Loudoun Imagery 1995 (BW)

Loudoun County has collected aerial imagery for many years. While imagery has been captured in digital format since the mid-2000s, prior years were captured with film cameras. Rolls of film from previous years were available at the County in film canisters, but unavailable for viewing on modern digital GIS systems. The County initiated a project to have these film rolls scanned into a digital format. The film was then processed using modern photogrammetric techniques to produce a digital orthomosaic of the County. Images from 1979, 1986, 1988, 1990-2007 were processed and delivered as tiled uncompressed GeoTIFF and MrSID mosaic formats. 1979, 1986, and 1988 were captured and produced as color (R,G,B) images, while the remainder were captured and produced as panchromatic (B/W) images. The MrSID mosaics use a 50:1 compression ratio. Approximate seamline locations containing the scan name attribute have also been provided. The aerial imagery generally encompasses the full land area of the County of Loudoun, VA. Images were flown in varying temporal and ground conditions and will exhibit some variation in tone, quality, and positional accuracy. "Digital Orthophotography compiled by Sanborn Map Company, Inc. Colorado Springs, CO"劳登县（Loudoun County）多年来持续收集航空影像数据。2000年代中期起，影像均以数字格式采集；而此前的影像则通过胶片相机拍摄完成。该县留存有早年拍摄的胶片卷，以胶片罐封装，但无法在现代数字地理信息系统（Geographic Information System, GIS）中直接查看。为此，该县启动专项项目，将这批胶片卷扫描为数字格式，随后采用现代摄影测量技术进行处理，生成覆盖全县范围的数字正射影像镶嵌图。本次处理并交付的影像涵盖1979年、1986年、1988年以及1990年至2007年的数据集，格式为分块未压缩GeoTIFF与MrSID影像镶嵌格式。其中1979年、1986年和1988年的影像为彩色（红、绿、蓝，RGB）影像，其余数据集均为全色（黑白）影像。MrSID影像镶嵌采用50:1的压缩比。同时，数据集还附带了包含扫描名称属性的近似接缝线位置数据。该航空影像数据集基本覆盖弗吉尼亚州劳登县的全部陆地范围。由于影像采集时的时间与地面条件存在差异，各影像在色调、成像质量以及位置精度上会存在一定差异。本数据集为科罗拉多州科罗拉多斯普林斯市的桑伯恩地图公司（Sanborn Map Company, Inc.）编制的数字正射影像。