Loudoun Imagery 1999 (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。自2000年代中期起，影像均以数字格式采集，而此前年份的影像均采用胶片相机拍摄。过往年份的胶片卷以胶片罐形式存放于该县，但无法在现代数字地理信息系统（Geographic Information System，GIS）中调取查看。该县启动专项项目，将这批胶片卷扫描转换为数字格式。随后，借助现代摄影测量技术对扫描后的胶片进行处理，生成覆盖全县域的数字正射镶嵌图（digital orthomosaic）。本次处理覆盖1979年、1986年、1988年以及1990年至2007年的影像，最终以分幅未压缩GeoTIFF和MrSID镶嵌格式交付成果。其中，1979年、1986年及1988年的影像为彩色（R、G、B）影像，其余年份影像均为全色（黑白）影像。MrSID镶嵌图采用50:1的压缩比。同时还提供了附带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区域。由于影像采集时的时间跨度与地面条件存在差异，影像在色调、质量及位置精度上会存在一定波动。本数字正射影像由科罗拉多斯普林斯市的桑伯恩地图公司（Sanborn Map Company, Inc. Colorado Springs, CO）编制。