| description:
|
This data represents a land use survey of Trinity County conducted by the California Department of Water Resources, Northern Region Office staff.Data development: Trinity County was surveyed using the 2005 one-meter resolution National Agriculture Imagery Program (NAIP) digital aerial photos from the U.S. Department of Agriculture's Farm Services Agency as a base for line work. Digital 7.5’ quadrangle sized images were created from the 2005 NAIP imagery. In the spring of 2006, DWR's Northern Region staff digitized land use boundaries using AutoCAD Map software.The digital images and land use boundaries were copied onto laptop computers that were used as the field data collection tools. Staff visited all accessible fields to positively identify agricultural land uses. These site visits occurred between June and August 2006. Land use codes were digitized directly into the laptop computers in the field using AutoCAD Map (using a standardized digitizing process). Some staff took printed aerial photos into the field and wrote land use codes directly onto these photo field sheets. The data from the photo field sheets were digitized using AutoCAD Map back in the office. For both data gathering techniques, any land use boundary changes were noted and then corrected in the office. The primary focus of this land use survey is mapping agricultural fields. Urban residences and other urban areas were delineated using primarily aerial photo interpretation, so some urban areas may have been missed. In some rural residential areas, urban land use was delineated by drawing polygons to surround houses or other buildings along with a minimal area of land surrounding these structures. These footprint areas represent the locations of structures but do not represent the entire footprint of urban land. Sources of irrigation water were not mapped in this survey. The linework and attributes from each AutoCAD drawing file were brought into ArcInfo and both quadrangle and survey-wide coverages were created, and underwent quality checks. The coverages were converted to shapefiles using ArcView. After quality control procedures were completed on each file, the data was finalized.Before final processing, standard quality control procedures were performed jointly by staff at DWR's Northern District, and at DPLA headquarters under the leadership of Jean Woods, Senior Land and Water Use Supervisor. After quality control procedures were completed, the data was finalized. The positional accuracy of the digital line work, which is based upon the 2005 orthorectified NAIP imagery, is approximately 6 meters, but in some areas linework may be 10 meters from the actual location. The land use attribute accuracy for agricultural fields is high, because almost every delineated field was visited by a surveyor. The accuracy is 95 percent because some errors may have occurred. Possible sources of attribute errors are: a) Human error in the identification of crop types, b) Data entry errors. |