The number and distribution of sampling points was determined according to the USGS/NPS Vegetation Mapping protocols. A stratified random sampling approach was used, distributing the sampling effort across the 16 vegetation associations. Polygons labeled with modified Anderson level II categories were not included in the thematic accuracy assessment sampling. The polygons labeled as mosaics were grouped with the association that was the least abundant between the two associations in the mosaic. The number of samples per association varied according to the rarity of the vegetation type, both in terms of number of polygons and polygon size. The following rules were used to determine the number of points assigned to each association:
Scenario A: The class is abundant. It covers more than 50 hectares of the total area and consists of at least 30 polygons. In this case, it is recommended that 30 polygons be selected at random from the set of association's polygons. One sampling point will be assigned to each of the 30 selected polygons.
Scenario B: The association is relatively abundant. It covers more than 50 hectares of the total area but consists of fewer than 30 polygons. In this case, it is recommended that 20 polygons be selected at random from the set of association's polygons, and that 1 sampling point be assigned to each of the 20 selected polygons. If the association contains less than 20 polygons, some polygons will contain multiple sampling points. The number of sampling points assigned to each polygon is determined by the relative area of that polygon compared with the other polygon in that association.
Scenario C: The association is relatively rare. It covers less than 50 hectares of the total area but consists of more than 30 polygons. In this case, it is recommended that 20 polygons be selected at random from the set of association's polygons. One sampling point will be assigned to each of the 20 selected polygons.
Scenario D: The class is rare. It has 5 - 30 polygons and covers less than 50 hectares of the area. In this case, it is recommended that 5 polygons be selected at random from the set of association's polygons. One sampling point will be assigned to each of the 5 selected polygons.
Scenario E: The association is very rare. It has fewer than 5 polygons and occupies less than 50 hectares of the total area. In this case, it is recommended that one sampling point be assigned to each polygon.
Scenarios B, C, D, and E were used in the sampling design. Scenario A was not used for this sampling design because no association met the criteria of that scenario. The number of sampling points for Northern Red Oak - Northern Hardwood Forest was doubled from that recommended by the protocol in order to better assess the accuracy of that type. In order to randomly select the polygons in Scenarios B, C, and D, the Create Random Selection tool in Hawth's Analysis Tools was used in ArcGIS. For all of the scenarios, the Generate Random Points tool in Hawth's Analysis Tools was used to randomly determine the location of the sampling points in the polygons. The total number of thematic accuracy assessment sampling points assigned through this procedure was 539.
In June 2006, each accuracy assessment point was located in the field using a Trimble GeoXM global positioning system (GPS) unit, with the datum set to North American 1983 (Conus) and the coordinate system set to Universal Trans-Mercator (UTM) zone 18.
Biological Informatics Program 
U.S. Department of the Interior |
U.S. Geological Survey
