Thursday, May 29, 2008

Mapping Accuracy

The focus of today’s post is the importance of mapping accuracy and how it relates to geospatial data. Personally I think accuracy is one of the most difficult and misunderstood aspects of our industry. How many people out there in the geospatial industry have used data (vectors, orthos, or other spatial data) where you have no definite information about the accuracy of the data? How often can people confidently state that their data conforms to an accuracy standard? Furthermore, how many people have detailed information on the lineage of their data? For example, if you have a “roads” vector layer – where did it come from? Was it collected from high-accuracy aerial photography (where you know the triangulation results) in a stereo feature extraction system? Or was it digitized in 2D from a scanned map of indeterminate origins? These are critical questions for applications that require accurate data for making business decisions. The importance of accuracy is also application-specific. For example, in the USA the DOT’s (Departments of Transportation) require high-accuracy vector mapping data. If a road is built incorrectly, it is extremely costly to correct. Conversely, for an application such as a tourist map of a downtown area, often the relative accuracy is the only important factor. The precise coordinates aren’t so important, as long as everything lines up correctly and the person reading the map understands where they need to go.

For high-accuracy geospatial projects, customers (the organization purchasing the data) specify the accuracy requirements. Right now there are several standards for terrain, orthophoto, and mapping data. The USGS National Geospatial Program Standards are a good example. Other standards such as Federal Geospatial Data Committee standards can be found on the ASPRS Standards page. For a detailed look at accuracy standards, check out the Idaho Geospatial Committee's document for review on Aerial Mapping and Orthophoto Standards. It discusses accuracy in terms of the end-to-end photogrammetric workflow, from planning through final product generation.

I would also like to highlight Fugro EarthData’s recent Spring 2008 newsletter. One of the pieces, “The Call for a New Lidar Accuracy Reporting Framework”, features a discussion by industry experts Lewis Graham (GeoCue Corporation) and Karen Schuckman (Penn State University). This is a good read and I would recommend it for anyone interested in the topic of terrain accuracy standards. One of the key points is that many customers still define their project requirements in the form of contours: 2D cartographic representations of 3D data. Other topics explore horizontal accuracy, accuracy reporting standards, spatial variability of error, lidar processing errors, and more.

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