COMMENT ON THE LIKELY ACCURACY OF THE DATA AND ANY LIMITATIONS ON GIS
The final important component and the backbone of GIS is data. Knowing what data types are available and understanding their limitations in the GIS application is very important. Once the data has been entered, it may need to be edited for accuracy or processed in other ways. The data is then manipulated and analyzed in accordance with the goals of the project. This data contains either an explicit geographic reference, such as a latitude and longitude coordinate, or an implicit reference such as an address, postal code, census tract name, forest stand identifier, or road name.
Accuracy is the degree to which information on a map or in a digital database matches true or accepted values. Accuracy involves the quality of data and the number of errors contained in a dataset or map.
The level of accuracy required for particular applications are diverse. Highly accurate data can be very difficult and costly to produce and compile. The more accurate the map, the more it costs in time and money to develop. Inaccurate data can result in inaccurate models and maps, skewing the results of the analysis and ultimately resulting in poor decisions. Too much accuracy can be as harmful to the success of a GIS project as too little.
Many factors besides resolution, influence how accurately features can be depicted, including the quality of data source, the map scale, the drafting skill and the width of lines drawn on the ground. In addition to this, human drafting errors will occur and can be compounded by the quality of the source maps and materials.
The mapping potential of each source of digital data is measured by its obtainable accuracy, which can be relative or absolute. Relative accuracy refers to the displacement between two points on a map, compared to the displacement of those same points in the real world, it is often more important and easier to obtain than absolute accuracy because users rarely need to know absolute positions. More often, they need to find a position relative to some known landmark, which is what relative accuracy provides. Users with simple data requirements generally need only relative accuracy.
Absolute accuracy refers to the relationship between a geographic position on a map and its real world position measured on the surface of the earth, it is primarily important for complex data requirements such as those for surveying and engineering based applications. A feature positioned on the aerial photo which has an accuracy of plus or minus 2.5 feet, is within plus or minus 2.5 feet of its true position in the real world.
Attribute accuracy refers to the precision of the attribute database linked to the map’s features. Attribute accuracy is most important to users with highly complex data requirements.
Maps are still useful for decision-making and analysis even though they are not entirely accurate. A map accurate for one purpose is often inaccurate for others since accuracy is determined by the needs of the project as much as it is by the map itself.
Users of GIS should be made aware of the limitations of the technology. For example, a typical GIS can technically adjust the scale of any map to user specifications. It is up to the user not to violate the precision of the data, even if the software will allow this function to be performed. For example, if data are mapped at a relatively small scale of 1:250,000, the user must decide if rescaling the map to a larger scale of 1:15,840 presumes more precision or accuracy than really exists in the data.
GIS provides a rigorous structure for spatial and attribute data, but it will not change the nature of spatial and attribute data, nor will it correct basic inaccuracies in data. While every effort is made to ensure that the data contained in the records is correct, errors do occur. Sometimes, much of the data is derived from third-party sources and may not have been conveyed accurately, or may be open to misinterpretation. The bottom line is, just because data are in a GIS does not mean that they are correct.
Credit:ivythesis.typepad.com
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