Incorporating imagery into a GIS workflow
The potential uses of imagery and the ArcView Image Analysis extension
within the ArcView GIS environment are nearly limitless. The software is
most often used for natural resource applications such as forestry and
agriculture, environmental assessments, infrastructure management, and
updating and maintaining GIS databases. Whatever application area you are
working in, the use of imagery tends to fall into four workflow
categories: display, map input and revision, analysis, and change
detection.
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| Applications of ArcView Image Analysis are likely to fall into four workflow categories. |
The most basic use of imagery is to display it and look at it. In fact, visually analyzing an image is typically a necessary step before the image is used in any other way. Visual analysis can be cursory or detailed. At the least, you might ask, "Am I looking at the correct image?" To answer this question, you’d probably identify a few major geographic features in the image, such as a city, river or waterbody, a unique highway intersection, or a range of hills or mountains.
Chances are, though, you’ll perform a more thorough visual examination of the image. A great deal of information can be gained from visually analyzing an image, particularly if you’re familiar with the area or have been trained in image interpretation techniques. For example, you might be able to guess the season of the year by the state of the vegetation and your knowledge of the local climate. In spring, for example, deciduous trees may not have leaves, and agricultural fields may be fallow.
While images are powerful, and often beautiful, to look at on their own, they can provide orientation and supplemental information when displayed as a background behind other GIS map layers.
You will be examining a variety of images as you go through this
course. Along the way, you’ll gain experience and knowledge that will
improve your ability to interpret what you see in an image.
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| Images used as a background on maps can provide orientation and aid in map interpretation. |
Imagery, especially aerial photography and satellite imagery, is commonly used to create and update map databases. For example, streets, highways, building footprints, pavement markings, and other types of infrastructure are conveniently mapped using imagery. Imagery is used to compile and update natural resource inventory maps such as forest stands, habitats, and so on, especially in remote regions. Imagery is also an ideal source to update maps after a natural disaster has occurred or in areas where extensive development has taken place.
There are many types of imagery to choose from with a wide range of
scales, spatial and spectral resolutions and data accuracies that can be
used for map input and revision. Using the tools in Image Analysis and
ArcView, you will be able to use imagery to identify and update changes in
your database, as well.
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| Imagery is commonly used to update GIS vector data sets such as adding this new development to a city street shapefile. |
Analysis represents the next level of sophistication in the imagery workflow. Behind the many shades and colors of an image displayed on a computer screen are numbers. The digital nature of imagery allows us to use mathematics to perform both quantitative and qualitative analyses on the data. For example, with ArcView Image Analysis you can calculate a vegetation index from an image, create categories from an image, smooth or sharpen an image, and compare two different images.
Performing quantitative analysis on imagery allows you to take
advantage of the power of the computer. When used in concert with other
techniques such as visual analysis and ground
truthing, quantitative image analysis can provide highly accurate
information about an area. Quantitative analysis of satellite imagery for
extensive areas such as continents, countries, or ecosystems enables you
to create and update small-scale maps relatively quickly. The growing
season map of Africa shown below is an example of an environmental
baseline map that was developed through image analysis.
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| Growing season map of Africa. Satellite imagery is ideal for creating environmental baseline maps like this one because of its extensive coverage and repeatability. |
The fact that imagery can be acquired repeatedly and archived allows us to take image analysis a step further: to detection and monitoring of changes over time. Change detection can be performed at any level, from visually comparing two images to performing extensive quantitative analysis on the images and then detecting changes between the analysis results.
Using imagery to monitor changes is a widely used management tool. For
example, it is used to manage natural resources such as forests,
watersheds, coastal zones, agricultural soils, and rangeland. Change
detection is also key to local and regional land use planners and those
monitoring natural hazards.
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| Identifying and quantifying changes to the landscape over time is a key application of image analysis. |