcPad knows where to look in a file to retrieve exactly the right data to reconstruct an image. In addition no decompression step is needed. This format is suitable in terms of speed for both small and large images alike. Its only disadvantage is the file size and therefore it's impact on cost and size of memory cards needed to make such data portable.

TIFF/CCITT is tailored for black/white fax transmissions. It assumes that a large portion of your document is white and assigns a highly compressed binary code for such an area. Such a technique can yield up to 100:1 compression ratios, however, in practice, 50:1 may be more realistic. This format is often coded in a single strip which makes it behave like a sequential file format. This means its will suffer from the same limitations that exists in GIF and PNG in that when you zoom in to the centre of the image large portions of off screen imagery needs to be decoded before ArcPad reaches the portion needed.

TIFF/LZW and TIFF/DEFLATE are both good. They both achieve decent 10:1 compression, require small file sizes, use little memory to decompress and are relatively fast. They are strip based formats so that ArcPad can decompress the image that it needs by decompressing the right strips. The most ideal layout is to have exactly 1 row per strip. This will guarantee that ArcPad decompresses the least amount of data to reconstruct the image. TIFF/LZW is widely supported by various software applications whereas TIFF/DEFLATE (sometimes called ZIP-in-TIFF) is supported mainly by Adobe software.