Loudoun Imagery 1991 (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，影像以数字格式采集；而此前年份的影像均采用胶片相机拍摄。早期拍摄的胶片卷以胶片盒形式保存于该县档案中，但无法在现代数字地理信息系统（GIS, Geographic Information System）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。随后通过现代摄影测量技术对扫描影像进行处理，生成覆盖全县的数字正射影像镶嵌图（digital orthomosaic）。本次处理覆盖1979年、1986年、1988年及1990-2007年的航空影像，最终交付的格式为分幅未压缩GeoTIFF与MrSID镶嵌影像。其中1979年、1986年及1988年的影像为彩色（RGB）影像，其余时段的影像为全色（黑白，pan/B&W）影像。MrSID镶嵌影像采用50:1的压缩比。数据集同时附带了包含扫描名称属性的近似接缝线位置数据。本次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。由于影像采集于不同的时间与地面条件下，数据在色调、质量与位置精度上存在一定差异。本数据集为科罗拉多州科罗拉多斯普林斯市桑伯恩地图公司（Sanborn Map Company, Inc. Colorado Springs, CO）编制的数字正射影像。