Generating Low Confidence Polygons

In the new ASPRS Positional Accuracy Standards for Digital Geospatial Data, low confidence areas within LIDAR data are defined to be where the bare earth model might not meet the overall data accuracy requirements. Generally speaking with LIDAR data this can occur where there is heavy vegetation that causes poor penetration of the pulses. If the Vegetated Vertical Accuracy (VVA) cannot be met then low confidence area polygons should be created and explained within the metadata. “For elevation data derived from LIDAR, the low confidence areas would include dense cornfields, mangrove or similar impenetrable vegetation. The low confidence area polygons are the digital equivalent to using dashed contours in past standards and practice.”

There are two types of low confidence polygons. The first kind of low confidence areas are those identified in advance of the collection. These are done by the data producer and are areas where the identification of bare earth is unlikely to occur. These are typically mangroves, swamps and other wetland marshes. The second are those identified post collection, and are typically valid VVA areas, such as forests. In these areas checkpoints should still be collected and accuracy assessments performed, however, these areas are delineated subsequent to classification and are easily identifiable by the reduced density within the bare-earth points.

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