Using images as a data source for a GIS: Two different approaches
Michael Simmons
Vice President International
Tydac Technologies Inc. Bangkok, Thailand
It has unfortunately become conventional wisdom in GIS that digital maps are expressed only as vectors. If this limiting would, prior to use, either have to be converted to vector format or be used only to amend an existing vector map file. Fortunately for those GIS users who use image data, approaches are available which are not restricted to vector formats. These new methods open some very efficient and effective possibilities for the use of images in GIS.
General approach to image use by GIS:
The image is raw data; that is before processing it has neither positional information nor does it convey information about the earth's surface or sub-surface. These tasks are performed by an operator with the help of an image analysis system. The resulting processed image is geometrically corrected, positioned accurately on the Earth's surface with reference to a specified projection, and through the use of appropriate classification techniques the data may convey information on vegetation cover, geology, land use, water depth, etc. These classified, geometrically corrected images are maps.
Less and less is the photographic classified image product seen as an end in itself. These photographic products are now used primarily for visual interpretation of images prior to digital input as map data to a GIS.
In areas in which map data is scarce and even in areas in which rapid change is occurring but which are otherwise well mapped, these images may be a most significant source of map data. In extreme cases imagery may provide the only source of map data.
For maximum benefit images should always be used with conventional maps, usually produced from aerial photographs and ground surveys. These maps are converted to digital format through vector based digitising techniques. Data may already have been digitized for another purpose, and need to be transferred from a GIS different from that selected for the project under consideration.
To summarize: image analysis has come to assume a role of data input, and sometimes-primary data input, to GIS. The traditional photographic image products are now of secondary importance to digital image products. Geo-positioning of images has become of crucial importance, otherwise the classified images can not be co-registered with other digital map products in a GIS.
Conventional vector map to Image Integration
The representation of maps as vectors and the representation of images as rasters have presented difficulties for those interested in integration of maps and images. An obvious first step is to super impose vector line files over an image. This, of course will only have meaning if there is a relationship between the two data sets. A justification often made for use of imagery is to amend older maps with more current information. Examples cited include new residential streets and changes in forest vegetation. A new map is made by adding new vectors to the previous map file and perhaps by also adding new entries to the associated database.
The approaches developed to achieve this objective of map amendment are of two main types Image analysis systems have added "bit" mapping functions which allow a raster "map' to be constructed by the operator using a mouse to trace a one pixel wide white line around the boundary of regtons on the image. GIS, on the other hand, have used the image as a backdrop or reference over which the operator can trace a vector, also using a mouse. More recent improvements permit previously constructed vector files to be displayed and the vectors "dragged" to new positions reflecting the data displayed on the image.
The result of both approaches is creation of a new or amended vector file, which can be introduced to a GIS as a new map.
