Phil Austin, Positive Systems, Inc

GIS users are faced with a bewildering array of sources of imagery, with confusing and often contradictory specifications. Prior to a purchase decision, the information content, accuracy specifications, currency and cost of imagery must be evaluated to ensure that current and future needs are met.

This paper will examine the role of digital imagery within a GIS, beginning with the question "Why use images?", and follow with a full discussion of the factors which should be taken into account before purchase.

Why Use Images

Vision is mankind's primary sense of environment.

Images are international, and require no language or symbols.

Images are information rich.

GIS image layers that are spatially accurate can be used as a base layer to verify derived data from a wide variety of sources.

Information content has both subjective and qualitative elements, and is often difficult to assess from imagery specifications. Imagery which may appear on the surface to have similar specifications can have widely varying information content. Users need to consider the resolution, source, acquisition date, and resolving power to ensure imagery will meet their specific requirements.

Imagery from a wide array of sources is available at various levels of spatial correction, from raw uncorrected images through extremely high accuracy digital ortho quads. In addition to evaluating whether the imagery meets information content needs, the user must also consider the geo-correction accuracy, and how it will relate to existing and future data layers.

Many off-the shelf imagery products are available for immediate delivery, while a custom acquisition may take many months of flight operations and spatial correction preparation. Users will need to balance budgetary constraints with time to delivery of a final product and it's utility.

Scale is a concept all of us are familiar with from the realm of printed maps and hard copy images. When moved into the digital world, this issue becomes confusing - digital imagery in and of itself no longer has a scale - it is merely a collection of digital bits stored on a computer hard drive. We will examine the traditional concept of scale for film photography, how this changes in a digital world, and how specifications can impact information content.

Spatial processing of imagery for use in a GIS is often the most time consuming and expensive part of the process. We will describe emerging methods which can decrease costs of upgrades, speed time to delivery, and improve co-registration between subsequent image data sets, thus facilitating change detection.

We will examine a variety of imagery sources and their application to various GIS needs.

Imagery is playing an increasing role in GIS due to it's high information content and ease of interpretation. Imagery is available from a wide variety of sources, pricing, specification and age. This paper highlights how GIS users can evaluate any particular source of imagery as to its suitability for their particular application.