There are a variety of uses for digital orthophotographs (DOPs), as was discussed in the Applications module. And there are a variety of DOP providers, as was discussed in the Sources module. Depending on the application, one DOP resource will be more suitable than the others in an area with multiple coverage. This module addresses the factors that differentiate DOPs when a selection must be made, and offers some advise toward making the best choice.
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What are the categories of DOPs?
DOPs can be categorized into three general groups based on the provider: federal, state/local, and historical. These groups are described in length in OrthoFinder's Sources module.
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Why shouldn't I just go with the DOQ for my area of interest?
If there were only one type of ortho coverage, say DOQs, for the entire state of Wisconsin, then this module would be very short. (For that matter, the entire OrthoFinder project would lose most of its value.)
While DOQs are the most abundant source of DOPs in Wisconsin, there are some areas of Wisconsin where DOQs are not yet available. (For a display of Wisconsin's DOQ availability, see this USGS Status Graphics page.)
Even if a DOQ coverage exists, it is possible another available DOP would be a better choice, depending on the application. Year of production, resolution, See the next section for details.
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What should I keep in mind to make my DOP choice?
The following list summarizes the key issues to be addressed when searching for DOPs:
- What is my area of interest?
- What are the photo acquisition years of interest?
- What resolution best fits my needs?
- Do I prefer a color DOP if it is available?
- What level of metadata exists per source?
This is the initial and often the limiting question. It the study area has multiple DOPs available, than the following questions become pertinent.
If the project is a study of the most recent conditions, there will be a point where a DOP is too old to be useful. If the study is concerned with change over time, say decades, then historical DOPs become invaluable.
Clearly if the study is concerned with feature details down to 1-foot, a 1-meter DOQ will be insufficient. On the other hand, working with DOPs that have a higher resolution than necessary becomes inefficient due to increased storage space and processing time. Thus this is a situation where one must weigh the pros of higher resolution versus the cons.
Color DOPs are typically more easily interpreted than black and white. However, color DOPs cost more to make (which will be reflected in its purchase cost) and require approximately three times the storage space of a black and white DOP of the same resolution. Another situation where pros must be weighed against the cons.
Federal and State metadata standards are quite similar, and extensive. The USGS provides Federal metadata standards online. DOPs that are made by the USGS should carry with them this metadata. Similarly, Wisconsin also provides their standards online. Here are the standards for DOPs in Dane County.
Check out the orthofinder application.
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So I made a choice of what kind of DOP I need, now where can I find them?For Wisconsin, this is discussed more on our sources page. For sources outside Wisconsin, the following list will provide a start for your search.
After you make a choice, click here:
Formats: D= Downloadable V= Viewable C= Contact info
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