3.4 Images and Graphics

Table Showing Main Differences Between GIF, JPEG and PNG Image Formats

	PNG	JPG	GIF
Format	24-Bit	24-Bit	8-bit
Colours	16 Million	16 Million	256
Compression	Lossless	Lossy	Lossless
Best For	Multiple Edits	Photographs	Simple Images

GIF (Graphics Interchange Format) is an image format developed by CompuServe in 1987. Its method of indexing using a colour lookup table and specifying the value of each raster cell as a position within the table makes it particularly suitable for images containing large areas of limited colours, as repetitions of sequences of bytes can be referenced according to their previous occurrence thus saving space. GIF is not a suitable format for complex images such as photographs. Complex images with smooth variations in colour and tone such as these are best suited to the JPEG file format, such named as it was developed by the Joint Photographic Experts Group. The 24 bit format allows for over 16 million colours. However, the compression does sacrifice some of the original data making it unsuitable for important images or those which will undergo multiple edits. The PNG (Portable Network Graphics) format seeks to combine the benefits of the jpg format with the compression rates of GIF by using a 24-bit format and the indexed colour method of storage. Because this format is lossless it is particularly suited to images which need to be preserved in their entirity.

GIS makes use of all three of these image formats both in bringing images into GIS for spatial enrichment (georeferencing) and exporting map documents for printing or use in other applications such as Microsoft PowerPoint. In addition GIS systems make use of other image formats such as BIL, IMG and JPEG 2000.

Below is the same map layout in JPEG, GIF and PNG image formats. All files are 99KB in size. As you can see, the JPEG has not coped well with the resizing.
                                                                             JPEG

GIF

                                                                             PNG