Location Errors from Primary and Secondary Data Sources (Especially for GATE-Geospatial 2022)

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We can examine location errors by referring to the data source used for digitizing.

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Finding Location Errors Using Secondary Data Sources

If the data source for digitizing is a secondary data source such as a paper map, the evaluation of location errors typically begins by comparing the digitized map with the source map. We can make a check plot of the digitized map at the same scale as the source map; superimpose the plot on the source map.

Map Shows Secondary and Tertiary to Primary Locations

How Well Should the Digitized Map Match the Source Map?

There are no federal standards on the threshold value. A geospatial data producer can decide on the tolerance of location error.

Spatial features digitized from a source map can only be as accurate as the source map itself. A variety of factors can affect the accuracy of the source map. The accuracy of a map feature is less reliable on a scale map than on a scale map.

For example, a meandering stream on a large-scale map becomes less sinuous on a small-scale map. Therefore, we must consider the map scale when we evaluate the location accuracy of digital maps.

Location Errors Using Primary Data Sources

Although paper maps are still the most common source for spatial data entry, new data entry methods using global positioning systems (GPS) and remote sensing imagery can bypass printed maps and map generalization practices.

The resolution of the measuring instrument determines the accuracy of spatial data collected by GPS or satellite images: map scale has no meaning in this case.

The spatial resolution of satellite images can range from less than 1 meter to 1 kilometer. Similarly, the spatial resolution of GPS point data can range from several millimeters to 20 meters or higher. These are the common sources of error.

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